Cold hardening in winter wheat
Wheat in Kansas has gone through a bit of a roller coaster ride so far this fall and winter. Temperatures took a sudden dive on November 11 and stayed unusually cold for quite a while. This burned back a lot of topgrowth and shocked the wheat into preparing for winter. Since then, there have been periods of mild weather and moisture – enough that a little new growth of leaves and tillers could be seen in some fields. Where this is the case, is that wheat in condition to survive the winter?
Figure 1. Wheat in northwest Kansas during week of December 15-18. Photo by Jeanne Falk Jones, K-State Research and Extension.
In short, the answer is “probably so.” Where new growth occurred in December, that doesn’t mean the wheat will have lost its winterhardiness. The wheat may not be as cold tolerant now as it could be, however. As long as temperatures are at or below freezing at night, there won’t be much new growth. And the new growth that has occurred can still re-harden.
It helps to know how winter wheat typically survives the winter. Winter wheat is never truly dormant, but it does gradually go through cold acclimation in the fall until it is able to withstand cold temperatures (down to a point).
During the fall, winter wheat seedlings spend the first month or so of their lives developing their first leaves, the crown, and a secondary root system. All the while, the seedlings are building and storing the energy they will need to go through the cold acclimation process and survive the winter. Normally seedlings will need at least 2-3 true leaves and a tiller or two to have built up enough stored energy reserves to survive the winter. The seedlings will have a better chance of winter survival if their crowns are well developed in firm soil, about a half-inch below the soil surface.
Winterhardiness or cold tolerance is a physiological process triggered by gradually cooling temperatures in the fall. During the process of cold acclimation, certain genes within winter wheat begin to initiate the production of “anti-freeze” type substances to protect the cell membranes.
The process of cold acclimation within a sufficiently developed wheat seedling begins when soil temperatures at crown depth fall below about 50 degrees F. Photoperiod also plays a role in the process of cold hardening, with shorter days and longer nights helping initiate the process. Winter survival depends on the crown remaining alive, and the substances that produce cold acclimation are most needed within the crown.
It takes about 4 to 6 weeks of soil temperatures below 50 degrees at the depth of the crown for winter wheat to fully cold harden. The colder the soil at the depth of the crown, the more quickly the plants will develop winterhardiness.
Cold hardiness is not a static state, however. After the cold hardening process begins in the fall, wheat plants can rapidly unharden when soil temperatures at the depth of the crown get above 50 degrees. But the plants will re-harden as crown temperatures cool below 50 degrees again. By the time winter begins, winter wheat will normally have reached its maximum level of cold hardiness. Wheat in Kansas normally has its maximum level of winterhardiness from mid-December to mid-January, unless there are high temperatures during that period.
Even during the depths of winter, winter wheat is still respiring and roots may be growing – as long as the ground is not frozen. It is not unusual to find a much more developed crown root system in early February than existed in early December.
It is not unusual to see some green leaves intermingled with straw-colored or pale leaves in the winter. The fact that some of the leaves have some green color does not mean the wheat is not cold tolerant.
Once winter wheat has reached the level of full cold hardiness, it will remain cold hardy as long as crown temperatures remain below about 32 degrees– assuming the plants had a good supply of energy going into the winter.
If soil temperatures at the crown depth rise to 50 degrees or more for a prolonged period, there will be a gradual loss of cold hardiness, even in the middle of winter. The warmer the crown temperature during the winter, the more quickly the plants will start losing their maximum level of cold hardiness. Winter wheat can re-harden during the winter if it loses its full level of winter hardiness, but will not regain its maximum level of winterhardiness.
Even at its maximum level of winterhardiness, winter wheat can still be injured or even killed by cold temperatures if temperatures at the crown level reach single digits. There are varietal differences in winterhardiness. As soil temperatures at the crown level rise to 50 degrees or more, usually in late winter or spring, winter wheat will gradually lose its winterhardiness entirely. Photoperiod also plays a role in this process. When the leaves switch from being prostrate to upright, the plants will have completely dehardened.
Jim Shroyer, Crop Production Specialist Emeritus