Winter Survival

Supporting images I
Winter-Hardiness Potential of Cereal Species
Winter-Hardiness Potential of Wheat Cultivars
Breeding for Improved Winter Hardiness
Importance of Snowcover in Overwintering Cereals
Critical Management Practices -
[ Date of Seeding | Seeding Depth | Phosphorus and Nitrogen Fertilization ]
Assesment of Winter Damage
Supporting images II

INTRODUCTION

Winter wheat overwinters as a seedling. Seedlings must survive numerous stresses during the overwintering period. In order to cope with these stresses, winter wheat has evolved adaptive mechanisms which are temperature regulated and involve acclimation processes that can be reversed. An understanding of winter stresses, and the methods winter wheat has evolved to withstand them, can greatly assist the producer in reducing the risk of winterkill and in assessing crop condition during the winter and early spring.

Low temperature damage to the crown of the winter wheat plant during periods of cold is the main cause of winterkill on the Canadian prairies. The crown is normally located less than two inches (5 cm) below the soil surface. Consequently, it is the soil temperature at this depth that determines winter wheat survival.

COLD ACCLIMATION

When growth starts in the early fall, winter wheat plants will not survive subfreezing temperatures any better than spring cereal plants. However, winter wheat grown under cool fall temperatures will cold acclimate or "harden off". For example, the minimum survival temperature for Norstar is normally near -3°C at the beginning of September and -19°C or lower by the end of October ( Fig. 1 ). Under field conditions, eight to 12 weeks of growth is usually required for the full development of winter hardiness. The first four to five weeks is a period of active growth that takes place when average daily soil temperatures at a depth of two inches (5 cm) are above 9°C. Both the cold acclimation process and winter survival require energy and this period of warm temperature allows for the establishment of healthy vigorous plants ( Fig. 2 ). Plants with well developed crowns before freezeup are most desirable. However, plants that enter the winter with two to three leaves are usually not seriously disadvantaged.

Cold acclimation of winter wheat plants begins once fall temperatures drop below 9°C. A translocatable substance that promotes cold acclimation is not produced when winter wheat plants are exposed to acclimating temperatures. Consequently, the cold hardiness level of different plant parts, such as leaves, crowns and roots, is dependent upon the temperature to which each part has been exposed. Because the crown contains tissues that are necessary for plant survival, it is the soil temperature at crown depth that determines critical cold acclimation rates.

Plant growth slows considerably at temperatures that promote cold acclimation. In the field, soil temperatures gradually decrease as winter approaches and four to eight weeks at temperatures below 9°C is usually required to fully cold harden plants. Rate of cold acclimation during this period is dependent upon crown temperatures. As an example, winter wheat plants will cold acclimate twice as fast at crown temperatures of 0 compared to 5°C.

During the fall acclimation period, exposure of the winter wheat crown to soil temperatures above 9°C results in a rapid loss of cold hardiness. Dehardening rate is approximately three times faster than the rate of acclimation and it is dependent upon the temperature to which the crown is exposed. For example, dehardening proceeds twice as fast at 18 compared to 13°C. At this stage, plants that have been exposed to crown temperatures above 9°C will resume cold acclimation once they return to temperatures below 9°C.

Winter wheat normally does not realize its maximum cold hardiness potential until after freezeup in the late fall. In Saskatchewan, full acclimation is usually achieved by the middle to the end of November ( Fig. 1 ).

MAINTENANCE OF COLD HARDINESS

Once cold acclimation has been completed, winter wheat can maintain a high level of cold hardiness provided crown temperatures remain below freezing and the plants have an adequate energy supply ( Fig. 1 ). In the fall, winter wheat will cold acclimate when exposed to crown temperatures colder than 9°C. In contrast, prolonged exposure of fully acclimated plants to winter temperatures above freezing results in a gradual loss of cold hardiness. The warmer the crown temperature during the winter, the shorter the period that maximum levels of cold hardiness can be maintained and, once started, the more rapid the rate of decline in cold hardiness.

