Microclimate Research

Many vegetable crops do not perform to their full potential in short, cool, Saskatchewan growing season. Producers can, however, modify the environment a small scale, creating microclimates more suitable for growing high value, warm-season crops.

Growers have a number of options for altering the growing environments for their crops. Below is a collection of articles outlining some of the Vegetable Program's research into creating microclimates that benefit specific vegetable crops. The articles are available as HTML documents and PDF files.

NEW! The 2010 Vegetable Cultivar and Cultural Trials are now available online in PDF format. Visit the Cultivar and Cultural Trials page for the latest information from the Vegetable Program.

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Plastic mulch research

Weed Control in Row Centers PDF
The trial evaluated several herbicides for their potential to provide long-term weed control between rows of plastic mulch.

Evaluation of Biodegradable Mulches PDF
In trials conducted from 2007-2009 the performance of several colors of biodegradable mulch was evaluated.

Mulching Options for Sweet Corn PDF
This trial evaluated the performance of sweet corn on various mulching treatment.

Enhancing Crop Growth with Colored Plastic Mulches PDF
Trials conducted in 2000 and 2001 evaluate the performance of several high value warm season vegetable crops grown on a range of plastic soil mulches. The use of reflective mulches is also examined.

Influence of Mulches on Crop Establishment and Yield of Direct Seeded and Transplanted Pumpkins PDF
Two varieties of pumpkin were grown to evaluate the effects of plastic soil mulch on crop performance.

Potential to Double-Crop Plastic Mulch.. Canadian Journal Plant Science. 88: 187-193. Waterer, D., Hrycan, W. and Simms, T. 2007.

Influence of Soil Mulches and Method of Crop Establishment on Growth and Yields of Pumpkins. Canadian Journal of Plant Science 80: 385-388. Waterer, D.R. 2000.

Rowcover research

Tunnel Materials for Warm Season Vegetable Crops (2008-2010) PDF
Previous trials have demonstrated the benefit of using row covers to enhance early growth of warm season crops like melons and peppers. There are many types of covers available and it is important to select the type of covering material that best suits the needs of the crop. This trial examined the performance of several warm season vegetable crops (peppers, melons and tomato) grown with different types of row cover.

Plant Growth Control by Photoselective Filters PDF
Photoselective filters developed to alter the ratio of red to far-red light in incoming sunlight offer the possibility of growing compact plants without affecting yield. Trials in 2000 and 2001 compare photoselective row covers to traditional polyethylene row covers.

CO2 Levels in Low Tunnels PDF
Low tunnels restrict air movement to create warm and sheltered microclimates for warm season crops, but reduced airflow also creates CO2 deficits. This study looks at CO2 flucuations in the low tunnel and explores the benefits of adding CO2.

Microclimate Related Presentations

Tips on...Mulches and Rowcovers (57.1 MB)

High Tunnel Technology - Taking the Next Step (3.2 MB)
The risks and benefits of high tunnels are discussed High tunnel technology is compared with low tunnels and other crop covering technology.

Extending the Growing Season (36.2 MB)
Presents techniques for extending the growing season suited to use by back yard gardeners.

Weed Control in Row Centers

The rows of many horticultural crops are widely spaced to provide room for eventual crop growth and/or access by equipment and pickers. Consistent weed control in the space between rows (row middles) is necessary; otherwise the weeds will compete with the crop and/or interfere with the harvest. At present, most growers rely on tillage to control weeds in the row middles. However, tillage tends to stimulate the germination of weed seeds – necessitating repeated tillage operations over the course of the growing season. Repeated tillage is time consuming, burns fossil fuels, degrades the soil structure and contributes to the depletion of soil organic matter reserves. Tillage between the rows also becomes progressively less practical as the crop grows into the row middles.

Fig. 1. Weed free row middles

Use of soil applied herbicides that provide long-term residual weed control may represent a more effective, cost efficient and environmentally benign approach for achieving consistent and persistent weed control in row middles. As the herbicides are not applied directly to the crop (but rather way from the crop into the row middles) they are not subject to the rigorous approval standards required of products that are designed to be applied to the crop. The overall objective of this project was to demonstrate the potential to use herbicides to achieve long-term weed control in row middles. The trial were conducted at the U of S Horticulture Field Research Station in Saskatoon. This site has been in long term production of horticultural crops and therefore the weed population and species spectrum is typical of commercial horticultural operations.

June trial – the clay soil at the Saskatoon site was rotovated in early May to simulate standard production practices. This tillage would have controlled emerged winter-annual weeds but would also have triggered germination of spring annuals. Repeated rain events from mid-May through early June provided near ideal soil moisture conditions for weed germination. The herbicides were not applied until early June – as this is when growers would begin to focus on weed control in the row middles. Some spring annual weeds had begun to emerge at this time. To test the herbicides for control of newly emerged weeds as well as for the prevention of germination of weeds, each plot was divided in half – with one half rotovated again to produce a weed-free starting point and the other half left with the newly emerged weeds intact.

The range of herbicides tested was based on previous U of S work and suggestions from the industry;

a) Weedy control – no tillage beyond the initial field prep
b) Clean Start = Aim (carfentrazone) @ 14ml/a + Roundup (glyphosate) @ 0.5 l/a
c) Chateau (flumioxazin) @ 75g/a
d) Sencor (metribuzin) @ 200 g/a
e) Lorox (linuron) @ 1.9 L/a
f) Authority (sulfentrazone) @ 100 ml/a
g) Edge (ethafluralin) @12 kg/a
h) Goal (oxyflurfen) @ 500 ml/a
i) Corn gluten @ 1200 kg/a

All rates represent the amount of product applied.

