Chapter 5 - Temperature, Frost, Hail

Temperature, Frost and Hail

Canola is reasonably widely adapted and performs well in many areas under variable temperatures. However, B. napus and B. rapa vary in their ability to respond to favourable and adverse conditions. Reduce the risk of weather losses by matching production practices and variety selection to environmental conditions.

Temperature

Heat and Growing Degree Days

Plant functions such as evapotranspiration, photosynthesis, water and nutrient absorption and transport, enzyme activity, and other biological and chemical activities are regulated by temperature. For this reason, the development of the crop is more closely related to the amount of heat the crop is exposed to than calendar days. Other factors such as moisture, light (day length), nutrition and variety also play a role, but they generally have less influence.

Growing Degree Days (GDDs) and Corn Heat Units (CHUs) are both measures of heat accumulated by a crop over a period of time. CHUs are a more sophisticated and accurate measure of accumulated heat and are important for warm season crops like corn. GDDs are much simpler to calculate and prove a good estimation of accumulated heat for most cool season crops like canola and wheat. GDD, however, does not adequately account for the effects of extreme temperatures (high or low). Despite this disadvantage, GDDs provides a useful tool for predicting or measuring development of cool season crops.

Heat energy can be measured and used to track daily plant growth progress. This requires keeping a daily record of minimum and maximum air temperatures for calculating daily GDDs and maintaining a summary of these GDDs.

GDDs are calculated by averaging daily maximum (Max T) and daily minimum (Min T) temperatures for each day, and subtracting the assumed minimum temperature (Base T) required for growth to proceed. For canola, there is some debate as to whether the base temperature should be 0° or 5°C. For this discussion, we have chosen to use a 0°C base, in part because it is simple to use.

The formula for daily growing degree days (DGDDs) is:
DGDDs=(Max T + Min T)/2 “ Base T
Where:
Max T = highest temperature of the day. The warmer it is, the faster the plant will develop up to a point. Canola is a relatively cool season crop in that its best growth occurs above 12°C and below 30°C. The optimum temperature for maximum canola growth and development has been estimated at 21°C by Canadian research. However, Australian research indicates a 25°C maximum. B. juncea and B. rapa appear to have a higher optimum temperature than B. napus. Temperature above the optimum will slow the rate of plant growth. The more the optimum temperature is exceeded, the greater the development rate is slowed until reaching maximum temperature where all development stops. Heat stress research has been a bit inconclusive on threshold temperature, reporting critical temperatures for heat stress from as low as 25 to 27°C to as high as 30 to 32°C in Canada and 35°C in Australia for B. napus. These heat stress effect studies were often confounded by variety differences, light and water stress.

Min T = lowest temperature of the day”temperatures less than 0°C are set to 0°C. The minimum and maximum temperatures are for the 24-hour period of midnight to midnight.

Base T = 0°C”a temperature below which no development occurs for a given plant species. At temperatures above the minimum, plant development growth rate increases as temperature increases to optimum. While there is only limited plant growth at temperatures slightly above freezing, germination and seedling growth do occur at temperatures between 0 and 5°C. While the majority of previous research used a canola base temperature of 5°C, recent research indicates a more accurate base temperature is from 0 to 1°C. For a cool season crop like canola grown in western Canada, 0°C is often the best base temperature for predicting development.

Example: Assuming the maximum daily temperature is 15°C and the minimum is 5°C, the number of daily DGDDs is:
(15+5)/2) - 0 = 10

This value represents the daily heat useful for canola growth. Daily calculations resulting in values greater than zero are added together to determine the accumulated weekly, monthly or yearly GDDs. When the daily DGDDs of any two or more days are added, this is referred to as accumulated GDDs. An example of a 30-year average for DGDD is shown for Lacombe, AB (Table 1).

Long-term GDD records for a farm or an individual field combined with seeding dates and records of growth stages can be useful. GDDs provide a means of estimating growth stage during the growing season and become another tool for diagnostic work in solving problems. Minimum and maximum temperatures to calculate DGDDs can be obtained daily for a location or a nearby location from www.weatheroffice.com. Calculations improve if the farm has actual maximum/minimum records.

Temperature Effects on Canola Growth

Canola plants require a specific number of GDDs to develop from growth stage to growth stage between emergence, flowering and maturity. The GDD requirement for canola can vary considerably as shown by research data from Agriculture and Agri-Food Canada (AAFC) Scott, SK and Swift Current, SK Research Centres (Table 2).

