Since January 24, the majority of Kansas has experienced warmer-than-average air temperatures for this time of the year. During the week of January 24-30, the eastern half of the state had temperatures departing from the normal by as much as 11 degrees F (Figure 1). The week of February 1-7 was cooler than the previous week, but still warmer than normal by as much as 4.7 degrees F (Figure 1). The forecast indicates that temperatures will also be above normal for the period of February 10-12. The effects of these warmer-than-average temperatures to the wheat crop are concerning producers and crop consultants.
How warm have soil temperatures been?
The physiology of wheat vernalization and cold hardiness suggests that soil temperatures at the crown level rather than air temperatures should be the primary driver leading to increased or decreased cold hardiness during the winter. Soil temperatures will be influenced by the amount of residue on the soil surface, as well as soil moisture. Soils that with a thick residue layer on the surface often have lower temperatures than bare soils, as the residue blocks direct exposure to sunlight and reduces soil evaporation, generally conserving more moisture. Moist soils require more energy to cause any change in temperature than dry soils; thus, any increase or decrease in temperature occurs more slowly than in dry soils. As a result, moist soils with heavy residue will heat more slowly than dry bare soils.
Soil temperatures at the 2-inch depth ranged from less than 32°F in the north central portion of the state, to as high as 41°F in the southeast quarter of the state (Figure 2). The warmest soil temperatures were in the southern three tiers of counties, east from Comanche/Kiowa/Edwards counties.
Although average temperatures did not seem to come close to the 48°F threshold for winter wheat to start losing cold hardiness, daily soil temperatures actually came very close to this threshold, and sometimes even surpassed it in several locations (Figure 3). In particular, locations such as Parsons, Hutchinson, Stevens County, Harper County, and Hill City, were consistently near the 48°F threshold.
Understanding the vernalization process in winter wheat
Winter hardiness or cold tolerance is a physiological process triggered by gradually cooling temperatures in the fall. During the process of cold acclimation, there is a reduction in moisture content in the cells of the crown, which slows growth processes and the accumulation of soluble carbohydrates, all of which protect the cell membranes from freeze damage.
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.
Temperature fluctuations during the winter and its effects on wheat cold hardiness
Cold hardiness is not a static state. 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 winter hardiness from mid-December to mid-January, unless there are high temperatures during that period.
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.
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.
Possible consequences to the wheat crop
The effect of the high soil temperatures in the late-January to early-February period on the wheat crop will depend on several factors, but will particularly depend on temperatures during the remainder of February and early March.
Where temperatures were consistently close to 50°F and with little fluctuation during the recent period, such as in many areas throughout the southeast quarter of the state, the high temperatures should cause a gradual loss of cold hardiness. The warmer the crown temperature got during this recent period, the more quickly the plants will start losing their maximum level of cold hardiness.
The forecast indicates that temperatures should decrease to low forties across the state in the week to follow, which would be beneficial to the crop and possibly help hold crop development back. Winter wheat can re-harden to some extent during the winter even if it loses its full level of winter hardiness, but will not regain its maximum level of winter hardiness. Thus, in the regions where soil temperatures have been warmest, the crop may be less tolerant to low temperatures for the remainder of this winter, becoming more susceptible to freeze injury if temperatures decrease to single digits in the near future. While some producers expressed the concern about a possible early spring greenup, it is very unlikely that even varieties characterized by short vernalization requirements will go into spring greenup at this time. The vegetative condition map (Figure 4) shows only a slight increase in photosynthetic activity compared to the 27-year average for the period of Jan. 31 through Feb. 6.
Romulo Lollato, Wheat and Forages Specialist
Mary Knapp, Weather Data Library