All of the herbicide treatments were applied at the high end of label recommended rates for weed control within crops. The corn gluten represents an organic weed control option. All products except the Edge and the corn gluten were applied using a CO2 power small plot sprayer equipped with 80-02 flat fan spray nozzles. The Edge granules and corn gluten were evenly spread over the plot area and then lightly incorporating using a rake.

Each treatment block was 2m wide (width of a typical row middle) and 5 m long. The treatments were replicated 4 times in a randomized complete block design. Rainfall or irrigation within 1 week of application is required to activate a number of the herbicides. In 2010 several light rain events occurred soon after treatment of the plots.

Efficacy of the herbicide treatments was determined by evaluating the number of weeds present, the weed species present and the % of the plot surface covered by weeds at increasing intervals after application of the herbicide treatments (14, 28 and 56 DAT).

August trial – this second trial was used to confirm the efficacy of the "best" treatments identified in the June trial. The trial was established August 21 on two plots of recently rotovated land at the U of S Horticulture Research Station in Saskatoon. The plan was to have one of the plots managed in the standard manner, while the other plot would serve as a dryland site – to test the importance of moisture in activating the herbicides. However there were repeated rainfall events after establishment of this trial and no irrigation was required to activate the herbicides.

The herbicide treatments tested in this trial were;

a) Control = Roundup
b) Clean Start = Aim (carfentrazone) + Roundup
c) Chateau (flumioxazin) + Roundup
d) Sencor (metribuzin) + Rounpdup
e) Lorox (linuron) + Roundup

The earlier trial had indicated than the herbicide treatments that had provided the best long-term weed control were relatively ineffective at controlling emerged weeds. As many dandelions had survived the rotovation step used to prepare the plots prior to spraying, it was decided to add Roundup to all treatments as a means to deal with these emerged weeds. The herbicides were applied at the same rates using the same equipment as previously described. Each plot was 2 m wide and 5 m long, with 4 replicates of each treatment arranged in a randomized complete block design. Weed control was evaluated as previously described at 14, 28 and 56 DAT.

Results

June trial - The dominant weeds in the non-sprayed control treatments were typical of the vegetable growers' fields in Saskatchewan - common groundsel (Senecio vulgaris), annual sow-thistle (Sonchus oleraceus), dandelion (Taraxacum officinale), portulaca (Portulaca oleraceae) and red-root pigweed (Amaranthus retroflexus). At the first evaluation (14 DAT), weed growth (% ground coverage) in the non-tilled control plots was substantially greater than in the adjacent tilled plots. This trend persisted through the 2nd evaluation at 28 DAT but by the final evaluation at 56 DAT the ground was almost completely covered by weeds in both the tilled and non-tilled control treatments. At 14 DAT the weed control provided by the Clean Start, Sencor and Lorox treatments was superior to the non-treated control in both the tilled and non-till plots. At the 2nd evaluation (28 DAT), the Clean Start, Sencor and Lorox treatments were still providing a very high degree of weed control, with the Authority and Dual II treatments providing a lesser but still significant degree of control.

At the final evaluation (56 DAT) only the Lorox and Sencor treatments were still providing a level of weed control that was significantly superior to the non-treated control in both the tilled and non-tilled plots. Portulaca and common groundsel represented the majority of weed escapes in the treatments that had earlier provided some degree of weed control. The Sencor treatment appeared to provide a superior level of portulaca control relative to all other treatments.

August trial – wet cold weather from mid-August through mid-September delayed development of the weeds in this trial. At the 14 and 28 DAT evaluations there was basically no weed growth in any treatment. Good weather from mid-September through early October allowed some weed growth (24% ground coverage in controls) by the final evaluation at 56 DAT. Dandelion, common groundsel and annual sow-thistle were the dominant weeds in the control treatments. Treatment responses were similar at the two sites and the data were therefore combined. All of the selected herbicide treatments provided a very high degree of weed control (Fig. 2) – although none of the treatments was consistently effective at controlling established dandelions.

Fig. 2. Influence of herbicide treatments on % ground cover by weeds – August trial. * = % ground cover is significantly different (P=0.05) from the control.

Conclusion – this project illustrated that a high degree of control of the broad spectrum of weeds commonly found in vegetable fields could be achieved by applying long-lasting soil active herbicides such as linuron and sencor to the row middles. These products were most effective when applied prior to weed emergence but very effective control of emerged weeds could be achieved by mixing the sencor or linuron with a non-selective contact herbicide such a glyphosate or carfentrazone. The maximum duration of efficacy observed in this project was about 2 months. This would be more than enough time to allow a vine crop like cucumbers or pumpkins to become well established with the vines filling in the row centers, effectively eliminating the need for any further weed control. In slower growing or more upright crops like tomato or peppers a second application might be required in mid-July to keep the row middles weed free for the duration of the growing season. Late season applications of herbicides with a long soil residence time can raise concerns about the potential for herbicide damage to sensitive crops seeded into the treated area early in the next growing season.