Death of the crown tissue will result if the soil temperature falls below the plants minimum survival temperature ( Fig. 1 ). Exposure of winter wheat plants to crown temperatures that are 2 to 3°C warmer than their minimum survival temperature will cause immediate damage and a reduction in cold hardiness. Longer periods of exposure to temperatures approaching the minimum survival temperature can quickly reduce the plants ability to tolerate cold stress. For example, 50 hours exposure to -18°C in controlled environment rooms reduces the minimum survival temperature of Norstar winter wheat from -24 to -18°C, a cold hardiness loss of 6°C.

SPRING DEHARDENING

A return to crown temperatures above 9°C in the spring accelerates plant growth and eventually results in a complete dehardening of winter wheat ( Fig. 1 ). Growth rate and rate of dehardening are both temperature dependent. Therefore, because frozen soils warm slowly in the spring, several weeks of warm air temperatures are required to re-establish and completely deharden winter wheat plants that have survived without winter damage.

CRITICAL LOW TEMPERATURE STRESS PERIODS

Critical low temperature stress periods for winter wheat can be identified by comparing long term soil temperature records with the general pattern of cold acclimation and maintenance of cold hardiness described above. These comparisons indicate that prolonged spells of cold weather in January and February are the most damaging periods for winter wheat in western Canada.

In the late fall (prior to Dec. 20), the soil has a large heat capacity and decreases in soil temperature lag considerably behind decreases in air temperature. Plant cold tolerance is at a maximum at this time and low temperature damage to winter wheat is improbable ( Fig. 1 ).

During the calendar winter (Dec. 22 to Mar. 20), prolonged periods of cold weather increase the potential for low temperature damage to winter wheat. In the absence of a protective snowcover, the soil gradually loses its ability to buffer the effects of low air temperatures and winterkill occurs when soil temperatures fall below the minimum crown survival temperature for winter wheat ( Fig. 1 ).

Prolonged periods of extremely cold weather are rare in the spring (after Mar. 20). Therefore, there is a low probability that spring soil temperatures at crown depth will drop below the minimum survival temperature for winter wheat ( Fig. 1 ).

WINTER-HARDINESS POTENTIAL OF CEREAL SPECIES

Winter cereal cultivars do not all possess the same ability to withstand the extremes of winter. In field trials there is often complete winterkill of some cultivars while others survive the winter undamaged. These observations demonstrate that differences in cultivar winter- hardiness potential can be greater than 100 percent.

Survival data from field trials that have been subjected to different levels of winter stress have been used to produce a comparative measure of cultivar winter-hardiness potential. This measure of winter-hardiness potential is known as the cultivars' Field Survival Index (FSI).

Table 1. Winter cereal field survival indices (FSI) estimated from fully acclimated cultivar or strain minimum survival temperatures (MST).

a) Unacclimated
Cultivar/Species FSI* MST(°C)
Spring or winter wheat 70 -2.5
Spring or winter rye 90 -3.5
b) Acclimated
Cultivar or Strain Species FSI* MST(°C)
Random Spring oats 115 -7.7
Bonanza Spring barley 115 -7.7
Manitou Spring wheat 160 -9.6
Gazelle Spring rye 210 -11.6
WIR 46870 Winter durum wheat 255 -13.5
Penium Winter oats 275 -14.3
WIR 46870 + Cougar Winter triticale (6X) 275 -14.3
Compactum Winter oats 290 -14.9
Dover Winter barley 300 -15.3
Cappelle Desprez Winter wheat 306 -15.5
Dicktoo Winter barley 355 -17.5
Novamichurinka Winter durum wheat 370 -18.1
Kharkov + Puma Winter triticale (8X) 460 -21.8
Ulianovkia + Kodiak Winter triticale (8X) 480 -22.6
CDC Kerstel Winter Wheat 497 -23.3
Karkov 22MC Winter wheat 499 -23.5
Ulianovkia Winter wheat 530 -24.7
Sangaste Winter rye 550 -25.5
Kodiak Winter rye 575 -26.5
Cougar Winter rye 620 -28.3
Frontier Winter rye 735 -33.0
Puma Winter rye 735 -33.0

FSI* (Field Survival Index) - Differences in cultivar FSI reflect average differences expected in field survival, e.g., Ulianovkia versus Kharkov 22MC = 530 - 499 = 31% difference in expected winter survival potential.