Table 1. Daily Normal GDDs at Lacombe, Alberta Based on a 30-year Average Temperature in °C

Day	Month
	April	May	June	July	August	Sept
	Average Daily DGDDs, at 0°C Base Temperature
1		8.6	13.4	14.5	16.1	12.8
2		8.9	12.9	14.4	16.4	12.6
3		8.6	12.5	14.4	16.2	12.6
4		8.4	13.3	14.8	17.1	12.0
5		9.0	13.3	14.8	16.9	11.7
6		8.4	13.0	14.3	16.6	11.5
7		9.2	13.1	14.8	16.3	11.2
8		9.2	13.8	15.3	16.1	10.8
9		9.3	13.1	15.6	16.1	10.9
10		9.3	13.8	15.7	16.1	10.5
11		9.4	13.4	15.4	15.3	9.9
12		10.0	13.3	15.4	15.1	10.2
13		10.2	14.2	15.4	14.8	10.4
14		10.6	13.5	15.4	15.5	10.5
15	6.0	9.7	13.9	15.1	14.6	10.7
16	6.2	10.1	14.0	15.0	14.5
17	6.4	10.5	13.9	15.2	14.5
18	5.9	10.1	13.9	15.3	14.7
19	6.6	9.3	14.2	15.5	14.1
20	7.0	10.4	14.2	15.8	14.2
21	7.5	10.4	14.8	16.5	13.9
22	7.6	11.2	14.7	16.3	14.2
23	7.8	11.4	14.4	16.4	14.5
24	7.3	11.5	14.2	15.9	13.8
25	7.5	11.9	14.0	16.5	13.5
26	7.3	11.6	14.3	16.7	13.1
27	7.9	11.4	14.6	16.8	14.0
28	7.7	11.0	14.3	16.9	13.9
29	7.4	11.9	14.8	16.4	13.5
30	7.7	12.5	14.5	16.9	12.9
31		12.2		16.6	12.7

Table 2. GDDs Required for Canola Growth Stages in Western Canada Using a 0°C Base Temperature

Growth Stage	Growth Stage Description	GGDs for B. napas	GGDs for B. rapa
1.0	Emergence-cotyledoms completely unfolded	152-186	102-143
1.2	Two leaves unfolded	282-324	201-254
1.4	Four leaves unfolded	411-463	300-365
6.0	Flowering begins”at least one open floret on 50% or more plants	582-666	467-554
6.5	Flowering 50% complete	759-852	630-726
7.1	Seed fill begins”10% of seeds have reached final size	972-1074	826-934
8.1	Maturity”seed begins to mature”10% of seed has changed colour	1326-1445	1152-1279
8.4	Swathing”40% of seed on main stem has changed colour” optimum swathing stage	1432-1557	1249-1382

An alternative source of ongoing GDD maps that illustrates canola GDDs with a 5°C base temperature for the western provinces are available from the Agrometeorological Centre of Excellence Web site at www.aceweather.ca/canola.cfm.

Under drought and/or heat stress, canola GDD requirements will be toward the low end of the reported range for each stage. Several factors such as dry seedbeds, excess moisture or irrigation, high fertility, frost or hail and low plant populations could slightly delay crop advancement and GDD values would approach or exceed the high end of the range. In fact, the GDD requirement for canola often exceeds the ranges shown in Table 2 due to these other factors. In five years of Canola Council of Canada Crop Production Centre trials across western Canada, the GDDs for AC Excel ranged considerably as shown in Figure 1.

The primary cause of year-to-year and location-to-location variation in days to maturity of the same canola variety is temperature variation.

The total GDDs required for B. napus to mature is similar to that of spring wheat while B. rapa is similar to barley. GDDs are more accurate in predicting growth stages than are calendar days. In Figure 1, AC Excel matured in 83 to 120 days (average 99 days) or 1,380 to 1,718 GDDs (average 1,560 GDDs) after seeding. For calendar days, there is a potential error of plus or minus 21 days. Using GDDs, the potential error is about eight calendar days. This difference is important in estimating maturity risk for varieties. Growing late maturing B. napus varieties in areas with marginal GDDs will increase the risk of late maturity and the probability of fall frost damage. Based on Table 1, a B. napus variety sown on May 10 at Lacombe, AB would, on average, just meet the requirement of 1,560 GDDs for maturity by August 29. Therefore, in some years with lower GDDs there would be a high risk of fall frost damage”the crop would not reach maturity until well into September. Growers in the Lacombe area report that the risk is about one in five years for fall frost damage to a B. napus crop sown on May 10. Unfortunately, the GDDs cannot be forecast. However, the use of existing records for past trends can provide a more accurate means for predicting canola growth.