This project was made possible by support from the ADOPT program of Saskatchewan Agriculture

Canadian Journal of Plant Science

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Volume 90, Number 5, September 2010
ISSN 1918-1833

Evaluation of biodegradable mulches for production of warm-season vegetable crops

D. Waterer
Page(s) 737-743  | doi:10.4141/CJPS10031 | Published by the Agricultural Institute of Canada

Abstract: While plastic soil mulches are widely used in vegetable crop production, removal and disposal of these plastics at the end of the growing season is costly and environmentally unsound. This trial assessed the field performance of several colors of corn-starch-based biodegradable mulches for the production of warm season vegetable crops (sweet corn, zucchini, cantaloupe, pepper and eggplant) over three cropping seasons in Saskatchewan. The clear and wavelength selective types of mulch most commonly produced beneficial effects on the rate of crop development and yields. This may be attributed to these mulch types enhancing soil temperatures, especially early in the growing season. There were no appreciable differences in the soil temperatures or crop growth and yield responses on the biodegradable mulches as compared with the same color of standard low-density polyethylene mulch. The biodegradable mulches were easy to apply and were readily incorporated into the soil at the end of the growing season. Although the clear and to a lesser extent the wavelength selective forms of biodegradable mulch tended to break down well before the end of the growing season, this early failure did not negatively impact the performance of any of the crops tested, as long as supplemental weed control was provided. Supplemental weed control would be more important for slow-growing, erect crops like peppers and eggplants than for the more robust and sprawling crops like corn and melons. Although the biodegradable mulches are more expensive than the corresponding standard polyethylene-based plastics, this added cost is more than offset by the costs to remove and dispose of the standard plastic mulches.

Key words: Sweet corn, pepper, zucchini, eggplant, cantaloupe, Biotelo

Résumé: Si l’on utilise abondamment les paillis de plastique pour le maraîchage, leur retrait et leur élimination à la fin de la période végétative coûtent cher, et l’environnement en subit les conséquences. L’essai devait servir à évaluer la performance sur le terrain d’un paillis en fécule de maïs biodégradable de couleur variée pour la production de fruits et de légumes de la belle saison (maïs sucré, courgette, melon brodé, poivron, aubergine), durant trois périodes de production, en Saskatchewan. Le paillis translucide et ceux sélectionnant une longueur d’onde ont des effets bénéfiques sur la rapidité du développement et sur le rendement des cultures. On l’attribue au fait que ces paillis haussent la température du sol, surtout au début de la période végétative. On ne note aucun écart appréciable entre la température du sol ou la croissance et le rendement des cultures obtenus avec le paillis biodégradable et le paillis ordinaire de même couleur, en polyéthylène de faible densité. Le paillis biodégradable s’applique facilement et est prêt à être incorporé au sol à la fin de la saison. Bien que le paillis biodégradable translucide et, dans une moindre mesure ceux de couleur, ait tendance à se désagréger longtemps avant la fin de la période végétative, cette détérioration n’a aucune incidence négative sur le rendement des cultures testées, pourvu qu’on adopte des mesures supplémentaires de lutte contre les mauvaises herbes. De telles mesures revêtiraient plus d’importance avec les cultures lentes, à port érigé, comme le poivron et l’aubergine, qu’avec les cultures plus robustes et rampantes tels le maïs ou le melon. Si les paillis biodégradables coûtent plus cher que le polyéthylène usuel, la dépense supplémentaire est plus que compensée par les frais découlant du retrait et de l’élimination du paillis de plastique.

Mots clés : Maïs sucré, poivron, courgette, aubergine, melon brodé, Biotelo

Mulching Options for Sweet Corn

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Germination and early growth of sweet corn can be enhanced by the soil warming provided by plastic mulches. Clear plastic provides the greatest degree of soil warming but weed growth can be a problem under clear mulch. Wavelength selective (IRT) mulches provide almost as much soil warming as the clear plastic while also providing a high degree of weed control. Planting corn through any type of plastic mulch requires specialized machinery. A lower cost option used by some growers is to create a shallow trench, and then to seed the corn into the trench using standard seeding equipment. Clear plastic mulch is then laid over the seeded rows. The trench provides sufficient space for the corn seedlings to develop for a week or two within the greenhouse-like environment provided by the clear mulch. The plastic must be removed before temperatures in the trench become excessive.

This trial evaluated the performance of sweet corn on various mulching treatments.

Materials/Methods

The trial was conducted in 2006 at the University of Saskatchewan Horticulture Field Research Facility in Saskatoon. The heavy clay soil at the test site was prepared by rotovating one week prior to laying the mulch and seeding the crop. The treatments tested were;

• 1) Crop mechanically seeded using a push type small plot seeder. No mulch used
• 2) Crop mechanically seed into a trench (ca 30cm wide * 10cm deep) cut by hand. Clear plastic mulch (1.1 mil) then laid over the trench
• 3) Crop hand seeded through clear plastic mulch (1.1 mil)
• 4) Crop hand seeded through IRT mulch (1.1 mil)

The cultivars tested were 'Navajo' (early maturing Se type) and 'Fantastic' (later maturing Sh2 type). For each mulch treatment, each cultivar was seeded in twin 4m long rows spaced 20cm apart, with 15cm between plants within a row. The trial was seeded on May 23. Drip irrigation tape was used to supply water as needed under the various mulch treatments.

Results

The crop emerged more quickly in the trench and clear mulch treatments than when the IRT mulch or no mulch was used. The clear plastic was removed from the trench treatment on June 14. By that time the plants had grown to about 20cm in length but were held close to ground by the clear plastic mulch. Although temperatures were in the mid-20's on several days prior to removal of the clear covering, there were no indications of high temperature stress to the corn plants in the trench. However, growth of the plants in the trench treatment appeared to slow after removal of the plastic covering. Considerable populations of common groundsel and portulaca developed in all treatments except the IRT.

The mulch treatments had no effect on the rate at which 'Navajo' matured, but both of the standard mulch treatments accelerated maturity of the later maturing 'Fantastic' (Table 1). The clear produced the highest yields (cob number and weight), followed closely by the trench treatment for cob weight and the IRT treatment for cob number (Table 1). The no-mulch treatment produced the lowest yields of both cultivars. Cob quality and flavor were not affected by the mulch treatments.

Conclusion

This trial again demonstrated the potential benefits that can be obtained from using mulches during production of sweet corn. Although it had no clear effect on the rate of crop development, clear mulch appeared most beneficial in terms of enhanced yields. This may reflect a better stand or healthier cob development on the clear mulch. The trench + clear treatment produced yields that were equivalent to the clear mulch, but without the added labor or machinery cost associated with planting through the plastic.