[ Back to Table 1]

Differences in cultivar FSI represent the average percent differences expected in field survival. The higher a cultivar's FSI, the greater its winter-hardiness potential ( Table 1 ). For example, following high stress winters, Ulianovkia (FSI = 530) is expected to have a 31 percent (530 - 499 = 31) higher survival rate than Kharkov 22MC (FSI = 499).

There is a close relationship between FSI ratings from field trials in western Canada and minimum survival temperatures of fully acclimated cultivars frozen in the laboratory under controlled conditions [FSI = (-74.7) - 24.5 (minimum survival temperature)]. Based on this relationship, FSI have been estimated for a number of winter cereal species. Cultivars that represent the winter-hardiness range within each species are listed in Table 1. These estimates demonstrate that, although there is an overlap among species, cultivars of rye have the best winter-hardiness potential. Cultivars of common wheat and triticale are next in line followed by durum wheat, barley and then oats.

WINTER-HARDINESS POTENTIAL OF WHEAT CULTIVARS (VARIETIES)

While estimates of cultivar minimum survival temperatures ( Table 1 ) are more easily obtained, differences in winter survival under field conditions provide the most reliable measure of winter hardiness for establishing varietal recommendations.

Table 2. Winter wheat field survival indices (FSI) derived from field trials.

Year Commercially Available in North AmericaCultivar or StrainArea of AdaptationFSI
N/A Cappelle Desprez France 306
1969 Ionia Michigan 345
1972 Arrow New York 345
1967 Blueboy North Carolina 358
1968 Yorkstar New York 360
1971 Fredrick Ontario 368
1965 Nugaines Washington 376
1973 Tecumseh Michigan 386
1961 Gaines Washington 389
1975 Lancota Nebraska 392
N/A Besostoia 1 USSR 401
1946 Rideau Ontario 405
1971 Centurk Nebraska 433
1991 AC Readymade Alberta 437
1970 Scoutland Nebraska 444
1967 Scout 66 Nebraska 444
1922 Cheyenne Nebraska 445
1960 Warrior Nebraska 446
1961 Winalta Alberta 463
N/A Mironovskaja 808 USSR 466
1986 Norwin Saskatchewan 476
1968 Froid Montana 488
1902 Minhardi Minnesota 492
1932 Yogo Montana 493
1965 Hume South Dakota 493
1991 CDC Kestrel Saskatchewan 497
1912 Kharkov 22MC Quebec 499
1977 Norstar Alberta 514
N/A Alabaskaja USSR 527
See Table 1 for application of FSI

The FSIs given in Table 2 are an average of winter wheat cultivar performance observed in over 100 field survival trials grown in Saskatchewan during the period from 1972 to 1992.

The cultivars listed in Table 2 are representative of several winter wheat producing areas of the world. This group includes the most winter-hardy strains that have been identified to date. It should be noted that improvements in winter-hardiness potential have been small and, in most traditional winter wheat growing areas, the FSI of new cultivar releases has decreased with time. This reduction was made possible by improved management practices that have allowed farmers to capture the maximum winter-hardiness potential of cultivars adapted to their region.

BREEDING FOR IMPROVED WINTER HARDINESS

The total North American winter wheat breeding effort expended since the introduction of Crimean wheats in the late 1800's (Minhardi and Kharkov 22MC are selections from Crimean introductions) has produced only a marginal improvement in winter hardiness ( Table 2 ). Inspection of the pedigrees of the winter-hardy wheat cultivars reveals an extremely narrow genetic base. This suggests that a lack of additional exploitable genetic variability has been the main reason for the absence of significant improvement in winter hardiness of wheat. A similar situation exists for barley and oats.