Figure 1. GDD”Base T 0°C”For AC Excel at Crop Production Centre Sites across Western Canada 1997-2001

GDDs do not limit canola production in northern areas as much as might be expected since longer daylight hours partially compensate for lower temperatures. For example, Fort Vermilion, AB has 538 daylight hours in July and a mean temperature of 16°C, while Lethbridge, AB has 488 daylight hours and a mean temperature of 19°C. The longer daylight hours reduces the overall GDD requirements in the Alberta Peace region by about 150 GDDs.

Temperatures Required for Germination and Emergence

Temperature, light and water are the major environmental factors determining the success of germination and early seedling development. Germination is also influenced by the genetics of the variety, growth conditions as the seed matures, how the seed was stored and seed treatments.

Seeding early can reduce the risk of damage from a fall killing frost. However, cold soil conditions may result in emergence problems. Spring soil temperatures are frequently less than optimum for canola, especially in the short season frost-free areas where B. napus varieties must be sown early. Air and soil temperatures in the spring vary from location to location and from year to year. The year-toyear soil temperature variation can be very large, as shown by data from the AAFC Lacombe Research Centre for soil under conventional tillage for three years (Table 3). The temperature at the 5 cm (2") soil depth was colder than at the 2.5 cm (1") depth.

Table 3. Soil Temperatures for Conventional Tillage (May 15) for 1979-1981

Soil Depth	Temperature °C
	1979	1980	1981
2.5 cm (1")	6.7	25.4	6.7
5.0 cm (2")	6.4	16.8	6.4

Soil temperatures can be determined for any field by taking an average of readings with a soil thermometer inserted at seeding depth at 8:30 a.m. and 4:00 p.m.

Generally, temperatures below 10°C result in progressively poorer germination and emergence. Various research studies have shown that both B. napus and B. rapa canola will imbibe water and germinate at constant temperatures of 2°C. Sustained low temperatures for both B. napus and B. rapa, however, damage the seed embryo, which reduces germination and growth. Low temperature impairs the production of proteins required for proper germination and early seedling development through reduced metabollic processes. Limited seedbed moisture slows uptake of water by the seed, slowing the speed and number of seeds that germinate and emerge. Seedbed moisture generally declines the longer seeding is delayed. Any factor that reduces the rate of emerging seedlings may make them more susceptible to seedling disease.

The number of days to 50% germination is a useful indicator as the first 50% of plants to emerge account for a majority of the yield from a field. Similar to previous studies, AAFC Beaverlodge, AB Research Centre found much lower and slower germination at low temperatures for both species compared to warmer temperatures (Figures 2 and 3). Temperatures of 4°C or higher had little effect on total percent germination in B. napus, however, the number of days to 50% germination increased dramatically at temperatures below 6°C.

Figure 2. Effects of Temperature on Germination of B. napus

Figure 3. Effects of Temperature on Germination of B. rapa

The number of days to 50% germination in B. napus was only three days at 8°C compared to nearly 13 days at 2°C. This low temperature effect of slower and lower germination was even more pronounced with B. rapa canola (Figure 3). In B. rapa, there was greatly reduced germination at 3°C, and at 2°C, even after 20 days, 50% emergence was not reached.

This research was carried out with only one seed lot for each variety and species. Therefore, expect differences between varieties and seed lots due to differing abilities to germinate and grow at lower temperatures.

A more recent study of temperature effect on canola emergence at Colorado found t hat knowing the actual heat unit requirement or GDDs for emergence was more useful than the minimum temperature at which seed will germinate (Figure 4).

To reach 50% germination for all temperatures 3°C or higher requires 75 to 120 GDDs (Base T 0°C) in B. napus and at temperatures above 8°C over 115 GDDs for B. rapa. The days to 50% germination or emergence date for canola can be reasonably predicted from knowing how many GDDs have accumulated since planting. Count the GDDs beginning with the day after seeding.