Table 1. Influence of various mulching options on development and yields of sweet corn in 2006

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Enhancing Crop Growth with Colored Plastic Mulches

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The use of black or IRT soil mulches for weed control and clear mulches for increasing soil temperatures are common practices in vegetable crop production. The development of new colors of mulch may allow growers to alter other aspects of crop growth. Red mulch has been shown to increase early and total yields of tomatoes. Blue mulch improved both the growth and flavor of turnips. Yellow and silver mulches have been shown to repel certain insect pests. Trials conducted in 2000 and 2001 evaluated the performance of several high value warm season vegetable crops grown on a range of mulch colors.

The trial was conducted at the Department of Plant Sciences Horticulture Field research station in Saskatoon in 2000 and 2001. In 2000, clear, black, wavelength selective (IRT), blue, yellow and white mulch were tested. In 2001, silver and red mulches were added to the trial. The mulches were laid about a week prior to establishment of the crop. The peppers, tomatoes and melons were transplanted while the zucchini was direct seeded. Ten plants of each crop were tested. Drip irrigation was used to supply the crops’ moisture requirements. The plots were harvested twice weekly once the fruit reached the following stages; peppers - 50% red, tomatoes - breaker stage, melons - full slip, zucchini - 20 cm long. Just prior to the first killing frost, the plots were completely harvested and total fruit yields determined.

Conclusion

The responses to the mulches varied with the crop and the cropping season. In 2000, red and blue mulches appeared to enhance growth and yields, while yellow and white mulches performed poorly. By contrast, in 2001, the most reflective mulches (white and silver) produced the best yields. Colored mulches are substantially more expensive than standard types. Growers should consider both the added expense and the lack of consistent responses before selecting to grow with colored mulches.

Light Reflection from Silver Mulch

Silver mulches have been reported to increase plant growth by reflecting light striking the ground back into the undercanopy of the crop. In 2002, peppers (Paladin) and tomatoes (Sunbrite) were grown on standard black plastic mulch. Another strip of mulch was laid adjacent to each side of the crop row. In one treatment, the additional rows of mulch were black plastic while the other treatment was silver. The crops were harvested just prior to frost and the fruit was graded for maturity and freedom from defect.

Results

Positioning strips of silver mulch adjacent to the row increased yields of both crops. In-season measurements of light levels demonstrated that the silver mulches did reflect light back into the canopy. This may explain the increased productivity. Silver mulch is costly and laying multiple layers of mulch in close proximity is impractical. However, this study does demonstrate the merit of using reflective mulches to increase crop growth.

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Influence of Mulches on Crop Establishment and Yield of Direct Seeded and Transplanted Pumpkins.

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Pumpkins are a high value warm season crop which should respond well to the benefits supplied by soil mulches. Pumpkins can be established by either direct seeding or via transplanting. Direct seeding represents a lower cost option however, soil temperatures in the spring are rarely near the optimum for germination of pumpkins. Soil mulches should enhance germination of pumpkins as they increase soil temperatures. Higher soil temperatures under mulches should also accelerate crop development, increase yields and/or accelerate crop maturity. The benefits of mulch should be most apparent when slow developing cultivars are grown or when growing conditions are less than ideal.

In 1997 and 1998, the influence of mulches on two cultivars of pumpkin established from seeds or transplants was evaluated at the Plant Science Department Research Station in Saskatoon. The cultivars tested were;

• “Spirit” -a relatively early maturing cultivar which produces excellent yields of medium sized fruit.
• “Howden” - a later maturing cultivar which produces large fruit with excellent color

The mulch treatments were;

• a) bare ground
• b) black plastic mulch
• c) clear plastic mulch

The crop was established in late May by either direct seeding through the mulch or by transplanting 10 day old seedlings. Plants were spaced 50 cm apart within each row. The mulch was applied about one week prior to crop establishment to allow for some soil warming. In the direct seeded treatment, a minimum of two seeds were placed in each planting hole. The crop was thinned to one seed/hole at three weeks after emergence. The crop was protected with a floating row cover until early June. Drip irrigation was used to supply water and nutrients to the crop. The fruit were collected in a once-over harvest in early October. Fruit were counted, weighed and evaluated for maturity based on degreening. Each treatment was replicated four times and each 5m long plot contained 10 plants.

Results

Both growing seasons were warmer than normal, with accumulated growing degree days (base 10°C) over the production season 33% and 17% greater than normal in 1997 and 1998 respectively. In 1997, the first killing frost was 4 weeks later than normal. In 1998, April and May were exceptionally warm resulting in substantial soil warming prior to planting.

Plant stands for both cultivars were more complete in 1998 than in 1997 (Table 1). The exceptionally early spring in 1998 produced conditions better suited to establishment of pumpkins than in 1997. Both crop establishment methods produced excellent stands. Transplanting may have been more advantageous if conditions prior to and after seeding had been less favorable. For both cultivars, establishing the crop on plastic mulch improved the stand relative the non-mulched control (Table 1). Mulches conserve soil moisture and raise soil temperatures; changes beneficial to establishment of both the direct seeded and transplanted crops. For cv. Howden, the clear plastic mulch produced a better stand than the black plastic. Howden had a much lower germination rate than cv. Spirit even under the greenhouse conditions used to establish the transplants. Germination of cv. Howden may be sensitive to unfavorable environmental conditions and might benefit from the warmer soil temperatures provided by the clear mulch.

