Environmental effects

Soil moisture and temperature are the most important environmental factors controlling stand establishment. Physical damage from wind, hail and excess water can also be a concern for canola plants at the early stages of development. Understanding the impacts of these environmental effects can help prepare growers to make informed management decisions, and set the crop up for success later in the growing season.

Important tips for best management

  • Canola needs a moist seedbed with temperatures of two degrees Celsius or warmer for germination, the start of root growth, and ultimately emergence. Ideal temperatures for rapid germination and emergence are 10 degrees Celsius or higher.
  • Early seeding in late April or early May can provide yield benefits in spite of cooler temperatures, as long as an adequate plant population survives. Starting to seed when soil temperatures in the seed zone reach four to five degrees Celsius is a reasonable compromise, or even earlier if the forecast is for temperatures to continue to increase following seeding.
  • When conditions are dry, direct seeding and low disturbance openers will conserve seedbed moisture and improve germination and emergence.
  • Consider the local frost risk when choosing a seeding date. The frost-free period varies considerably from location to location in Canada, and each year is different. Extreme variations of the dates of final spring frost and first fall frost may occur between years.

Moisture

Moist (not wet) seedbeds are ideal for canola germination. Semi-dry seedbeds often result in slow, uneven germination and increased abnormal seedlings.

Soil type influences the soil’s moisture-holding capacity. Clay soils have higher moisture storage capacity and tend to have better and quicker emergence than sandy loam soils, which have lower moisture storage capacity at 100 per cent field capacity (F.C.) (Table 1) 1.

Table 1. Emergence of canola at two soil moisture levels

Note: F.C. refers to field capacity

If irrigation is available prior to seeding canola, prepare the field for seeding then irrigate heavily enough to firm the surface and wet the soil to the level of sub-surface moisture, usually one to two centimetres (0.5-0.75 inch). Leave the field to become surface dry, then seed. Do not irrigate between seeding and emergence, due to potential soil crusting. Depending on soil type and environmental conditions, a single irrigation to promote germination and emergence is usually all that is necessary until the crop is in the four- to six-leaf stage. Avoid over-irrigation during this period which can reduce the rate of crop growth and increase the level of seedling disease.

Physical damage

Excess water

Canola is quite susceptible to water logging and shows a yield reduction after only three days. Wet soils cause an oxygen deficiency, which reduces root respiration and growth. This reduces nutrient uptake, and if conditions persist, plants can die or prematurely senesce.

Wet conditions can also cause leaching and denitrification of nitrogen and leaching of sulphur. Before investing in a fertilizer top up of nitrogen and sulphate sulphur, wait to see how the crop recovers from saturated soils. With wet conditions, roots may be shallow and not able to access nutrients once the soils begin to dry, so it is important to monitor closely.

Even if soils do dry up quickly and the crop recovers, think twice before applying high rates of nitrogen top dress. Nitrogen will stimulate growth, and can make a delayed crop even later, increasing the risk of grade and yield loss from fall frost.

Crusting

Wet conditions followed by warm sunny days may cause soil crusting, creating a physical barrier that stops seedlings from emerging. If a few plants have emerged, it may be best to leave them rather than take measures to break up the crust. Two plants per square foot in late May or early June can be a viable crop if managed properly. However, if early in May and no plants have emerged and dead seedlings are found below the soil surface, reseeding can be an option.

If no plants have emerged and the majority of the seeds have not germinated, a light harrowing might help but there is a lack of research to support this. Take care to ensure that the harrowing operation is not damaging germinated seedlings that are trying to emerge.

Using a roller may be worse than harrowing. Instead of cracking up the soil surface, a roller could compact the topsoil and make the situation worse.

Wind

Wind can blow seed and seedlings out of the ground or create a sandblasting effect. This can cause plants to tip over or break at the point of injury. This is more common under dry conditions and in livghter textured soils, where the crop will already be stressed. It can also be worse on hilltops and side slopes facing into the wind. Assess the whole field before making a decision to reseed, or consider reseeding patches. If the wind damage only affects small patches or if the plant stand is still above 10-20 plants per square metre (one to two per square foot), then leaving the stand could be a better choice than reseeding the whole field.