While the opportunity for improvement has been restricted, there is evidence that small gains in winter hardiness should be possible using conventional breeding methods for winter wheat improvement in western Canada. The two Russian strains, Albaskaja and Ulianovkia, represent potential sources of additional cold hardiness that may be used in western Canadian breeding programs ( Table 2 ). Part of this variability was made available to Canadian farmers when the cultivar Norstar, which was selected from a cross between Winalta and Alabaskaja, was released for commercial production in 1977 ( Fig. 3 ). Further improvement in the winter-hardiness potential of western Canadian cultivars should be possible using conventional plant breeding methods. However, from what is known of the genetics of cold hardiness, an intensive plant breeding effort would only be expected to yield cultivars with a 15 FSI unit increase over Norstar. Unfortunately, a 15 FSI unit improvement does not represent a large enough increase in winter-hardiness potential to have a major impact on winter wheat production in western Canada.

Related species provide another potential source of genetic variability that has been considered in attempts to improve the winter hardiness of cultivated cereal species (See Chapter 8 for a more detailed discussion of the use of related species in winter wheat breeding). The hardiness of rye provides proof that the potential of cereals to survive winter stresses is much greater than that presently known in wheat, barley and oats. However, the transfer of this potential from one species to a related species is not likely to be an easy task. For example, the cold hardiness advantage of rye is suppressed when winter wheat and rye are combined to produce winter triticales ( Table 1 and Table 3 ). Further research is required before winter wheat breeders will be able to access this source of alien genetic variability.

Biotechnology has provided new methods and tools which can be used to expand the plant breeders efforts to bridge the barrier that has prevented major improvements in the winter hardiness of wheat in this century. Application of this new technology has already added greatly to our understanding of the genetic mechanisms that control cold hardiness. There is strong evidence that the genes controlling cold hardiness exist in clusters on specific chromosomes. It is also probable that many of these gene clusters have remained unaltered in the evolution of different plant species. The similarity of genetic systems suggests that the differences we see in the cold-hardiness potentials of species is due to differences in gene regulation rather than the presence or absence of cold hardiness genes. Further exploitation of this new technology promises to produce a more objective approach to plant breeding and expanded opportunities for the manipulation of the genes controlling cold hardiness in cereals. The acquisition of better tools and a more complete understanding of genetic control of cold-hardiness may one day lead to the development of super-hardy winter wheat cultivars.


Table 3.
Table 3

THE IMPORTANCE OF SNOWCOVER IN OVERWINTERING CEREALS

In the spring, winter cereals can regenerate from undamaged crown tissue. Death of the plant will occur if the soil temperature falls below the minimum survival temperature of the crown at any time during the winter ( Fig. 1 ). Therefore, the temperature to which plant crowns are exposed is the critical factor that determines the winter survival of cereals.

The crown of the plant is normally located less than two inches (5 cm) below the soil surface. As noted earlier, the soil has a tremendous capacity to buffer temperature change. However, prairie winters are harsh and, outside of the chinook area in southwestern Alberta, a protective snowcover is usually required to prevent soil temperatures from falling below the minimum survival temperature for wheat (See Chapter 5 for a detailed discussion of snow management).

Table 4. Minimum cultivar field survival indices (FSI) required for undamaged winter cereal stands nine out of ten years in Saskatchewan.
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ConditionFSI
Bare summerfallow >650
2 in. (5 cm) snowcover 540
4 in. (10 cm) snowcover 430
>6 in. (15 cm) snowcover <420