The transplanted crop out-yielded the direct seeded crop in both cultivars. Since both establishment methods produced comparable stands, the yield difference must have been linked to the transplants enhancing canopy growth and/or fruiting. For cv Spirit, the clear plastic mulch produced yields substantially greater than either black plastic or the non-mulched control. For cv. Howden, both mulches improved yields relative to the control. The benefits of mulching were likely related to enhancement of soil temperatures. Clear plastic is more effective for soil warming than black, but this difference only appeared to benefit yields for cv. Spirit.

Complete degreening of pumpkins is crucial to successful marketing. For cv. Spirit, a greater proportion of the crop had matured to orange prior to harvest in 1998 than in 1997 (Table 1). This was unexpected, as 1997 was warmer than 1998 and had a long frost-free fall; both these factors should have been conducive to fruit ripening.

Table 1. Main Effect Means for cv. Spirit and Howden Pumpkins Grown Using Two Crop Establishment Methods on Various Mulches in 1997 and 1998.

Relatively few of the slow growing cv. Howden, fruit were mature by harvest time irrespective of the year (Table 1). Transplanting and mulching did not significantly enhance the maturity of either cultivar. By substantially increasing the fruit load on each plant, these management practices may have retarded the rate of development of each fruit, thereby effectively canceling out the expected advancement in maturity.

In summary, maximum yields of pumpkins were obtained when the crop was grown on plastic mulches using transplants as a means for crop establishment. Production or purchase of transplants and laying of mulch both represent added production costs; growers need to examine the relative costs versus benefits before adopting these more intensive management practices.

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Tunnel Materials for Warm Season Vegetable Crops

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Previous trials have demonstrated the benefit of using row covers to enhance early growth of warm season crops like melons and peppers. There are many types of covers available and it is important to select the type of covering material that best suits the needs of the crop. This trial examined the performance of several warm season vegetable crops (peppers, melons and tomato) grown with different types of row cover.

The trial was conducted at the University of Saskatchewan Horticulture field Research Station in 2008 and 2009. This site features a Sutherland Series heavy clay soil which is slow to warm in the spring. The site was prepared by rotovating one week prior to transplanting the test crops. Three days before transplanting a biodegradable wavelength selective plastic mulch was applied to the test plots. Drip irrigation lines were located beneath the mulch. Three week old cantaloupe (cv. ‘Strike’ in 2008 and ‘Athena’ in 2009) or 6 week old pepper (cv. ‘Red Start’) transplants were planted out into the mulched rows once the risk of frost had passed in early June. Tomato (cv. Celebrity) (6 wk old transplants) was also tested in 2009. The melon and pepper plants were spaced 30 cm apart within each row, while the tomatoes were spaced 0.5 m apart. Each test plot was 3 m long and the rows were 2m apart. Immediately after transplanting, row covers were installed on all plots except for a non-covered control. The cover materials tested were;
a) perforated clear polyethylene – transmits about 85% of the light in the visible spectrum. Clear perforated polyethylene creates a very warm environment.
b) non-perforated clear polyethylene – would be expected to create an environment even warmer than the equivalent perforated clear poly.
c) perforated green polyethylene - filters out about 41 % of incoming light within the visible spectrum, resulting in cooler temperatures. Filtering certain wavelengths out of the incoming light may also alter the growth pattern of the covered crops.
d) perforated white polyethylene - filters out about 26 % of the incoming visible light.
e) Novagryl woven polyester - filters out about 20 % of the incoming visible light and is also highly perforated.

The covers were supported by metal hoops (45 cm tall at peak) installed over each row to create a low tunnel. Each treatment was replicated 3 times in a randomized complete block design. Temperatures and crop condition were monitored inside the tunnels. The tunnels were removed once the crop started to flower - except for the non-perforated treatments where the tunnels had to be removed earlier due to problems with overheating and crop stress. No problems with insect pests or diseases were observed in 2008 trial and no pest management measures were required. In the 2009 trial some of the pepper plants were lost at mid-season to root rot. Problems with this disease were uniform across the tunnel treatments. Mouse damage to the fruit was a problem in all treatments in the tomato trial.

The peppers were taken in a once-over harvest in mid-September. The fruit were graded into red ripe, mature green and immature categories. The melons were harvested weekly, with the fruit picked at half slip maturity. Any melons remaining after the first killing frost were taken in a once-over final harvest (Sept 22 in 2008, Oct 5 in 2009). Tomatoes reaching the breaker stage of maturity were harvested weekly. Any fruit remaining on the plants were harvested just after a killing frost in early October.

Results

2008 growing season – temperatures in May and June were cooler than normal but there were no frosts after the trial had been established. Temperatures in July and August were close to normal, while September was abnormally warm. The frost free season (May 26 to Sept 26) was about 4 days longer than the 30 year average.

2009 growing season – May and June of 2009 were exceptionally cool. Frost was recorded on four occasions after the trial was established (June 1, 4, 6 and 9). Some frost damage was observed on the plants covered with clear polyethylene as well as the non-covered plants. This frost damage coupled with wind damage resulted in the loss of about 10% of the plants in the treatments that were not protected by a row cover. Temperatures remained slightly below normal through July and August, but September was exceptionally warm, with the first frost delayed by two weeks relative to the longterm norm. The near-perfect fall conditions allowed the otherwise delayed crops to mature. The total length of the frost free season in 2009 was about 10 days shorter than normal.

