Today, the temperature is supposed to be around 70. Last week low temperatures were in the single digits and the week before record highs in the eighties were reached. Naturally this weather rollercoaster has spurred discussion about the condition of the winter wheat crop and its status. Has the wheat broken dormancy? Was the wheat damaged by the temperature swings? Were wheat stands reduced? To answer these questions, it’s helpful to review what winterhardiness is and how it works.
• Our wheat is a winter annual grass and has several important properties. First until it joints in the spring (hollow stem), the growing point is below the soil surface and protected from air temperatures by the soil. Second, as day length and average temperatures decrease in the fall, wheat undergoes a series of physiological changes. Cell contents change and the plant essentially produces antifreeze to help protect cells from freezing. Third fall growth tends to be prostrate, close to the ground. In the spring, after breaking dormancy, wheat growth is vertical. Being closer to the ground helps plant cells by benefitting from soil heat relative to air temperatures. Lastly, winter wheat must accumulate a certain amount of cold before it is able to flower. A process termed vernalization. Wheat varieties differ in the amount of cold accumulation necessary. When wheat enters dormancy in late fall or early winter, it can lose all its top growth without any real damage to the plant or yield as long as the growing point is undamaged.
• Wheat is most winterhardy at the start of dormancy. As winter progresses and daytime temperatures reach the 50s or above and lows stay at or above freezing, wheat will green up and start to grow. Wheat breaking dormancy is less winterhardy and more susceptible to cold temperature damage.
• If there is an extended period of warm temperatures followed by cold, the possibility of damage depends on how the cold occurs. If temperatures gradually decrease, wheat adjusts and is normally fine. If, however, temperatures drop radically and dramatically, the wheat is much more susceptible to freeze damage. Once wheat has jointed it is much more sensitive to freezing air temperatures.
• Wheat can break dormancy and reestablish winterhardiness again but each time it is less cold tolerant. This is why when the wheat crop sustains cold damage, it is typically in the late winter/early spring with much less severe cold.
• Other factors that help wheat withstand the ups and downs of temperatures are soil moisture and the extent of wheat development when it enters dormancy. Good soil moisture helps buffer the soil form rapid temperature swings and allows the root system and growing point to better able withstand cold. A well-established wheat crop helps cover the soil and protect the growing point. So damage is much more likely when soils are drier, and the wheat stand is thin and poorly tillered due to conditions or late planting.
Now to answer the questions in order. A windshield survey indicated that fields in the area had started to break dormancy and were greening up. The temperature retreat from the record highs wasn’t abrupt but gradual so the wheat should have had time to harden. We won’t know if there was stand loss until later but if there is, it may have more likely been caused by the extended cold around Christmas than the past two weeks, at least in our area.