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Fall Wheat Emergence and the Vernalization Process

By Dennis Pennington and Jenna Falor

The fall weather so far has led to delay in the harvest of many crops across Michigan, which in turn made it difficult for some to get their wheat planted in a timely manner. As of Nov. 5, 2023, only 80% of Michigan wheat had been planted according to the United States Department Agriculture National Agricultural Statistics Services Crop Progress and Condition Report, which is 17% behind the same date last year but up 11% from Oct. 29. This means a lot of farmers are getting their wheat planted later than normal. Many are also pondering, 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.

Hardening off, vernalization and emergence

Winter wheat goes through a period where the plant hardens and adjusts to the colder winter temperatures. The plant’s ability to withstand very cold temperatures is built over time as the temperature slowly cools in the fall.

Hardening is the level of tolerance a wheat plant has to cold temperatures. The hardening 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 to resist frost damage.

The hardening off period begins in the fall once temperatures at the crown (growing point, generally 1-2 inches below the surface) drop below 48 degrees Fahrenheit and continues as temperatures decrease.

Warm periods over the winter start to reverse the hardening process. Once temperatures cool off again, the hardening process begins again.

When warming events occur over the winter, the level of cold hardiness is reduced each time. Wheat plants will never reach the same level of cold hardiness as was first achieved early in the winter.

Each time the weather warms, the bar is raised regarding how cold of temperatures the plants can tolerate. Often, winterkill occurs after the plants have gone through a series of warm spells followed by a very cold night.

Vernalization is a physiological process that wheat plants must undergo to reach reproductive stages (produce seed). Vernalization requires a six- to eight-week period of temperatures below 48 F.

Without the cold period, winter wheat will remain in the vegetative growth stages and never produce seed. The time and temperature required for complete vernalization varies by variety and day length.

Emergence. Remember that germination is not the same as emergence. 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; emergence 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. At seeding depths of 1 and 2 inches, 130 and 175 GDD, respectively, are required 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 degrees Celsius. 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 we can answer the million-dollar 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.

Generally, there is adequate soil moisture in the fall in Michigan for germination. The only concern would be very late-planted wheat planted into very dry soils. Michigan State University Extension recommends further scouting and tracking the crop over winter and spring.

While it is optimal to plant wheat in September and early October, late-planted wheat can survive, vernalize and produce a normal yield come July. Fall tillering will be lower—if there is any—and yield potential will be lower. But for those who have wheat in the ground and are wondering if it needs to emerge to vernalize, this should clear things up.

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