Temperature profiles - Temperature profiles under the various tunnel materials for the period when the tunnels were in place in 2008 (June 5-July 2) are presented in Fig 1, while the temperature profiles for 2009 (June 3-July 3) are presented in Fig 2. Temperature monitors in the Novagryl treatment failed in 2009 and no data were collected. Temperature profiles for the various tunnel treatments were similar in the two years of testing. On sunny days, all of the covering materials increased air temperatures inside the tunnels relative to having no cover. The tunnels had little impact on temperatures at night or on cloudy days. Temperatures under the opaque materials (white, green and Novagryl) were comparable, running about 5C warmer than air temperatures in 2008, with slightly less temperature enhancement seen during the cooler conditions encountered in spring of 2009. Temperatures inside tunnels constructed of clear poly were far higher than for the other materials tested. The perforated clear poly tunnels produced daytime temperatures that were a few degrees cooler than the non-perforated covering.

During the frost events that occurred in 2009, none of the tunnel treatments provided any significant degree of frost protection and in many cases temperatures under the tunnels were marginally lower than when no cover was used. As polyethylene is both a poor insulator and fairly transparent to long wavelength radiation its potential to retain heat through the night is limited.

The clear poly coverings were clearly beneficial to growth of the crops early in spring when conditions were cool. However, by mid-June of 2008 temperatures inside the non-perforated clear poly tunnels were becoming excessively high, especially for the peppers which were showing signs of heat stress (marginal firing of leaves). In the 3rd week of June of 2008, outdoor temperatures reached the mid-20oC range and none of the peppers and few of the melons in the non-perforated clear poly tunnel survived, as temperatures in the tunnels exceeded 65oC for several days in a row. Some heat stress damage was also observed on peppers in the perforated clear poly tunnels where peak temperatures reached 60oC. There were fewer indications of heat stress under the clear poly tunnels during the cooler conditions experienced in 2009. Nonetheless temperatures in excess of 55oC were recorded under the clear poly tunnels in 2009. There were no obvious differences in crop growth under the various opaque coverings. At the time of tunnels removal, all covered crops appeared to be larger than the crop grown without a covering but it did not appear that flowering was advanced or enhanced by the tunnels.

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Evaluation of Rowcover Treatments for Warm Season Crops

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Although the benefits of using row covers to protect warm season crops like peppers and melons are well established, the best type of rowcover to use is still a valid question. Perforated clear polyethylene covers result in the greatest enhancement of air temperatures in the vicinity of the crop. Although this may be beneficial when conditions are cool, temperatures under clear poly covers may become excessive on warm sunny days. Porous rowcover materials such as Reemay allow for more air movement resulting in more moderate conditions inside the tunnels.

Recently the concept of zip tunnels has been introduced in an effort to combine the best of both types of rowcovering. Zip tunnels are constructed of clear non-perforated polyethylene to produce maximum warming, but the tops of the tunnels are always open ... allowing some ventilation. A drawstring system allows the size of this opening to be varied, depending on prevailing conditions and the needs of the crop.

This trial evaluated the performance of two cultivars of bell pepper (King Arthur and Double Up) grown using clear poly, Reemay or zip tunnel systems and two cultivars of cantaloup, (Earligold and Gold Express) grown using clear poly or zip tunnels. Transplants of each cultivar were planted out in late May. Wavelength selective soil mulch was used along with drip irrigation. Sections of each row were covered with tunnels constructed by draping perforated clear polyethylene or Reemay rowcover over metal hoops. The zip tunnels are supported beside and above the plants using plastic stakes stapled onto the tunnel material.

The Reemay and clear poly tunnels were left in place until early July, by which time temperatures were more consistently favorable to growth of the test crops. The zip tunnels were left in place for another 2 weeks in melon crop and were never removed in the peppers. This decision was based on the premise that overheating would not be a problem as the zip tunnels as supposed to provide superior ventilation. Pepper fruit were harvested once they reached 50% red, with a final harvest just prior to the first killing frost in late September. The melons were harvested at full slip stage, with a final harvest just prior to frost.

Results

The zip tunnels were initially set to have minimum ventilation, as the days after the crop was transplanted were consistently windy. Although the test site was fairly sheltered, the zip tunnels blew apart on several occasions and had to be re-installed. The plastic stakes used to support the sides of the tunnel were not sufficiently strong to prevent the zip tunnels from being blown virtually flat by high winds. The standard tunnels withstood these wind events with minimal damage. The drawstring system designed to allow the tunnels to be opened or closed did not work well. This prevented us from testing the efficacy/practicality of opening or closing the tunnels in response to prevailing conditions.

Aside from the damaging winds encountered at transplanting, growing conditions were excellent in 2003. An unusually severe infestation of aphids occurred early in the season while the pepper plants were still under the row covers. This delayed timely detection of the problem and also made treatment difficult. However good control of the problem was eventually obtained by applying a systemic insecticide through the drip system. Temperatures inside the zip tunnel closely matched those in the clear poly tunnel.

Peppers

King Arthur was far earlier than Double Up. The first fruit ripened to mature red about 5 days earlier in the Reemay treatments than in the other treatments. The enhanced earliness in the Reemay treatments was even more clearly illustrated by the fact that over 25% of the fruit in that treatment turned red prior to frost, as compared to only 7-12% in the other treatments. Total yields of marketable fruit (mature green + red) were similar in all three treatments, but the value of the crop in the Reemay treatments would have substantially greater as red fruit command a substantial price premium. Although the warmer conditions characteristic of the clear polyethylene rowcovers and zip tunnels appeared to enhance early growth of the pepper transplants, it also appear to retard fruit set. The pollen of peppers is damaged at temperatures exceeding 25°C - while temperatures under clear polyethylene tunnels can exceed 40°C even on relatively cool days. The more porous woven Reemay covers appear to produce more stable and moderate growing conditions - eventually leading to higher yields.

