Environmental Effects

Table of contents

    Important tips for best management

    • Soil moisture and temperature are the most important environmental factors controlling stand establishment. Canola needs a moist seedbed with temperatures of 2 °C or warmer for germination, the start of root growth, and ultimately emergence. Ideal temperatures for rapid germination and emergence are 10 °C or warmer.
    • When conditions are dry, direct seeding and low disturbance openers will conserve seedbed moisture and improve germination and emergence.
    • Generally, temperature declines below 10 °C result in progressively poorer germination and emergence, but early seeding in late April or early May will often provide yield benefits in spite of this as long as an adequate plant population survives. Starting to seed when soil temperatures in the seed zone reach 4-5 °C is a reasonable compromise, or even earlier if the forecast is for temperatures to continue to increase following seeding.
    • Consider the local frost risk when choosing a seeding date. The frost-free period varies considerably from location to location in Canada, but each year is different. Extreme variations of the dates of final spring frost and first fall frost may occur from year to year.


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

    Soil type influences the soil’s moisture holding capacity. Clay soils with higher moisture storage capacity tend to have better and quicker emergence than sandy loam soils with lower moisture storage capacity at 100% field capacity (F.C.). (Table 1) [1]

    Table 1. Emergence of Canola at Two Soil Moisture Levels
      % Emergence of B. napus
      6 Days After Seeding 13 Days After Seeding
    Soil Type 100% F.C. 100% F.C.
    Tisdale Clay 74 76
    Sylvania Sandy Loam 33 50

    When spring moisture is limited, take steps to conserve soil moisture during seedbed preparation. Maintain a firm seedbed to reduce the loss of moisture to the surface.

    If irrigation is available, centre pivot systems provide greater flexibility as irrigation can be used to aid seedbed preparation. 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 1-2 cm (0.5-0.75"). 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 4- to 6-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.


    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 can blow seed and seedlings out of the ground or create a sandblasting effect causing plants to tip over or break at the point of injury. This is more common under dry conditions and in lighter textured soils, in which case 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. If the wind damage is patchy or if the plant stand is still 20 to 40 plants per square metre (2 to 4 per square foot), then leaving the stand is likely better than reseeding.

    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. An average stand can be reduced to fewer than 40 plants per square metre (4 per square foot) 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. However, with fewer plants maturity of the crop can be delayed.


    Seeding early can reduce the risk of damage from a fall killing frost, but cold soils will delay emergence. This delay increases the risk of seedling disease damage, which reduces plant populations. Slow and uneven seed germination and emergence can result in poor stands and later uneven 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, temperatures declines below 10 °C result in progressively poorer germination and emergence. [2] Various studies have shown that canola can germinate at constant temperatures of 2 °C, but sustained low temperatures can damage the seed embryo, which reduces germination and growth.

    Cold soils (< 5 C) at seeding, and for 2 weeks following, can increase mortality by 10% to 20%. But given the yield benefit of early seeding, soil temperatures of 4-5 C is a reasonable starting point. 

    When seeding based on soil temperature, keep in mind that:

    • Varieties and seed lots will differ in their ability to germinate and grow at lower temperatures.
    • Calendar date is not a reliable indicator of soil temperature. The year-to-year and field to field soil temperature variation can be very large.

    Growing degree days (GDDs)

    Days to 50% 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 0 C) to reach 50% germination for all temperatures 3 °C or higher. B. rapa requires 115 GDDs to reach 50% emergence at temperatures above 8°C. [3] Count GDDs beginning the day after seeding.

    Factors that influence seedbed 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.


    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, as well as 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 3- to 4-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.

    When a frost does blacken the cotyledons and/or leaves, no action should be taken for 4 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. [5]

    Under good growing conditions, green re-growth from the growing point should occur in 4 to 5 days. Under poor growing conditions — 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 a diagonal path across the field and evaluate all plants in a quarter square metre (or some square or hoop of known dimension) every 20 paces and note each sample. This should result in 50 to 100 samples. Use these counts to calculate the percentage of the field that has an adequate density of plants recovering.

    If 60% of the field has a minimum of 20 to 40 recovering healthy plants per square metre (2 to 4 per square foot) 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. (Link to reseeding section.)

    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 may require 5 to 8 days longer to mature, but a reseeded 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. With a moderate weed population that cannot be effectively controlled, the plant stand should be 60 to 70 recovered plants per square metre (6 to 7 per square foot) as the weed pressure may limit the ability of the canola plants to compensate through increased yield per plant.

    How frost damage works

    Low temperatures injure plants primarily by inducing ice formation between or within cells. Water that surrounds 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 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. [6] 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 -3 °C to -4 °C.

    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 1. Significant variations usually occur on a local scale and extreme variations of the dates of spring and fall frost may occur from year to year.


    [1] “The Effect of Seeding Depth, Soil Moisture Regime, and Crust Strength on Emergence of Rape Cultivars,” W. F. Nuttall, Agronomy Journal 1982. (Researchers from Agriculture and Agri-Food Canada’s centre in Melfort, Sask., working with Grey Wooded and Dark Grey Wooded soils in northern Saskatchewan.)

    [2] Growth chamber study. Christensen, J.V., W.G. Hegge, R.M DePauw, A.M.F. Henning, J.S. McKenzie, B. Siemens, and J.B. Thomas. 1985. Effect of seeding date , nitrogen and phosphate fertilizer on growth, yield and quality of rapeseed in northwest Alberta. Can. J PIt. Sci. 65:275-284.??? Agriculture and Agri-Food Canada in Beaverlodge, Alta., found much lower and slower germination at low temperatures for both B. napus and B. rapa compared to warmer temperatures (Figures 2 and 3). Temperatures of 4°C or higher had little effect on total percent germination in B. napus, but the number of days to 50% germination increased dramatically at temperatures below 6°C.

    [3] A study of temperature effect on canola emergence at Colorado found that knowing the actual GDDs required for emergence was useful in predicting days to emergence (Figure 4). Vigil, M.F., R.L. Anderson, and 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., “Freezing in oil-seedBrassicaspp.: some factors affecting injury” The Journal of Agricultural Science(1985), 104 : pp 513-518.

    [5] A 2004 Canola Council study showed a 7.4 bushel loss when reseeding a crop versus leaving a crop thinned due to frost stand through to harvest.

    [6] Studies at the University of Manitoba, University of Saskatchewan and at Agriculture and Agri-Food Canada in Beaverlodge, Alta., have shown that early-seeded canola seedlings that had undergone hardening could withstand -8 C to -12 C temperatures.