Effect of Frost on Flowering Canola
Frost at flowering will delay maturity but results in only minor reductions in yield. Frost after flowering, however, can result in significant yield reductions and grade loss. Frost during flowering usually causes flower abortion. Researchers have observed plants in which only those flowers open at the time of the frost were affected. Pods lower down on the stems and unopened buds continued to develop normally. Several days after the frost injury, gaps of aborted pods were evident on the stems. The injury is distinct in that all open flowers at the time of the frost showed the injury.
The amount of fall frost damage to canola depends on its stage of maturity. Three degrees of frost is enough to kill immature seeds containing 50 to 60% moisture while those ready to swath at about 35% moisture will normally escape damage. Thus it is important to have uniform stands which ripen uniformly early. Uneven stands, with a significant portion of late immature seeds, may produce seeds of lower quality as the frozen immature seeds will retain their green color which will reduce the grade.
Source. Canola Growers Manual of Canola Council of Canada