Melons

Crop growth was quite poor in the trial irrespective of the covering treatment. The plants were slow to start fruiting and the fruit were small and slow to mature. Rodent and disease damage to the fruit were severe. Although total yields under the two types of crop cover were similar, the poly covers allowed twice as many of the fruit to reach maturity prior to frost than in the zip tunnels. As the two types of cover produced similar temperatures, this yield difference must be related to some other variable. It is possible that leaving the zip tunnels in place for an additional two weeks interfered with bee activity, thereby delaying fruit set.

Conclusion

Although the zip tunnels theoretically represent the best of both the clear and woven rowcover systems, we found them to be costly and difficult to manage, with no appreciable advantages over the standard types of rowcover system.

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Plant Growth Control by Photoselective Filters

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Many warm season vegetable crops benefit from the cover provided by low tunnels. The period that plants can be left in this protected environment is limited - as the plants quickly outgrow the available space. Recently, photoselective filters have been developed that alter the ratio of red to far-red light in incoming sunlight. Plants grown under this filtered light are short and robust - making them ideally suited for a confined growing environment. We conducted trials from 1999-2001 to determine if growing crops under row covers constructed of a photoselective type of polyethylene would produce a more compact growth habit - allowing the covers to be maintained over the crop for extended periods.

In 2000, Melons (cv Earligold) were transplanted into rows of IRT mulch in early June. At transplanting, sections of each row were covered with tunnels constructed of either standard perforated clear polyethylene or photoselective filter (Photomorphogenesis Control Film - YXE10 Mitsui Chemicals Inc.). The tunnels were supported above the crop using wire hoops. The ends of the tunnels were closed.

The period after transplanting was very warm and the plants in the both treatment suffered severe heat stress. The plants under the photoselective tunnels were particularly hard hit as this material was not perforated - resulting in extremely high temperatures in the tunnels. Due to this crop damage, this trial was terminated and the crop was replanted in late July. The lateness of the season was not relevant as the objective was to evaluate early crop growth. The ends of the tunnels were kept open in this trial to prevent overheating. At seven weeks after transplanting, five plants from each treatment plot were harvested and measured for vine length and fresh weight. The plants were just beginning to set fruit at this point.

In the 2000 trial, melon vines grown inside wavelength selective tunnels were longer and weighed less than those produced in the standard perforated poly tunnel. The changes in crop development caused by the photoselective cover in the 2000 trial were not desirable from the perspective of controlling elongation while maintaining crop health. As the wavelength selective covering was not perforated, it is possible that the results in 2000 were related to overheating.

In the 2001 trial, melons (cv. Earligold) and peppers (cv. Legionnaire) were transplanted into black mulch in early June. At transplanting, sections of each crop row were covered with tunnels constructed of either non-perforated clear polyethylene or photoselective filter The tunnels were supported above the crop using wire hoops. The ends of the tunnels were closed.. The covers were removed in mid-July at which time three plants from each treatment replicate were measured and weighed. The trial continued until the first frost at which at time the fruit were counted and weighed.

Results

Using wavelength selective plastics as row cover materials provided no appreciable benefits in terms of altering plant growth or enhancing yields. The changes in crop morphology were largely negative and could be attributed to the wavelength selective films producing a hotter and more shaded environment than standard plastics.

Comparison of plant growth 7 weeks after transplanting for Earligold melons grown using wavelength selective or non-perforated clear polyethylene row covers. 2000 trial.

Comparison of plant growth 6 weeks after transplanting and final fruit yields for Earligold melons grown using wavelength selective or non-perforated clear polyethylene row covers. 2001 trial.

Comparison of plant growth 8 weeks after transplanting and final fruit yields for Legionnaire peppers grown using wavelength selective or non-perforated clear polyethylene row covers. 2001 trial.

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CO2 Levels in Low Tunnels

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Plants use the energy in sunlight to convert carbon dioxide (CO2) and water into the sugars that form the basic building blocks for plant growth. Plant growth rates are strongly influenced by the availability of CO2. Under normal field conditions, CO2 levels are typically between 350 and 400 ppm. Once CO2 levels drop to 200 ppm, photosynthesis and growth slow, while higher than normal CO2 levels can accelerate plant growth. Deficient CO2 levels commonly occur within the confines of greenhouses as the rate at which the greenhouse crops absorb CO2 exceeds the rate it is replenished by introducing fresh outside air into the greenhouse. When greenhouse growers add CO2 to overcome this problem, they can substantially boost their yields if they add CO2 well in excess (ca 1000 ppm) of the normal atmospheric levels.

In Saskatchewan many growers are using low tunnels constructed of polyethylene or woven fibre to create the warm, sheltered microclimate preferred by crops like tomatoes, peppers and melons. Like a greenhouse, the low tunnels work by restricting movement of air in the vicinity of the crop - but the covers may also restrict replenishment of the CO2 utilized by the crop. This study examined the CO2 levels inside a standard low tunnel and explored the potential for adding CO2 into the tunnels as a means to enhance productivity.

The trials were conducted in 2001 and 2002 at the Department of Plant Sciences Horticulture Field Research Facility in Saskatoon. Peppers and melons were selected for testing as they respond well to the low tunnels. The plots were prepared by rototilling and then rows of wavelength selective IRT mulch were laid over standard drip tube. Greenhouse grown seedlings of the test crop were then transplanted into the mulch rows. The crops were then covered by standard low tunnels (1 m wide and 40 cm tall) constructed by covering wire hoops with clear polyethylene (perforated and non-perforated) or spunbond fibre (Reemay). The edges of the coverings were buried in the soil. Each test plot of the various coverings involved a minimum of 10 plants spaced 30 cm apart within the row. Each cover treatment was replicated at least twice. The crops were irrigated as required using the drip lines.

CO2 levels in the low tunnels were monitored by either placing a small CO2 analyser (Bacharach Model 2800) right inside the tunnel or by collecting a sample of air from inside the tunnel and running it through the analyser.

