For winter wheat, survival through our cold Manitoba winters is directly influenced by fall management decisions, including variety selection, seeding date and depth, adequate plant stands, fertility and stubble height/density. Optimal winter survival can also be influenced by fall weather conditions and snow cover.
What is the ideal situation heading into the winter?
- Plant stage would be at the 3 to 4 leaf with 1 to 2 tillers, and well developed crown tissue.
- Cool conditions in the fall, where plants would grow for 4-5 weeks, followed by 4-8 weeks (October to November) of growth that allowed plant to acclimate (harden off) and vernalize (giving the plant the signal to flower next spring).
- A minimum of 4 inches of trapped snow cover through December to early March to buffer soil temperature changes and provide protection to the crown tissue.
What is cold acclimation and vernalization?
Cold Acclimation. The ability of the winter wheat plant to survive the winter often depends on its ability to withstand low temperatures. Under normal field conditions, eight to twelve weeks of growth is usually required for the full development of winter hardiness. The first four to five weeks is a period of active growth that takes place when average daily soil temperatures at a depth of two inches (5 cm) are above 9°C. Both the cold acclimation process and winter survival require energy and this period of warm temperature allows for the establishment of healthy vigorous plants. Plants with well developed crowns before freeze-up are most desirable. However, plants that enter the winter with two to three leaves are usually not seriously disadvantaged.
Cold acclimation of winter wheat plants begins once fall temperatures drop below 9°C. In the field, four to eight weeks at temperatures below 9°C is usually required to fully cold harden plants. However, regardless of the amount of cold acclimation, the wheat plant must receive insulating snow cover to survive the cold prairie winters.
Vernalization. During the period of cold acclimation, the low temperatures also initiate in the plant a physiological response called vernalization. During vernalization, the plant converts from vegetative to reproductive growth and the reproductive structures are developed. Because of this vernalization requirement, winter wheat produces only leaves for both the main stem and tillers aboveground in the fall in preparation for winter. The growing point and buds of both the main stem and tillers remain belowground, insulated against the cold winter temperatures. Once vernalization requirements are met, the growing point differentiates and develops an embryonic head. At this time, wheat head size or total number of spikelets per head is determined. What is important to note here is neither seedling growth nor tillering is required for vernalization to occur. This process can begin in seeds as soon as they absorb water and swell. Hence, late planted wheat that has not emerged prior to winter should be adequately vernalized. Or in extreme conditions, vernalization may occur under cool spring conditions.
Submitted by: Pam de Rocquigny, Provincial Cereal Crops Specialist, Manitoba Agriculture