In general, temperatures have been unusually mild in Kansas so far this fall. Some record high temperatures have even been recorded during Nov. 14-16. As of Nov. 18, however, temperatures are getting much colder and more seasonal. Given how warm it has been until now — and how actively the wheat has been growing in many areas – the question is: Will be wheat in Kansas be ready for a switch to very cold temperatures?
In short, the answer is “probably so.” Where wheat has been growing actively as recently as Nov. 17, that doesn’t mean the wheat will not have developed enough winter hardiness to withstand cold temperatures. The top growth will normally get “burned back” by a hard freeze (Figure 1), but the plants themselves should be adequately hardened to survive the kind of low temperatures predicted for the Nov. 18-20 period.
It helps to know how winter wheat typically survives the winter. 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 (Figure 2). 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.
Winter hardiness 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. Below this threshold, there is an inverse relationship of cold acclimation as affected by crown temperatures; in other words, wheat plants will acclimate twice as fast when crown temperatures are 32°F as compared to 40°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 winter hardiness. As indicated by the mean soil temperature map in Figure 3, soil temperatures at the 2-inch depth were generally above 50 degrees in most of Kansas as of the week of Nov. 11-17. Temperatures at the crown will be influence by the amount of residue as well as the amount of soil moisture. Wet soils with heavy residue will cool more slowly than dry bare soils.
Cold hardiness is not a static state, however. After the cold hardening process begins in the fall, wheat plants can rapidly un-harden 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 winter hardiness 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 winter hardiness.
Even at its maximum level of winter hardiness, winter wheat can still be injured or even killed by cold temperatures if temperatures at the crown level reach single digits or if plants are subjected to long periods when soil temperatures approach the minimum survival temperatures. Thus, not only the actual minimum temperature achieved can affect winter survival, but also the time period spent at those temperatures. There are varietal differences in winter hardiness. 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 winter hardiness entirely. Photoperiod also plays a role in this process. When the leaves switch from being prostrate to upright, the plants will have completely de-hardened.