Estimates of cultivar winter-hardiness levels required to produce undamaged cereal crop stands have been obtained from field trials that included cultivars with a wide range of FSI ( Table 4 ). The winter-hardiness requirements listed in Table 4 are mean values from 64 field trials grown at a number of locations in Saskatchewan for the period 1972-1977. Based on these values, a cultivar FSI of greater than 650 is required to insure an undamaged winter cereal stand on bare summerfallow. This means that, in the absence of snowcover, only the hardiest winter rye cultivars grown under optimum management have a good chance of surviving a Saskatchewan winter undamaged. Two inches (5 cm) of snow greatly reduces the winter-hardiness requirement, but the risk for wheat is still high. With four inches (10 cm) of snowcover, cultivars with a FSI greater than 430 are considered fair risks but a safety margin of at least 50 FSI units (FSI >480) is recommended. The winter wheat cultivars CDC Kestrel and Norstar fulfil this requirement ( Table 2 ) and they are both registered for production in western Canada. Test sites where the snow has been deeper than six inches (15 cm) have had Cappelle Desprez survive on occasion. In spite of this, cultivars with FSIs less than 420 are considered to have an unacceptably high winterkill risk in western Canada. Based on these observations, all presently available cultivars of winter oats, barley and durum wheat ( Table 1 )

do not have sufficient cold hardiness for production in Saskatchewan even under optimum conditions.

CRITICAL MANAGEMENT PRACTICES


General Information

Note: See Chapter 11 for a detailed discussion of the effects of management practices on winter wheat establishment in the fall.

Management practices can have a large influence on the ability of cereals to survive winter stresses. The risk of winterkill is minimized when properly fertilized winter cereals are seeded shallow with a no-till drill into a moist, weed-free field of standing stubble on the recommended seeding date. Management shortfalls in any of the above areas will results in a reduction in cultivar winter-hardiness potential.

The Field Survival Index (FSI) was developed to provide an objective measure of wheat cultivar winter-hardiness potential. The FSI also can be used to quantify the effects of management shortfalls on the winter hardiness of wheat cultivars. The units (%) used to measure the effect of suboptimal management on cultivar winter hardiness are the same as the cultivar FSI units ( Table 1 and Table 2a ). Therefore, the consequences of management shortcomings on winter-hardiness potential can be determined for each cultivar by simple subtraction. Comparison of the cultivar FSI, once it has been corrected for management deficiencies, with the minimum FSI required for undamaged stands ( Table 4 ) will then give an estimate of the wheat crop's chances for survival under different winter environments.

Date of Seeding

Winter cereals should be seeded early enough to allow for the establishment of a healthy, vigorous plant before freezeup. However, seeding too early can result in excessive fall growth and plants that are less resistant to winter injury and disease.

The recommended date of seeding for the traditional western Canadian winter wheat production area of southern Alberta is the second week in September. As one moves north and east on the Canadian prairies, cool fall weather usually arrives sooner and earlier seeding is required ( Table 5 ).

Recommended seeding dates for winter wheat in Saskatchewan are approximately August 27 for the north and September 6 for the extreme south of the agricultural area ( Table 5 ). Earlier and later dates of seeding produce wheat stands that are more susceptible to winter damage than those seeded during the optimum period ( Table 6 ). Seeding too early is usually not a problem with no-till winter wheat because removal of the previous crop rarely occurs before the optimum seeding date. As an example of the magnitude of the effect of late seeding on cultivar winter-hardiness potential, seeding Norstar (FSI = 514) on September 24 would be expected to reduce its FSI (514 - 38 = 476) to that of Norwin (FSI = 476) seeded on the recommended date of August 27 in the Parkland region ( Table 2, Table 5 and Table 6; Fig. 4 ).

Table 5. Optimum date for no-till seeding winter wheat into standing stubble:

Location Date
1. Lethbridge, AB

2. Maple Creek/Estevan, SK

3. Kindersley/Swift Current, SK

4. North Battleford/Saskatoon/Wynyard/Yorkton, SK

5. Meadow Lake/Prince Albert/Nipawin, SK

September 9

September 6

September 3

August 30

August 27


Table 6. Effect of seeding date on winter survival (subtract value from cultivar FSI).

Subtract
4 weeks early 31
3 weeks early 15
2 weeks early 3
1 week early 0
Recommended date 0
1 weeks late 4
2 weeks late 12
3 weeks late 25
4 weeks late 38
5 weeks late 38
6 weeks late* 15

*Yield potential is reduced when winter wheat is seeded six weeks later than the recommended seeding date.