Unlike these studies under controlled conditions, the speed of germination and emergence in fields with variable soil moisture could be slower by a few days. The longer it takes for seedling emergence, the greater the likelihood of seedling diseases occurring and, therefore, the greater the chance for reduced plant populations. Slow and uneven seed germination and emergence can result in poor stands and later uneven maturity. This also increases the potential for high temperatures during flower and pod formation. Low soil temperature and low soil moisture both delay germination and when combined have an additive effect. The worse case scenario would be sowing deep into a cold, dry seedbed. Field emergence studies have shown that the number of plants that actually establish in a canola field is much less than the number of seeds sown even under ideal conditions (usually 50 to 60%).

Figure 4. Effect of Heat Unit Accumulation on Emergence - B. napus

Seeding depth and heavy trash cover may also delay crop emergence by one to two days. Soil temperatures generally decrease with soil depth in springtime. Bare soil warms quicker than soils with surface trash or vegetation that reflects some of the solar radiation. Emergence in seedbeds without residues at the surface, at the same seeding depth and under the same climatic conditions, will be about one to two days sooner than in seedbeds with residues on the surface such as in a direct seeding system. Trash management is critical for both fall sown and very early spring sown canola fields. Tillage influences soil temperatures by reducing crop residue and drying the soil. Dry soil warms faster than wet soil, however, moist soil has a greater heat storage capacity. South-facing fields warm more quickly than north-facing fields or level land. Snow also melts earlier on south-facing fields. South facing fields usually have a slightly longer growing season.

The above studies on low temperature research were carried out with constant night and day air temperatures but fortunately soil and air temperatures in fields tend to increase after seeding. While temperatures above 10°C result in high germination, quick emergence, and rapid leaf development, a target soil temperature for an early seeding date would be 3°C or higher for B. napus varieties and 7 to 8°C for B. rapa varieties. Research at AAFC Scott, SK and Lacombe, AB Research Centres has shown that very early (April 25) spring sown B. napus canola in most years achieved higher yields than mid-May sown canola even with reduced germination and delayed emergence. From a practical standpoint, 10 to 12 days is not an unreasonable amount of time to wait for emergence in a crop. An early seeded B. napus sown in a 3°C seedbed would likely achieve 50% emergence in about 15 days. B. rapa, normally sown when soil temperatures are warmer, can also be sown early if soil temperatures are at least at 7°C. An emergence of about 35% of a normal seeding rate in 15 days should still result in an adequate plant stand with reasonable yield potential as long as there is little seedling blight and weeds are adequately controlled (Figure 5).

Figure 5. Effect of Heat Unit Accumulation on Emergence - B. rapa

Temperature Required for Seedlings, Flowering and Podding - Cold and High Temperature Stress

After emergence, canola seedlings prefer relatively cool temperatures up to flowering. The optimum range of temperature for leaf area development in canola has been estimated at 13 to 22°C (17°C mean temperature). Higher temperatures cause faster growth that results in shorter leaf area duration. Heat injury to seedlings can occasionally occur especially in the Brown and Dark Brown soil zones. Cold soil temperatures limit growth by reducing water and nutrient absorption by seedling roots.

Both low and high temperatures can adversely affect development prior to and during flowering. Low, but nonfreezing temperatures just prior to flowering slow the rate of plant development. The start of flowering is delayed or, if begun, the rate of flower opening is slowed and the amount of pollen shed is reduced. Low temperatures, however, do not reduce yield, except in the case of frost. Whereas high temperatures, especially in the late growth stages, do result in reduced yields as shown by an evaluation of Westar across western Canada co-operative trials for three years (Figure 6).

This research showed that as the average maximum temperature for July and August increased above 20°C, canola yields decreased.

Analysis of long term weather data shows that, on average, there are at least seven days in the summer when maximum temperatures above 32°C are experienced in the semiarid prairies. On sunny days, with air temperatures in the 30 to 35°C range, soil temperatures of 49°C have been recorded. Heat injury is commonly associated with drought injury, but excessive heat will also injure or kill plants even if moisture is plentiful (Figure 7).

High temperatures at flowering will hasten the plant™s development, reducing the time from flowering to maturity. High temperatures during flowering shorten the time the flower is receptive to pollen, as well as the duration of pollen release and its viability (Figure 8).

