Overnight temperatures dropped below freezing in parts of western and northern Kansas on the mornings of May 15-16. This is unusually late for freezing temperatures to occur in any part of the state. In low areas of the fields, temperatures will typically be lower than the officially recorded temperatures, and this will be where freeze damage will be most likely.
Whether actual freeze injury takes place depends on the low temperature reached, how long the temperatures stayed that cold, temperatures gradients in the field, wind speed, canopy density, and other microclimate factors. Soil moisture is another factor that is important in determining freeze injury.
We’ll break down the possibilities for freeze injury by stage of growth.
Boot: In the boot stage, wheat can be injured if temperatures drop down into the mid to upper 20’s for several hours. Injury is more likely if this occurs repeatedly and if it is windy at night.
To detect injury, producers should wait several days then split open some stems and look at the developing head. If the head is green or light greenish in color and seems firm, it is most likely going to be fine. If the head is yellowish and mushy, that’s a sign of freeze injury.
Freeze injury at the boot stage causes a number of symptoms when the heads are enclosed in the sheaths of the flag leaves. Freezing may trap the spikes inside the boots so that they cannot emerge normally. When this happens, the spikes will remain in the boots, split out the sides of the boots, or emerge base-first from the boots.
Sometimes heads emerge normally from the boots after freezing, but remain yellow or even white instead of their usual green color. When this happens, all or part of the heads have been killed. Frequently, only the male parts (anthers) of the flowers die because they are more sensitive to low temperatures than the female parts. Since wheat is self-pollinated, sterility caused by freeze injury results in poor kernel set and low grain yield.
Freeze injury to the lower stem at this stage of growth can also be a significant problem. This kind of damage may take a little longer to detect. Producers will eventually be able to find soft “lesions” on the lower stems. The damaged tillers may lodge. Even if they don’t lodge, the heads will not produce grain.
Awns beginning to appear: If the awns have begun to appear, there can be significant injury to the heads if temperatures reach about 30 degrees or lower for several hours. Tillers may go ahead and finish shooting up the head, but few, if any, of the spikelets may pollinate normally and fill grain. Damaged heads from a freeze at this stage of growth may seem green and firm at first glance, but the floral parts will be yellowish and mushy.
All may not be lost if the heads on the main tillers are damaged, however. If there are enough secondary tillers at an earlier stage of development when the freeze occurs, these tillers may be able to compensate and keep yield losses to a minimum. There have been spring freeze events in the past in which the secondary tillers have compensated well because of good spring growing conditions, resulting in good yields. It is getting late in the season now to expect much from secondary tillers, however. Odds are that the weather will get hot before these tillers can complete grain fill.
Heading. Wheat is particularly vulnerable to damage from freezing weather as the head starts to emerge through the flowering stage. Temperatures of 30 degrees or lower can damage anthers if they are exposed.
If the wheat was in the flowering stage at the time of the freeze in the early morning hours of May 16, you can determine if the anthers are damaged by examining them with a magnifying lens. Healthy anthers will be lime green. If they are damaged by a freeze, they will begin twisting within 2 to 3 days. Shortly afterward, they will begin to turn whitish or brown. The stigma in the florets may or may not also be damaged by a freeze. If the anthers are damaged by freeze, the flowers may fail to develop a kernel.
If you are unsure whether there has been freeze damage to the anthers, wait several days and determine whether kernels are developing normally. A week after flowering, kernels should be well-formed up and down the head under normal conditions.
If flowering has already occurred and kernels were forming at the time of the freeze, there still could be damage. Generally, it takes colder temperatures (about 28 degrees) to cause damage to kernel development than to floral structures.
Healthy, developing kernels shortly after flowering are greenish-white and as they grow, they turn more greenish. But if they are damaged, they will turn grayish-white, shriveled and rough and will not continue to enlarge. Producers might need to check heads several times to determine the amount of damage.
The comments above are general guidelines. Actual damage, if any, will not become apparent until temperatures have warmed back up for several days and growth has resumed.
For more information on freeze damage to wheat, see “Spring Freeze Injury to Kansas Wheat,” K-State Research and Extension publication C646.