Fall wheat emergence and the vernalization process

If late planted wheat doesn't emerge, can it still survive, vernalize and produce good yields?

Emerged wheat
Recently emerged wheat in October 2020. Photo by Ricardo Costa, MSU Extension.

The fall 2020 season has been much more conducive to timely harvest and planting of winter wheat. Much of the wheat crop was planted early or on time. While most of the crop is in good condition and has emerged, there are a few farms that planted in November and are raising questions about the hardening process over the winter. If a wheat crop does not emerge, will it vernalize and will it grow over the winter? Before answering the million-dollar question, we want to give you a more in-depth definition of a few essential terms.

Vernalization is the process where winter wheat develops the capacity to become a reproductive plant triggered by moisture and a period of cold temperatures. The hardening off period begins in the fall once temperatures at the crown (growing point, generally placed about 1-2 inches below the ground surface level) drop below 48 degrees Fahrenheit and continues as the temperature decreases. This process reduces moisture content in the crown's cells, which slows growth processes and the accumulation of soluble carbohydrates, all of which help the plant resist frost damage.

This will also trigger physiological processes in the plant that allow it to enter reproductive stages (produce seed). Without the cold period, winter wheat will remain in the vegetative growth stages and never produce seed. This can occur weeks after emergence, but in Michigan it often happens before emergence as soon as the seed imbibes water and is exposed to cold temperatures.

Emergence is not the same as germination. Wheat germination happens below the ground surface level and starts when the seed absorbs water and ends with the radicle's appearance. On the other hand, the emergence of wheat is when the seedling breaks through the soil surface, and it is affected by air, soil temperature and seeding depth. For seed planted 0.5 inches deep, it takes 105 growing degree days (GDD) to emerge. One and 2-inch seeding depths require 130 and 175 GDD, respectively, for emergence.

A GDD is calculated as the average of the daily high and low temperatures in degrees Celsius. For example, 50 F equals 10 C, and 40 F equals 4.4 C. So, on a day where the high temperature is 50 F and the low is 40 F, you would accumulate 7.2 GDD [(10 C + 4.4 C) / 2] or 14.4 / 2 = 7.2. Under these conditions, wheat planted 1 inch deep would take 18 days to emerge.

Now it is time to answer the question. Winter wheat does not have to be emerged for vernalization to occur. At a minimum, the seed must imbibe 35-45% by weight for germination to occur. Once the seed has swelled or imbibed this much water, vernalization can occur. For the vast majority of the Michigan winter wheat crop, we are well past this stage. The only concern would be very late planted wheat planted into very dry soils (a minimal amount of acreage in the state).

Most of the state has had adequate moisture for germination, but pockets of the state have remained very dry over the harvest season. These areas or sandy fields that don't hold water could be a concern in these late planted fields. Further scouting and tracking the crop over the winter and spring is recommended.

While it is optimal to plant wheat in September and early October, late planted wheat can survive, vernalize and produce an average yield next July. Fall tillering will be lower, if any, and yield potential will be lower, but for those who have wheat in the ground, wondering if it needs to emerge to vernalize, this should clear things up.

If you have any questions, please contact Dennis Pennington at pennin34@msu.edu or 269-832-0497.

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