Growth Rate and Days to Maturity
Canola development is aligned with growing degree days (GDD) and the amount of sunlight captured. Canola will grow at temperatures above 5°C, however the optimum temperature for growth is 18°C to 25°C. Growth rate and days to maturity are highly dependent on winter and spring conditions. High temperatures (i.e., above 28°C) at flowering cause flower and pod abortion and have a significant impact on yield. Drought and heat stress will shorten the days to reach maturity. Generally, winter canola matures around the same time as winter wheat in Ontario. Depending on conditions and where in Ontario the canola is grown, it may be harvested in late June through to early August. Winter canola in more northern locations may be in the field for nearly 12 months. Winter canola can be grown in areas south of Muskoka District down to the southernmost point of Ontario, and throughout Eastern Ontario on appropriate fields. Winter canola should not be grown in regions where winter conditions are long and extreme, where wet spring conditions are prolonged, and where winter wheat survival is generally poor. Winter survival is improved where winter conditions are mild.
Root System
Canola has a large main taproot that can extend up to 1.5 m (4.9 ft) deep into the soil under favourable growing conditions. Despite its taproot, canola is not capable of penetrating a soil hard pan and is susceptible to soil compaction. About 70% of the canola root system is in the top 15 cm (6 in.) of the soil profile. Early season root growth has a strong positive relationship to final yield. During early vegetative growth, moist topsoil and dry subsoil will result in a shallow root system. Canola roots will not grow in search of water or nutrients, they only intercept water or nutrients present in the soil. Soil compaction, weed competition, or dry soil can limit root growth and make potential canola yields more dependent on timely rainfall during flowering and pod fill. Root growth peaks at flowering.
Plant Development
Germination of canola is similar to other dicots. Emergence generally occurs between 4 and 10 days after planting, depending on soil and climatic conditions. The small seed will only sustain growth for approximately 7 days before accessing nutrients from the soil or through photosynthesis. Canola seedling mortality can be high unless quick, uniform emergence occurs. At emergence the seedlings are very small, and the root will be approximately 3–5 cm (1.2–2 in.) long. The growing point of canola is located between cotyledon leaves (seed leaves) and is susceptible to frost, insect and slug feeding and hail damage. The first true leaf appears about 4–8 days after emergence. Leaf area development is directly related to growth rate and final yield. Rate of growth depends on many environmental factors including soil temperature, air temperature, moisture, photoperiod, fertility, and pest feeding. Establishment and growth up to the 4-leaf stage may be slower than weed growth, making canola seedlings a poor competitor with weeds.
Canola develops a rosette with older leaves growing large at the base and new, smaller leaves forming at the center. Leaves are waxy and hairless. Winter canola should reach the rosette stage with a minimum of 6 to 8 leaves before overwintering. Plants with larger roots tend to have improved winter survival. However, as plants grow large and add new leaves, the growing point rises and becomes more exposed to winter conditions. Winter survival is improved when the growing point is close to the soil surface through winter. Outer leaves of the rosette may die over winter, but plants remain viable if the crown is not injured and the plants do not heave out of the soil. Canola that enters reproductive stages in fall are likely to die in winter.
Canola will produce anywhere from 9 to 30 leaves. Achieving and maintaining a high leaf area is associated with higher yield. At temperatures above 5°C the canola will resume growth in the spring. Flower buds form in the center of the rosette, quickly followed by stem elongation (“bolting”). A hard frost after the canola has entered reproductive growth stages can result in plant death, but modern winter canola hybrids tolerate frost well. In Ontario, winter canola has survived temperatures of -4°C to -5°C for up to two hours. See “Other Crop Problems” for more information.
Secondary branches develop from growing points in leaf axils along the main stem. At low plant populations, canola will produce a thicker main stalk and branch profusely. In very low populations the increased branching results in plants flowering over a longer period and taking longer to fully mature. Stems and branches continue to elongate through flowering.
At the onset of flowering, the main stem will be 30 to 60% of its maximum height. Final height is influenced by cultivar, fertility, moisture and population. Canola typically reaches between 100 and 175 cm tall (39 to 69 in.). The main stem will reach maximum height around the same time as peak flowering. The stem is an important source of photosynthate during pod and seed fill.
Flowering
Flowering begins with the opening of the lowest bud on the main stem and continues upward on main and secondary branches as they elongate. The first flower to open, and therefore the first pod to form, is at the bottom of the main stem. The last flowers to open are at the tips of the branches. Individual flowers remain receptive to pollination for 3 days after opening, and flowering continues for 14–21 days.
The bloom stage at a given time is typically described by how far through bloom the crop has progressed as a percentage, where halfway through the bloom period is referred to as 50% bloom. The bloom stage is approximated based on a visual assessment of the crop. There are approximately 10 flowers open on the main stem at 10% bloom, 20 flowers open on the main stem at 30% bloom, and greater than 20 open on the main stem plus some on side branches at 50% bloom. Canola can progress from first flower to 30% bloom in 6 to 10 days.
Canola produces more buds than can be developed into pods, and abortion of flowers and pods is normal. Canola is primarily self-pollinated but can cross-pollinate, depending on various environmental factors. Fertilization occurs within 24 hours of pollination. Temperature stress (>28°C) during this period causes flower abortion and is detrimental to yield.
Ripening
Pods contain two rows of seeds, separated by a thin membrane. Seed filling is complete 30–40 days after flowering. Pod ripening starts from the bottom of the plant. When seeds have turned green, leaf senescence begins, and the pod wall becomes the major source of photosynthate, although the stem is also important. Firm, green seeds are physiologically mature and are capable of germinating and supporting seedling growth. Temperature or drought stress during pod development can cause pod abortion.
Seed from mature Argentine varieties (Brassica napus) has a brown to black seed coat colour and are yellow inside. Seed colour change begins at the bottom of the plant and progresses up the main stem as moisture content decreases. When 30–40% of seed on the main stem has started to change seed coat colour, overall moisture content will be around 30%-35% and the seeds in the last formed pods are completing filling. Seed colour change advances by about 10% every 2–3 days; the rate increases in hot weather and may decrease in cool weather. The colour of the pod when seeds are mature will vary depending on the variety and environment, so the pod or plant colour visible across the field is not a good indication of seed maturity or moisture content. Fully mature pods may shatter and lose seed.