Possible consequences of below-freezing temperatures to the Kansas wheat crop
The extent of possible freeze damage to the developing wheat crop will depend on several variables. Minimum air temperature is a leading factor in any possible winter injury, as is the duration of the minimum temperature. It is important to remember that the crown is protected by the soil during this stage, so factors other than air temperature need to be considered. Several factors will influence the crop’s response to below-freezing temperatures at this stage:
- Crown insulation by the soil (influenced by seed-to-soil contact at sowing and depth of sowing)
- Crown root development
- Soil temperature and moisture status
- Amount of residue present (snow and/or crop)
- Degree of crop acclimation during the fall
With the majority of the wheat fields planted relatively late this season, there is concern for a lack of crown development.
How long were cold temperatures sustained?
The risk of freeze damage to wheat is a function of the minimum temperature and duration of time spent at potentially damaging temperatures. From December 6 – 8th, minimum temperatures below 15 degrees F were recorded throughout central and western Kansas. The number of hours below 15 degrees F varied according to geographical location within the state. Counties in southwest and north central Kansas were exposed to as many as 14 hours below 15 degrees F (Figure 1).
Figure 1. Total hours below 15 degrees F from December 6 – 8, 2017.
Freeze damage potential is a result of many interacting variables, however evaluating only air temperatures may not completely reflect the conditions experienced by the wheat crop. In this situation, soil temperatures can help determine the extent of the cold stress at crown level.
While air temperatures reached low levels and were sustained for several hours, soil temperatures at the 2-inch depth (slightly below where the wheat crown would normally be developed) were, on average, above 33 degrees F in northwest Kansas, and in most cases were between 41- 49 degrees F in other regions of the state (Figure 2). During the fall, most of the wheat winterkill occurs when temperatures reach single digits at the crown level. Higher soil temperatures may have helped buffer the cold air temperatures and minimized possible injury to the wheat crop.
Figure 2. Average weekly 2-inch soil temperatures during November 28 – December 5, 2017.
Potential effects to the wheat crop
Southwest and north central Kansas recorded temperatures below 15 degrees F for the longest period of time in the state. In addition to the low temperatures, the majority of the wheat region in Kansas is under moderate drought stress, and has gone without significant precipitation for weeks. While the majority of the Kansas wheat crop should be okay after this cold snap, the lack of soil moisture decreases the capacity of the soil to buffer temperature changes. Therefore, while weekly average soil temperatures were above 30 degrees F, minimum temperatures at any given point in time could have been much colder. A dry soil will cool down faster than a moist soil, thus increasing the chances of low temperatures at the crown level. The circumstances for concern with the crop’s ability to make it through these recent cold days include:
- 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 occur to 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 (death) (see Figure 3), 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 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.
Figure 3. Leaf tip burn from freeze damage. By itself, this is cosmetic damage only. Photo by Romulo Lollato, K-State Research and Extension.
Romulo Lollato, State Wheat Extension Specialist
Mary Knapp, Weather Data Library