Effects of Hail Damage to Canola

Fields with hail damage can turn yellow to green as young flower clusters are torn off. However, researchers at the Agriculture & Agri-Food Canada Research Station, in Saskatoon, 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 loss will be determined by how badly the hail has thinned out the stand. An average stand of 11 to 13 sq. ft. can be reduced to fewer than 4 sq. ft. 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, thereby 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 that are only bruised or torn suffer only partial loss, while those that are bruised on the main vein, or torn and broken and wilting, 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. Well-established root systems, and the ability to develop secondary flower clusters, help the plants recover. When buds and flowers are lost due to injury, the plant recovers rapidly by the development of flowers which normally would have been aborted. The plants also develop flowering branches from growth buds lower down on the plant; and in that way, replace (to a degree) the lost buds, flowers and pods. Seed yield loss will depend on the percentage of leaves and branches lost. Brassica napus varieties have shown a greater ability to recover from loss of flowering branches than Brassica rapa varieties.

If hail strikes during pod filling or ripening, plant recovery is not possible. Even if the plants do re-branch 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: Yield Loss Due to the Destruction of Branches During Flowering in Canola

 Source is The Canola Council of Canada Growers Manual