Figure 6. Effect of Temperature and Precipitation on Yield of Westar Canola, 1989-1991 (May, June and July Rainfall in mm)

Figure 7. Heat and Drought Injury at Elongation”Early Flower

Figure 8. Heat Injury to Flowers Resulting in Sterile Pods

High temperature can decrease total plant dry matter, the number of pods that develop, number of seeds per pod, and seed weight resulting in lower yields. Very hot weather combined with drought and high winds may cause bud blasting wherein the flower clusters turn brown and die resulting in serious yield losses (Figure 9). Flowering fields also tend to be off colour.

Figure 9. Heat Effect on Flower Clusters

Plants under water stress may have a reduced maximum temperature threshold due to decreased evaporative cooling. In the Brown and Dark Brown soil zones, high temperatures often coincide with drought during the critical flowering period.

Research at AAFC Swift Current, SK Research Centre on heat stress in canola showed significant yield losses at high temperatures for all canola species. Plants were grown in growth chambers with day/night temperatures of 20/15°C until flowering or early pod development and then subjected to high temperature stress of 28/15°C or 35/15°C for seven days then allowed to recover at 20/15°C (Figure 10).

Figure 10. Effect of High Temperature Stress on Canola Yield

This research showed that the early flowering growth stage was more sensitive to high temperature stress than the early pod stage. However, there were differences between the species and how they reacted to high temperature stress. More drastic yield reductions occurred in B. rapa with high temperature stress at 35/15°C during early flower and early pod than in B. napus or B. juncea. B. rapa responded to mild temperature stress 28/15°C at early flowering better than B. napus or B. juncea. All species were able to recover from the high temperature stress by continuing to flower after returning to 20/15°C. However, B. napus was least able to recover from severe stress at flowering as evidenced by the formation of many abnormal pods during recovery (Figure 11).

Figure 11. Abnormal Pod Formation in B. napus œQuantum in Response to 35/15°C Heat Stress at Early Flower

The main stem on the left shows aborted flowers and abnormal pods. Arrows indicate the beginning and end of the stress period. On the right close-ups of abnormal pods, seed arranged in abnormal pod and abnormal seeds (top to bottom).

Early to mid-May sown B. rapa flowers early and usually escapes the mid-summer heat. However, in the semiarid prairies, late seeded B. rapa canola or where reseeding with B. rapa is required, B. rapa crops may not escape the high temperatures of summer.

Canola is more tolerant to high temperatures when pods are formed than at flowering. Cooler night temperatures in August may also help the plant recover from extreme heat or dry weather. Higher rainfall and moderate temperatures (10 to 15°C) during seed development results in higher yield and oil content. Lower temperatures, rainfall and shorter days delay maturity increasing the risk of frost damage to seeds. High temperatures after flowering or during seed maturation result in reduced oil content. Drought stress after flowering severely reduces yield and oil content. Higher temperatures, drought and long days hasten maturity, and, in combination, can severely affect the formation of pods, seeds, seed size and oil content.

Frost

Frost-Free Periods

One of the major factors affecting canola production in western Canada is the short frost-free period. Killing frosts during seed development or during seed maturation are especially detrimental. The number of days between the last freezing temperature (0°C) of the spring and the first frost in the fall is the frost-free period. The frost-free period varies considerably from location to location in Canada. The long term average number of frost-free days is shown in Table 4. Significant variations usually occur on a local scale and extreme variations of the dates of spring and fall frost vary from year to year.

Table 4. Average Dates of Last Spring and First Fall Frosts and Average Frost-Free Days

Area	Average Date of Last Spring Frost	Average Date of First Fall Frost	Average Frost-Free Days
Guelph, ON	May 17	September 27	132
Kempville, ON	May 17	September 24	129
New Liskeard, ON	June 2	September 10	99
Kapuskasing, ON	June 10	September 5	86
Morden, MB	May 14	September 27	129
Winnipeg, MB	May 16	September 25	123
Brandon, MB	May 19	September 19	108
Portage La Prairie, MB	May 11	September 29	131
Regina, SK	May 24	September 11	109
Watrous, SK	May 25	September 10	107
Saskatoon, SK	May 21	September 16	117
Indian Head, SK	May 27	September 15	110
Scott, SK	June 1	September 7	97
Melfort, SK	May 28	September 7	101
Lethbridge, AB	May 23	September 17	116
Olds, AB	May 24	September 11	109
Lacombe, AB	May 31	September 8	99
Ellerslie, AB	May 24	September 11	109
Vermilion, AB	June 1	September 9	100
Beaverlodge, AB	May 26	September 4	101
Fort Vermilion, AB	May 28	August 30	94

To select a variety compare the heat unit accumulation and frost-free period with the days to maturity for the variety for the particular location. B. napus varieties are more suited to areas with higher heat unit accumulation and longer frostfree periods.