Winter wheat should be seeded shallow into a firm, moist seedbed. Deep seeding results in delayed emergence and weak plants that are more susceptible to damage from winter stresses ( Fig. 5 ). Reduced soil temperatures associated with late seeding dates increase the time winter wheat plants require to emerge. Consequently, the problems associated with deep seed placement are greater for late compared to optimum seeding dates.

In the severe winter of 1984-85, a difference in seeding depth of one inch (2.5 cm) compared to two inches (5 cm) often meant the difference between a crop and no crop in the spring. This represents a cultivar winter hardiness reduction of greater than 100 FSI units due to one inch (2.5 cm) deeper seeding.


Phosphorus and Nitrogen Fertilization

Phosphorus deficiencies and excesses reduce the winter-survival potential of winter wheat ( Table 7; Fig. 6 ). The phosphorus may not have a direct influence on the winter hardiness of the plant. Rather, when deficiencies are corrected, it may act by improving the spring recovery of wheat plants that have been winter damaged. As an example of the magnitude of the effect of phosphorous on winter hardiness, a 10 lb/acre P2O5 (11 kg/ha) deficiency for the cultivar Norstar (FSI = 514) would reduce its FSI (514 - 17 = 497) to that of CDC Kestrel (FSI = 497) sown with the recommended rate of seed-placed P2O5 ( Table 2 and Table 7 ).

Table 7. Effect of seed-placed phosphate fertilizer on winter survival of winter wheat. Subtract value from cultivar Field Survival Index (FSI).

Phosphate FertilizerSubtract (FSI)
15 lb/acre (17 kg/ha) deficiency 26
Minimum requirement met 0
15 lb/acre (17 kg/ha) excess 6
30 lb/acre (34 kg/ha) excess 10

The results of field trials have demonstrated that plant-available soil-nitrogen level does not normally affect the winter-hardiness potential of wheat unless the nitrogen has been applied in the seed-row at the time of planting ( Fig. 7 ). When placed in the seed row, both urea and ammonium nitrate can reduce seedling number and size, especially when the soil is dry at the time of seeding. Compared to ammonium nitrate, seed-placed urea causes more extensive damage to the germinating seed. Placement of both urea and ammonium nitrate a minimum distance of one inch (2.5 cm) from the seed will minimize seedling damage.

Similar grain yield responses for 30 lb of ammonium nitrate nitrogen/acre (34 kg N/ha) spring broadcast and seed-placed indicates that ammonium nitrate can be safely placed in the seed row at low rates. However, even at low rates, an increased level of winterkill has been observed for seed-placed ammonium nitrate following high stress winters ( Table 8 ).

Table 8. The effect of seed-placed ammonium nitrate (34-0-0) fertilizer on winter survival. The seed and fertilizer were placed in 3/4 inch wide rows spaced 8 inches apart in this study. Subtract value from cultivar FSI.
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Seed-placed NSubtract
(lb/acre)(kg/ha)(FSI)
0 0 0
30 34 17
60 67 34
90 101 51

*FSI = Field Survival Index


The concentration of fertilizer immediately adjacent to the seed is dependent upon fertilizer rate, drill row spacing, and opener design. Consequently, there is not a simple rule of thumb that can be used to determine the effect of seed-placed ammonium nitrate on the winter hardiness of wheat. However, the results of field trials with one of the more common row spacing-opener width combinations suggests that ammonium nitrate rates should be limited to less than 30 lb of nitrogen/acre (34 kg/ha) when wheat cultivars with marginal winter hardiness are utilized ( Table 8 ). High rates of phosphate fertilizer will not offset the effect that seed-row banded nitrogen fertilizer has in reducing winter hardiness.