Results

CO2 levels over several days inside a low tunnel constructed of perforated clear polyethylene. CO2 levels in the low tunnels varied with the time of day, outside conditions and the type of cover used. In general, CO2 levels peaked just before dawn and then declined through the mid-point of the day, and then began to recover (Figure 1 and 2). CO2 levels at mid-day were well below ambient levels, reflecting the photosynthetic activity of the plants within the confines of the tunnels. By contrast, at night, CO2 levels in the tunnels were well above ambient - this reflects the tunnel materials trapping the CO2 generated at night by the respiration of the plants and the soil micro-organisms. The extent of the diurnal fluctuation in CO2 levels depended on the porosity of the covering material used - it was most noticeable when the tunnels were constructed of non-perforated clear polyethylene and it was barely perceptible when the tunnels were constructed of the more porous woven materials. Wind rapidly eliminated the CO2 gradients, irrespective of the type of cover employed.

These results suggest that plants growing within the confines of low tunnels may be experiencing growth limiting shortages of CO2, particularly if the tunnels are constructed of relatively non-porous materials. Increasing the rate of ventilation of the tunnels would overcome this problem, but it would also interfere with the growth enhancing effect of the higher temperatures that occur within the confines of a closed tunnel.

The drip lines used to irrigate the crop represented a potential means for delivering supplemental CO2 to the crop within the covered rows. In this trial, the drip lines were occasionally disconnected from the water source and re-attached to a tank of compressed CO2. In-line regulators were used to control the rate flow of CO2 into the lines. Two rates of CO2 addition were tested ... the higher rate represented double the lower rate. The CO2 was typically injected for 1-2 hours commencing in mid-morning. CO2 was not injected on cloudy days, as bright sunshine is required for the plants to utilize the CO2. Similarly, CO2 injection was canceled on days when the wind speed exceeded 25 km/h, as preliminary trials showed that any CO2 injected into the tunnels under windy conditions was rapidly lost. Changes in CO2 levels in the tunnels in response to the added CO2 were monitored using the previously described system.
Levels of CO2 achieved within the low tunnels in response to the CO2 injection treatment varied with the duration of application, the rate of CO2 applied, distance from the injection point and the type of covering used.

An increase in CO2 levels in the tunnels was detectable within minutes of the start of the injection period (Figure 2). CO2 levels usually peaked within the first hour, with the level achieved at the plateau reflecting the amount being injected versus the amount being lost due to ventilation. A similar pattern in CO2 levels was observed when the CO2 injection stopped - with the rate of decline reflecting the rapidity of air exchange within the tunnels (Figure 3). Almost invariably, CO2 levels returned to ambient within 1-3 hour of termination of the injection. Throughout the injection period, the CO2 levels measured near the injection point were considerably higher than those further down the row (Figure 4). It appears that the drip lines are not delivering the CO2 uniformly - with the high rates adjacent to the delivery point reflecting higher gas pressures at that point. Differences in CO2 levels obtained with the two rates of injection were obvious (Figure 4). The higher rate consistently produced peak CO2 levels well above the maximum measurement capacity of the sensor used in this trial. The lower rate produced much more reasonable CO2 levels, particularly if there was enough wind to assist in the ventilation of the tunnels. CO2 levels in the non-perforated polyethylene tunnels were far higher than those observed for the perforated poly. CO2 levels in the woven fabric tunnels were never discernibly higher than ambient.

In 2001, health of the crops in the various treatments was observed but their growth was not formally evaluated. Crops grown under non-perforated polyethylene tunnels showed obvious signs of heat stress. The leave edges became chlorotic and then necrotic, the stems were thin and curled, and the flowers consistently aborted. Although the non-perforated covers were clearly superior in terms of their ability to generate and retain enhanced levels of CO2, the heat stress associated with the absence of adequate ventilation overcame any benefits associated with a CO2 enriched environment.

In 2002, the fresh weights of the pepper and melon plants were determined 8 weeks after they were transplanted into tunnels constructed of perforated polyethylene. The pepper plants grown with the supplemental CO2 were no taller than the controls but were on average 13% heavier. There was no significant effect of the CO2 treatments in the melons. It should be noted that in this trial, the melon plants were located further downstream from the CO2 injection point that the peppers and therefore would have received less CO2.

Conclusions

This study demonstrates that transient CO2 deficits occur within the confines of low tunnels, particularly if non-porous covering materials are used and the tunnels are located in relatively sheltered areas. The impact that these CO2 deficiencies have on growth of the covered crops is difficult to determine as it is confounded by the beneficial effects that the covers have on growth. The greatest deficiencies occur under the least porous covers, but these covers also provide the warmest and most sheltered microclimate.

Introducing supplemental CO2 into the low tunnels through the drip irrigation system was technologically simple - but the results were mixed. The levels of CO2 achieved were very non-uniform along the length of the drip tube, with excessive levels developing adjacent to the injection point and very little delivered only a few dozen meters further down the line. Drip lines are commonly employed for the delivery of CO2 in commercial greenhouses, but in those situations, the resulting delivery gradients are dissipated by fans creating a horizontal air flow pattern. As air flow within the low tunnels is limited, some other means of achieving a more uniform CO2 distribution pattern will need to be developed. More efficient control of the CO2 injection process could be achieved if the injection system and the monitoring system were electronically linked so that the CO2 was only injected when needed and at the appropriate dosages. These types of systems are in place in most greenhouses.

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Related Papers

Waterer, D. 1995. Multiseason use of plastic soil mulches in vegetable production. ADF Project No. 93000161 Final Report

Waterer, D. 1992. Demonstration of Multirow Floating Covers for Vegetable Crops. ADF Project No. 89000126 Final Report