Hail damage

If early season hail breaks off both cotyledons or snaps the stem, these plants usually do not survive. But while individual plants may die, a whole canola crop is fairly resilient to early season hail when it comes to overall yield potential. Canola can sometimes recover its yield potential at sub-optimal plant densities (below 30-40 plants per square metre or three to four plants per square foot) because the remaining seedlings take advantage of the reduced competition for light, moisture and nutrients. As a result, plants can grow larger, produce more branches, and develop more pods and seeds per pod, compensating for the lost plants. Regardless of that compensation, a thin stand will have less probability of achieving its maximum yield and it is likely to see delayed maturity, variable maturity and/or be less competitive with weeds.

Temperature

Seeding early can reduce the risk of damage to maturing plants from a fall killing frost, but cold soils (a potential challenge of early seeding) will delay emergence in the spring. This delay increases the risk of seedling disease damage, which reduces plant populations. Slow and uneven seed germination and emergence can result in thin, variable stands which can make it more difficult for timing in-season crop protection products (e.g. herbicides, insecticides or fungicides) and lead to uneven and/or later maturity.

To determine soil temperature, insert a soil thermometer at seeding depth and record the temperature at 8:30 a.m. and 4:00 p.m. over a few days. Take an average of those readings. This provides a more accurate overall estimate of soil temperature conditions the germinating seed and emerging seedling will endure.

Generally, soil temperatures below 10 degrees Celsius result in progressively poorer germination and emergence 2. Various studies have shown that canola can germinate at constant temperatures of two degrees Celsius, but sustained low temperatures can damage the seed embryo, which reduces germination and growth.

Cold soils (less than five degrees Celsius) at seeding and throughout the two weeks following can increase mortality by 10 per cent to 20 per cent. Given the yield benefit of early seeding, soil temperatures of four to five degrees Celsius is a reasonable starting point.

When seeding based on soil temperature, keep in mind that calendar date is not a reliable indicator of soil temperature. The soil temperature variation between years and between fields can be very large.

Growing degree days

Days to 50 per cent germination for canola can be predicted based on how many growing degree days (GDDs) have accumulated since planting, provided soil moisture is adequate to stimulate germination. B. napus requires 75 to 120 GDDs (base zero degrees Celsius) to reach 50 per cent germination for all temperatures three degrees Celsius or higher. B. rapa requires 115 GDDs to reach 50 per cent emergence at temperatures above eight degrees Celsius 3. When calculating, start count GDDs the day after seeding (not the day of seeding).
Factors that influence soil temperature:

• In spring, soil tends to be warmer closer to the surface.
• Bare soil warms quicker than soils with surface residue or vegetation that reflects some of the solar radiation. When seeding depth and climatic conditions are the same, emergence in seedbeds without residues at the surface will be about one to two days sooner than in seedbeds with residues on the surface.
• Tillage influences soil temperatures by reducing crop residue and drying the soil. Dry soil warms faster than wet soil but 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.

Frost

The temperature at which frost injury occurs varies with the plant’s stage of growth, the moisture content in the plant’s tissues, the length of time the temperature remains below freezing, the weather previous to the frost, and the soil surface’s ability to act as a reservoir for heat (minimizing the time the plants are exposed to freezing temperatures). Canola at the cotyledon stage can be more susceptible to frost damage than plants at the three- to four-leaf stage 4, which can usually withstand frost temperatures a couple degrees cooler. However, reported field observations have stated the opposite, where cotyledons have been less damaged than the larger plants, due to proximity to the soil surface which can act as a heat sink.

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. With wilting, there may be some yellowing or whitening of leaves, especially under drought conditions.

Assessing frost damage

One canola seedling killed by frost (on the left) and the other survived (on the right)

When a frost does blacken the cotyledons and/or leaves, no action should be taken for four to 10 days. It takes several days to determine the extent of damage and whether the growing points have been killed. If there is any green color at the growing points in the centre of the frozen leaf rosettes or the hypocotyls appear thick and not pinched off, most of the plants 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 (i.e. cold and/or dry) this may take up to 10 days.

Consider the percentage of plants killed the density and distribution of the plants that look like they will survive, the weed population and the time of year when evaluating frost damaged seedling fields.

To evaluate a frost damaged field, walk an “X” or “W” path across the field and note all plants that will survive in a quarter square metre hoop (or some square or hoop of known dimension) every 20 paces. This should result in 50 to 100 samples. Record an observation from each sample and use these counts to calculate the percentage of the field that has an adequate density of plants recovering.