ASSESSMENT OF WINTER DAMAGE

With a little experience, producers can continually update their assessment of the condition of wheat crops during the winter and early spring. Crops that have had a poor start in the fall have the highest risk of winter damage. Therefore, the health and vigor of the crop when it goes into the winter will be the first measure of its ability to withstand later stresses. As indicated earlier, the most important management decisions that affect winter survival of wheat are made before the seeding operation is completed. Farmers can have a large influence on winter survival by taking steps to assure that the crop is well established in the fall and that an adequate snow trap has been provided to protect the crop from the extremes of winter. Weather is beyond the control of the farmer and all that can be done during the winter is to estimate the impact of adverse conditions on the crop. Prolonged periods of cold will increase the risk of damage to plants and, in most instances, the areas of the field that have the least snowcover will suffer the greatest damage.

A record of crop condition in the fall, the winter stresses experienced, and the areas that were least protected by snow can be of valuable assistance in obtaining an accurate early assessment of winter damage. Without this information, the first impressions when the snow disappears in the spring can be quite misleading. Brown, dried leaves do not necessarily indicate winterkill, and an initial flush of green top growth is not a sure sign that the crop has come through the winter undamaged ( Fig. 8 ). In both instances the key to survival is the appearance of new white roots from the crown ( Fig. 9 ). The leaves that developed the previous fall can be killed off without reducing the plant's chances of survival. As long as the crown remains alive, new leaves and roots can be regenerated. If the crop starts to green up in the spring and then dies back, the crowns were probably damaged to the point where they were unable to regenerate roots ( Fig. 8, left ). In this instance, the leaves may continue to grow until the crown reserves are exhausted and then they will die off. This type of winter damage is often blamed on spring frosts but, in reality, the damage probably occurred during a prolonged cold spell in January or February.

Spring weather conditions can have a large influence on the ability of a damaged winter wheat stand to recover. Hot, dry weather, which results in a cracking and drying of the soil, can be very harmful to damaged plants that are struggling to regenerate new roots. In contrast, cool damp weather produces a much more favorable environment for plant recovery.

A quick method of determining if plants are alive is to remove a few from the field on a warm day. If this is done during the winter, be sure the air temperature is not below the minimum survival temperature or plants may be killed during removal giving a false impression of the degree of damage. Place the crowns in a moist environment (paper towel, sand, dirt, sawdust, etc.) and leave them in a warm room where they will be exposed to light for at least part of the day. Make sure the crowns of the plants do not dry out at any time during the period of assessment. Winter survival may also be assessed by placing the crowns of the plants in a small amount of water in the bottom of a bowl that is then covered with cellophane with a few small holes in it. Crown tissue that is severely damaged will quickly turn brown, while healthy tissue will remain white. At room temperatures near 20°C, healthy crowns will produce new white roots and green leaves in a few days ( Fig. 9 ).

The soil warms slowly in the spring. Consequently, one to two weeks of spring growth at warm temperatures should be allowed before a final estimate is made of the winter damage to plants growing in the field. Where severe damage is suspected, check for the production of new white roots. Do not be too hasty to plow down a damaged winter wheat stand. In most instances, a thin, poor-looking stand in May will look much better by the time the crop arrives in the bin.

The most difficult winter damage to assess occurs in fields where snowtrapping was ineffective. Untrapped snow will drift into dunes, or off the field completely, leaving areas that have little or no snowcover during the coldest part of the winter ( Fig. 10 ). Minor variation in snowcover over a distance of just a few feet can cause large differences in soil temperatures. Consequently, changes in wheat winter-hardiness requirements due to variable snowcover are often very abrupt ( Fig. 11 ). The usual consequences of variable snowcover and low temperatures is patchy survival patterns ( Fig. 12 ). If these fields are left in the winter crop, the winterkill patches usually end up as weed problem areas. In these instances, producers must make their own decision as to the percentage loss that will be accepted before the crop is written off and worked down. If it has been determined that damage is extensive and a decision to reseed is made, the surviving winter wheat plants should be cultivated out as soon as possible to minimize loss of soil moisture and a tie-up of soil nutrients.