The extent of possible winter damage to the developing wheat crop during the fall will depend on several variables. Minimum air temperatures achieved are the leading factor in any possible winter injury, as is the duration of the minimum temperatures. However, it is important to remember that the crown is protected by the soil during this stage, so factors other than air temperature also need to be considered. For instance, crown insulation by the soil (influenced by seed-to-soil contact at sowing and sowing depth), crown root development, soil temperature, soil moisture, snow residue, crop residue, and how well the crop acclimated during the fall, will all influence the crop’s response to below freezing temperatures at this stage.
How cold did it get?
Minimum air temperatures reached very low levels on December 8th and 9th across Kansas, especially in the northwest portion of the state (Figure 1). Most of the state was exposed to minimum temperatures below 20°F on both days. While minimum temperatures the eastern half of the state never reached single digits, the northwest portion of the state was exposed to negative temperatures (Figure 1).
How long were these cold temperatures sustained?
The risk of freeze damage to wheat is a function of the minimum temperature and duration of time spent potentially damaging temperatures. During December 8th and 9th, the number of hours below 12 degrees F varied according to geographical location within Kansas. Counties in the northwestern portion, neighboring Colorado and Nebraska, were exposed to as many as 17 hours below 12 degrees F during December 8th and 18 hours during December 9th (Figure 2). Minimum temperatures below 12 degrees F were registered throughout the half western portion of the state during December 8th, and across the entire northern two-thirds of the state on December 9th.
As freeze damage potential is a result of many interacting variables, evaluating only air temperatures may not completely reflect the conditions experienced by the wheat crop. In this situation, soil temperatures can help determining the extent of the cold stress at crown level.
While air temperatures reached critical levels, soil temperatures at 2” and 4” depth were above 27 degrees F in northwest Kansas, and in most cases between 30-35 degrees F in other regions of the state. During the fall, most of the wheat winterkill occurs when temperatures reach single digits at the crown level. Higher soil temperatures may have helped buffering the cold air temperatures experienced, minimizing possible injury to the wheat crop.
Potential effects to the wheat crop
Northwest Kansas recorded the lowest temperatures for a longer period of time compared to other regions of Kansas during this two-day period. In addition to the low temperatures, northwest Kansas is also under moderate drought, without significant precipitation for weeks. The lack of soil moisture decreases the capacity of the soil to buffer temperature changes. As a result, a dry soil will cool down faster than a moist soil will, increasing the chances of lower temperatures at the crown level. Thus, the circumstances in which I would be concerned with the crop’s ability to make it through these recent cold days include:
- Fields without substantial snow cover (less than ~2 inches)
- Extremely dry soils with poor root development
- Late-sown crops with delayed development (less than 4-5 leaves and 1-2 tillers)
- Shallowly-sown fields where the crown is closer to the soil surface
- Heavy-residue situations which may have precluded good seed-soil contact
Other than the above circumstances, most of the damage at this stage should only involve leaf tissue, which might give the crop a rough look for a few weeks. The first apparent sign of freeze injury will be leaf dieback and senescence (Figure 4), which should occur across most of the state regardless of damage to the actual growing point. Existing leaves will almost always turn bluish-black after a hard freeze, and give off a silage odor. Those leaves are burned back and dead, but that in itself is not a problem as long as newly emerging leaves are green. Provided that the growing point is not damaged, the wheat will recover from this damage in the spring with possibly little yield loss.
Romulo Lollato, Wheat and Forages Specialist
Mary Knapp, Weather Data Library
Christopher “Chip” Redmond, Kansas Mesonet Manager