Written by Bob Fanning, former SDSU Extension Plant Pathology Field Specialist.
There are two major differences between spring and winter wheat. With adequate cold acclimation, winter wheat can withstand freezing temperatures for extended periods of time at the seedling phase. Winter wheat also requires a period of exposure to cool temperatures to trigger its reproductive development. This process is called vernalization.
To achieve maximum cold acclimation or “hardening”, after germinating, producing several leaves and one to two tillers, the winter wheat plants are exposed to gradual decreases in average temperature before freeze-up occurs. Exposing the wheat plants to temperatures near 40 degrees F for at least three weeks is ideal, although the exact temperature and time period required varies by variety. The seed does not have to actually germinate and emerge to go through the vernalization process. At the minimum, all that needs to happen is for the seed to absorb moisture and swell, followed by exposure to cool temperatures for the required period of time.
The required temperature and time period for vernalization is closely related to the winterhardiness and maturity rating of the variety. More winterhardy and later maturing varieties tend to require lower temperatures for longer periods of time than less winterhardy and earlier maturing varieties. If the winter wheat plants do not go through the vernalization process, they will remain vegetative and not joint or produce a seed head.
On occasion, fluctuating temperatures in the fall has caused early maturing varieties to complete the vernalization process and enter the jointing stage in the fall in South Dakota. By the same token, winter wheat planted and/or germinating too late in the spring, particularly late-maturing varieties, may fail to complete the vernalization process and not produce a seed head.
An occasional question is, I planted my winter wheat in the fall but it didn’t come up until spring, will it produce a head? The answer is back in the statement, all that needs to happen is for the seed to absorb moisture and swell, followed by the exposure to cool temperatures for the required period of time. Even if planted into dry soil, like much of the wheat planted in the fall of 2012 is or will be; it’s almost certain that the seed will take up enough moisture to swell over the fall, winter and early spring to allow it to complete the vernalization process. The problem with late planted and late germinating winter wheat is outlined in the article Winter Wheat Planting Date is Important, delayed planting decreases yields.
For more information, review Chapter 4 of iGrow Wheat – Best Management Practices for Wheat Production.