Growing canola

An overview of the cultivation practices for this potentially lucrative crop.

Canola flowers produce pods from which seeds are harvested and crushed for their oil.
Photo: FW Archive

As canola seed is very small, the seedbed should be fine and without clods to ensure an even planting depth. Planting canola in dry soil is not recommended, as emerging seedlings are susceptible to soil crusting and soil moisture contact is crucial for rapid emergence.

Planting
Planting depth is shallower than for most grain crops. Canola needs the top 7,5cm of soil to be moist.

  • Density/spacing – Under irrigation, canola is typically grown in rows 15cm to 45cm apart. Consistent spacing optimises yield. Plant seed at 4kg/ ha to 6kg/ha for a plant population of 50 000 plants/ ha to 80 000 plants/ ha. Plant 3kg/ ha to 4kg/ha if the land is relatively free of weeds or has been sprayed with herbicide, the seedbed is moist with little crusting potential, and you use a mechanical planter for accurate seed placement.
  • Date – Plant canola from April to early June for the highest yield. If you plant after 15 June, the yield will be lower. Canola is very susceptible to temperatures above 27°C and drought stress when flowering. Planting in May reduces these risks. At the other end of the spectrum, seedlings can tolerate a temperature as low as -4°C.
  • Depth – If adequate moisture is present, do not plant canola deeper than 3cm. Quick seedling emergence enables the crop to get ahead of or keep up with weeds. Deep seeding delays crop emergence and weakened seedlings are less competitive.

Fertilisation
Nitrogen and sulphur are crucial for an improved yield.

Nitrogen – 
Canola has high nitrogen demands. About 55kg of nitrogen per hectare enables a yield of 1t/ha. A higher nitrogen rate stimulates leaf growth and increases transpiration rate and moisture use. Under irrigation, the recommended nitrogen fertiliser rate is 180kg/ ha on clay soils and 200kg/ ha on sandy soils. For dryland cultivation (Table 1).

  • Deficiency: When there is too little nitrogen, it is transferred from older leaves to younger growing tissue. For this reason deficiency symptoms first appear on mature leaves. With a severe deficiency, the oldest leaves and young plants are pale, with pink-coloured mid-ribs and petioles. Stems eventually turn reddish and older leaves turn yellow.

Phosphorus – The optimal phosphorus (P) level in topsoil should be 20mg/kg (Bray 1 method of analysis). Table 2 details adjustments to be made for lower or higher values.

  • Deficiency: During early development, a P deficiency will retard growth and result in dark-green leaves. Later, a pink-purple colour develops on the tips and margins of older leaves, changing to orange or red. The first symptom of P deficiency is puckering and wilting of the oldest leaves. A yellow-brown colour develops at the leaf tips and slowly spreads toward the mid-rib, which stays green the longest. The condition then spreads to younger leaves. 

Potassium
The optimum potassium (K) level of the topsoil is 80mg/kg for clay soil and 60mg/kg for sandy soil. Most soil has a high K level. Low K can be restored by following these guidelines: if the level is below 50mg/ kg, apply 40kg/ha; if 50mg/kg to 80mg/kg, apply 20kg/ha; if above 80mg/ kg, apply up to 20kg/ha.

  • Deficiency: The first symptom is a puckering of the oldest leaves with a dull sheen. The leaves become yellowish at the tips; this colour gradually spreads towards the mid-rib, which stays green longer.

Sulphur
Canola’s sulphur (S) requirement is 15kg/ha to 20kg/ha. Table 3 lists more detailed recommendations.

  • Deficiency: A severe S deficiency causes mottling of the leaves. In some cases a pink-purple colour develops on the underside of the leaves. High nitrogen availability can exacerbate S deficiency symptoms.

Other nutrients
Canola is susceptible to boron and molybdenum deficiencies. Copper, zinc, and manganese needs are higher than those of maize. Tables 4 and 5 detail application rates.

Method/time of application
Nitrogen and potassium should not come in direct contact with the seed. Rather broadcast or bandspread these nutrients in a band at least 5cm from the seed row. Divide nitrogen fertilisation into two to three applications if possible. On heavier soils, apply 40% to 60% at planting and the balance six weeks after emergence just before stem elongation. On sandier soils, apply 40% during planting and 60% in one or two equal applications. Apply sulphur with the nitrogen, especially on soil with a low pH, either during or shortly before planting.

Irrigation
Canola will suffer if the water supply is limited. The yield will increase if it is irrigated at critical times under moderately favourable conditions. The most important time for irrigation is during flowering and early pod development. As maximum oil deposition occurs in the pod stage until pod maturation, irrigation at this time will boost oil content further.

Weed control
Young canola seedlings are sensitive to weed competition. An effective weed control programme includes crop rotation, mechanical and chemical methods. Once established, canola is a good competitor. Its growth rate is higher than that of most weeds. Where broadleaf weeds are a problem, plant triazine-resistant cultivars. Canola is extremely sensitive to chemical drift from most broadleaf herbicides, so reduce or eliminate herbicide drift from canola lands.

Source: Canola Production Guideline, compiled by the Directorate Plant Production in collaboration with the ARC, published by the Department of Agriculture.