Frost damage can appear in patches, as light damage across a field, or complete stand death. Since frost tends to flow and settle across the landscape like water, low lying areas often see greater damage.

Canola seedling is recovering from frost damage; note the new growth.

With a light frost of zero degrees Celsius to minus two degrees Celsius, the risk of crop damage is minimal.

With a heavy frost that kills off the cotyledons or parts of true leaves, it will take a few days before new leaves will start emerging from the growing point. If no growth occurs within this time, the plant is likely dead. If the stem is pinched off or the plant flops over, the plant will likely die. The pinched off or broken stem cannot provide nutrients to the growing point. Dead cotyledons do not mean a dead plant. If the hypocotyl is green and healthy days after the frost, this plant has likely survived and will soon put out a first leaf.

The quickest way to accurately assess recovery is to mark individual plants that could recover and recheck these plants over the following few days.

If the majority of a field has a minimum of 20 to 40 recovering plants per square metre (two to four plants per square foot) with light weed pressure, the field should not need to be re-seeded. Fields with 10 to 20 plants per square metre (one to two plants per square foot) across the majority of a field can be adequate if managed carefully with today’s herbicide tolerant hybrid varieties. Thin stands such as these can sometimes yield up to 90 per cent of a normal stand seeded at an early date but will be later in maturity. 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. The surviving plants may require an extra five to eight days or longer to mature, but a reseeded crop will require an even longer growing period and have a greater risk of fall frost damage.

More frost scenarios and considerations are discussion on this Canola Watch podcast.

Frost Assessment

Frost damage details

Low temperatures injure plants primarily by inducing ice formation between or within cells. Water that surrounds plant cells freezes first (at about zero degrees Celsius), 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 becomes ice, more water vapor moves out of the cell and into the spaces around the cell where it freezes. 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, early 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.

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, like unhardened later-sown canola, be killed by temperatures of only minus three degrees Celsius to minus four degrees Celsius.

The frost-free period varies considerably between locations in Canada. Significant variations usually occur on a local scale and extreme variations of the dates of spring and fall frost may occur between years.

Frost and weed control

Weeds stressed and weakened by frost are not more susceptible to herbicide. The performance of some crop protection products declines below (or above) certain temperatures and life science companies may not be able to guarantee their products if applied too soon after a frost.
Review labels and performance restrictions on all crop protection products before using them.
After a light frost, spraying could resume when the following conditions are met:

• A minimum of one night with minimum temperatures of five degrees Celsius (the minimum for biological activity to occur).
• A minimum of one day of good growing conditions (warm and sunny) have passed.
• Good growing conditions (warm and sunny) are present at the time of spraying.
• There is no evidence of frost damage (blackening and water soaked appearance) on the crop or the weeds. Note that even herbicide tolerant hybrids require that the metabolism of the plant be working at full capacity to enable it to effectively process the herbicide and prevent injury.

After a heavy frost, check for damaged tissues such as water soaked and darkened leaves that eventually lead to necrosis (dead, dry tissue). If tissue damage is greater than 40 per cent of total leaf area, allow new leaves to grow before making herbicide applications.

Footnotes

  1. Nuttall, W. F. 1982. The Effect of Seeding Depth, Soil Moisture Regime, and Crust Strength on Emergence of Rape Cultivars. Agronomy Journal, 74(6), 1018-1022. doi:10.2134/agronj1982.00021962007400060020x
  2. Christensen, J.V., Legge, W.G., DePauw, R.M., Henning, A.M.F., McKenzie, J.S., Siemens, B., & Thomas, J.B. 1985. Effect of seeding date, nitrogen and phosphate fertilizer on growth, yield and quality of rapeseed in northwest Alberta. Canadian Journal of Plant science. Revue Canadienne de Phytotechnie. 65(2), 275-284. DOI: 10.4141/cjps85-040.
  3. Vigil, M.F., R.L. Anderson, & W.E. Beard. 1997. Base temperature and growing-degree-hour requirements for the emergence of canola. Crop Sci., 37, 844-849.
  4. Dhawan, A.K. 1985. Freezing in oil-seed Brassica spp.: some factors affecting injury. The Journal of Agricultural Science, 104, 513-518.

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