Frost Tolerance

Although frosts can occur in any month, it is usually those in late spring and early fall that are critical. The temperature at which frost injury occurs varies with the plant™s stage of growth, moisture content and the length of time the temperature remains below freezing. Frost cover (ice crystals) on a plant does not necessarily mean the plant has been damaged. Low temperatures injure plants primarily by inducing ice formation between or within cells. Water that surrounds the plant cells freezes first (at about 0°C), while the water within the cell contains dissolved substances that, depending on their nature and concentration, depress the freezing point of water several degrees. As the water around the cells become ice, more water vapour moves out of the cell and into the spaces around the cell where it becomes ice. The reduced water content of the cells further depresses the freezing point of the cell water. This could continue, up to a point, without damaging the cell, but below a certain point, ice crystals form within the cell, disrupt the cell membrane and injure the cell.

The length of time of freezing temperatures is important. A severe drop in temperature which only lasts a very short time may not damage canola plants, while a light frost of a few degrees that lasts all night may cause severe damage. The amount of frost injury will depend on moisture conditions, rate at which thawing occurs, the growth stage of the plant, and the amount of cold temperature hardening the plant has experienced.

After several days of near freezing temperatures, fall-sown and early spring-seeded canola will undergo a gradual hardening process that will allow the plants to withstand freezing temperatures without serious damage. It is likely that cold weather sets off a chain of plant gene activities that produce or degrade proteins that protect cells. Plants growing under these conditions are slower growing, producing smaller cells that have a higher concentration of soluble substances more resistant to frost damage. Studies at the University of Manitoba, Winnipeg, MB, University of Saskatchewan, Saskatoon, SK, and at AAFC Beaverlodge Research Centre, Beaverlodge, AB have shown that fall-sown and early-seeded canola seedlings that had undergone hardening could withstand -8 to -12°C temperatures.

Rapidly growing canola seedlings are more susceptible to frost damage than plants that are growing slowly under cold conditions, especially when there is ample moisture. Exposure to warm weather can cause cold hardened plants to lose frost tolerance and similar to unhardened later-sown canola, be killed by temperatures of only -3 to -4°C. Canola at the cotyledon stage is more susceptible to frost damage than plants at the three- to four-leaf stage which can usually withstand a couple of degrees more frost.

Figure 12. Frost Damage to Seeding

Hardened winter canola plants can survive short periods of exposure to temperatures between -15 and -20°C. Dehydration during sunny and/or windy days while the soil is frozen can cause extensive winterkill, even when the plants are optimally developed and fully hardened. Winter types tend to harden faster, achieve a higher degree of cold tolerance and unharden slower than spring types. Winter canola plants are best adapted to survive the winter in rosette stage with six to eight leaves. Smaller plants are not usually capable of surviving over-wintering, while plants with more leaves often start to elongate prematurely, exposing growing point tissue making it more susceptible to cold damage. The absence of snow cover during the coldest period of winter or the formation of ice on the soil surface can damage the crown area of plants and reduce the survival rate. Unhardening happens fairly fast in the spring after the plants initiate active growth.

Canola seedlings will usually recover from a light spring frost that does not damage the growing point of the plant. A light frost that wilts the leaves, but does not cause any browning, will not injure the plants. There may be some discolouration of the leaves, usually a yellowing or whitening especially under drought conditions (Figures 12 and 13). When a frost does blacken the cotyledons and/or leaves, no action should be taken for at least four to 10 days (Figure 14).

The extent of killing can be determined only by waiting several days following the frost. Time is required to determine the extent of the damage and whether or not the growing point has been killed. If there is any green colour at the growing point in the centre of the frozen leaf rosette, the plant will recover and yields will be higher than if the field is worked and reseeded. Under good growing conditions green re-growth from the growing point should occur in four to five days. Under poor growing conditions” cold and/or dry”this may take up to 10 days (Figure 15).

Figure 13. Frost Damage to Rosette with Whitening of the Larger Leaves

Figure 14. Frost Damage to Seedling with Blackening of the Cotyledons

Figure 15. Frost Damaged Seedling with Green Re-growth

Figure 16. Frost Damage to Seedling with Blackening of the Cotyledons

Figure 17. Seedling Killed by Frost

Consider the percentage of plants killed, the percentage recovered, the weed population and the time of year when evaluating frost damaged seedling fields (Figure 16 and 17).

To evaluate a frost damaged field, walk a diagonal path across the field and evaluate all plants in a 1/4 m² (3 sq. ft) every 20 paces and note each sample. This should result in 50 to 100 samples. Calculate the percentage of the field that has adequate plant recovering. For example, 60% of the field has a minimum of 20 to 40 recovering healthy plants per m² (18 to 36 per yd²) and a light weed population, and the remainder of the field has fewer plants (may even be none to spotty). This field still has a higher yield potential than one that is reseeded, especially if it™s the last week of May or first week of June. With a moderate weed population that cannot be controlled, the plant stand should be 60 to 70 recovered plants/m² (55 to 60 per yd²). The surviving plants will take advantage of the reduced competition for light, moisture and nutrients, and grow larger, producing more branches, pods and seeds per pod, compensating for the lost plants. The surviving plants will require five to eight days longer to mature, but a re-seeded crop will require an even longer growing period and have a greater risk of fall frost damage. Frost damage to seedlings in the spring has been only a minor problem in any one year across western Canada.

Frosts at flowering are rare and usually light and in low areas resulting in slightly delayed maturity and only minor reductions in yield. Frost during flowering usually causes flower abortion. Researchers have observed plants in which only those flowers open at the time of the frost were affected. Pods lower down on the stems and unopened buds continued to develop normally. Several days after the frost injury, gaps of aborted pods are evident on the stems. The injury is distinct in that all open flowers at the time of the frost show the injury.

Frost after flowering, however, can result in significant yield reductions and grade loss. The amount of fall frost damage to canola depends on its stage of maturity. Dry, mature seeds can be frozen with little or no effect on seed quality and viability. However, seeds with moisture content of 20% or more will suffer from frost injury. Generally, the higher the seed moisture content, the greater the chance of frost injury. A frost of “3°C is enough to kill immature seeds containing 50 to 60% moisture while those less than 20% moisture will normally escape damage. Frost does not damage all seeds to the same degree since the canola plant flowers progressively from the lower to the upper parts of stems. Frost damaged canola seed analyzed by the Canadian Grain Commission shows a range of seed damage (Table 5).

Table 5. Frost Damaged Seed Descriptions

Seed Description Categories	Sample Number, Degree of Frost Damage
	# 252 - No Frost	# 11 - Severe Damage	# 197 - Severe Damage
	Distribution of Seeds in Categories (%)
Normal seed, no white patches	90	22	27
Normal seed with white patches	0	13	15
Slightly shrivelled seed, no white patches	0	8	11
Slightly shrivelled seed with white patches	0	35	27
Severely shrivelled* seed, no white patches	0	3	1
Severely shrivelled* seed, no white patches	0	11	18
Cracked seed	10	8	1

Slightly damaged seeds were of normal diameter but had an angular appearance with obvious patches of white discolouration on the surface. Some severely damaged seeds had a reduced diameter compared to normal unfrozen seeds, and extensive white patches sometimes accompanied by white reticulation over much of the surface. Other severely damaged seeds were of reduced diameter but had collapsed inward, resulting in a fold. Pronounced white patches were often visible both inside and outside the fold. The most severely damaged seeds were of reduced diameter and extensively shrivelled. The seeds had white areas on the surface, particularly on the ridges of the folds, and a red-brown pigmentation of lighter intensity than normal. As the frost stops the development of the seed, the degree of shrivelling is an indicator of its stage of development at the time of the frost. The white patches on the seed surface are starch granules left after the seed enzyme systems were impaired.

Frost resulting in green seed is the major cause of downgrading in canola. Green seed occurs due to a failure of the seed to complete the normal chemical processes involved in degreening. A very heavy frost was experienced in 1992 in Alberta resulting in a severe drop in canola grades (Table 6).

Table 6. Canola Grades in Alberta for the Years 1991-1993

Grade	1991	1992	1993	Average 1984-91
	% of Crop in Each Grade
#1 Canada	94	32	64	78.6
#2 Canada	6	23	27	15.5
#3 Canada	0	26	13	4.4
Sample	0	19	3	1.5

Even the following year experienced lower than average grades for canola due to a cooler growing season that resulted in later maturity. Many late sown and late maturing crops were affected by a September 12 killing frost. Frost, even a light frost, can fix the green colour, preventing additional chlorophyll clearing, regardless of how favourable following weather conditions may become. Sublethal frosts from 0 to 1°C disrupt the biological enzyme system in seed nearing maturity that breaks down chlorophyll. Frost damaged seed dries down or desiccates very rapidly. Once seed moisture content is down to about 20% the biochemical activities within the seed have slowed to a very low level so that little or no further enzyme chlorophyll clearing can take place.

Thus it is important to have uniform stands, which ripen uniformly early. Uneven stands, with a significant portion of late immature seeds, may produce seeds of lower quality since frozen immature seeds will have lower oil content and retain their green colour, resulting in reduced grades.

Hail Damage

Fields with hail damage can turn yellow to green as young flower clusters are torn off. However, researchers at the AAFC Saskatoon, SK Research Centre have shown that canola plants have a remarkable ability to recover from hail damage at certain growth stages.

Canola plants injured in the seedling stage may have either one or both cotyledons missing, the seedling beaten down, or the stem broken at the soil line. Plants with both cotyledons broken or torn off, and those broken off below the cotyledons, usually do not survive. Yield losses can be determined by the per cent of the stand that the hail thinned out. An average stand of 120 to 140 plants/m² (109 to 127 per yd²) can be reduced to fewer than 40 plants/m² (36 per yd²) before yield losses exceed 10%. The crop recovers its yield potential because the remaining seedlings take advantage of the reduced competition for light, moisture and nutrients. As a result, plants grow larger, produce more branches, and develop more pods and seeds per pod, compensating for the lost plants.

Plants in the early vegetative stage are occasionally injured at the growing point and lost. However, the major injury is usually to the leaf canopy. Leaves only bruised or torn suffer partial loss, while those that are bruised on the main vein or torn, broken and wilted will be lost. Leaf area is very important for photosynthesis, therefore, leaf area loss will result in reduced seed yields. The loss in seed yield is equal to about 25% of the leaf area loss.

Plants injured in the late vegetative or early flowering stages seldom die (Figure 18).

Figure 18. Hail Damaged Plants at the Early Flowering Stage

A well established root system and the ability to develop secondary flower clusters help the plant recover. When buds and flowers are lost due to injury, the plant recovers rapidly by the development of flowers that normally would have been aborted. The plant also develops flowering branches from growth buds lower down on the plant, replacing to a degree, the lost buds, flowers and pods. Seed yield loss will depend on the per cent of leaves and branches lost, as shown in Table 7. B. napus varieties have shown a greater ability to recover from loss of flowering branches than B. rapa varieties.

If hail strikes during pod filling or ripening, plant recovery is not possible (Figures 19 and 20).

Figure 19. Hail Damaged Immature Seed

Figure 20. Hail Damage to Seeds at Podding

Even if the plants do rebranch and flower, there is insufficient growing season left to allow this new growth to reach maturity before the first killing frost. Seed yield losses for injury at the ripening stage depend directly on the loss of branches, individual pods and seeds (Table 7).

Table 7. Yield Loss Due to the Destruction of Branches During Flowering in Canola

% of Branches Lost	B. napus					B. rapa
	Days From First Flower
	-7	0	+7	+14	+21	-7	0	+7	+14
10	0	0	10	10	10	0	0	10	10
20	0	0	13	20	20	0	13	20	20
30	0	0	12	29	30	6	21	30	30
40	0	0	12	32	40	12	27	40	40
50	0	0	14	36	50	16	32	50	50
60	0	0	18	42	60	19	37	60	60
70	0	0	24	50	70	21	40	63	70
80	0	5	31	60	80	22	42	67	80
90	0	12	40	71	90	22	43	69	90
100	0	20	51	84	100	17	43	70	100

Summary

Know the growing degree days and frost-free period for your location to select the best adapted variety with the least amount of risk. Use late maturing varieties in areas with shorter frost-free periods and where lower heat accumulation poses a higher potential risk. Seed in early spring to reduce this risk.

Spring soil temperatures can be critical in establishing a uniform stand. Determine each field™s soil temperature before seeding. Keep in mind the optimum soil temperature for rapid germination and emergence, and the days to maturity of the variety selected. Waiting for warmer spring soil conditions may increase the risk of fall frost damage. Frost and hail damage at early growth stages must be carefully evaluated. Know the plant growth stage, and the effects of frost and hail on plant growth to help decide whether or not to leave the crop, reseed, or work under.

References