The effects of a freeze event to the wheat crop will depend on how the event matches up with critical sensitive stages of crop development. This is especially true for early maturing varieties which were already headed as of last week. Here are the possibilities for freeze injury by the most common stages of growth in the areas of the latest freeze:
Jointing to pre-boot: Jointing wheat can usually tolerate temperatures in the mid to upper 20s with no significant injury. The lowest official readings were all in that range or above on April 23. But, if temperatures in some low-lying areas fell into the low 20s or even lower for several hours, the lower stems, leaves, or developing head may have sustained injury. If the tillers were in this stage or earlier at the time of the freeze and the tillers are green and growing actively now, then the heads should be fine. If the head had been killed, the tiller would not be green and actively growing. If the leaves coming out of the whorl are chlorotic, then the head on that tiller is dead. Few fields in southwest Kansas are still at this stage, most fields have flag leaf emerging or are at boot stage.
Boot: In this stage, wheat can be injured if temperatures drop down into the mid to upper 20s for several hours. Injury is more likely if this occurs repeatedly and if it is windy at night. Temperatures at these levels were measured in parts of southwest Kansas. 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.
Flowering: It’s possible a few fields may be in the flowering stage where temperatures got below freezing on April 23, particularly early-maturing varieties. 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 the wheat was in the flowering stage at the time of the freeze, you can determine if the anthers are damaged by examining them with a magnifying lens. Healthy anthers will first be lime green, then yellow. 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.
Fortunately, wheat doesn’t flower all at the same time on the head. Flowering proceeds from florets near the center of wheat spikes to florets at the top and bottom of the spikes over a 3- to 5-day period. This small difference in flowering stage when freezing occurs can produce some odd-looking heads. The center or one or both ends of the spikes might be void of grain because those florets were at a sensitive stage when they were frozen. Grain might develop in other parts of the spikes, however, because flowering had not started or was already completed in those florets when the freeze occurred.
— Ryan Flaming is a Kansas State Research and Extension agent for Harvey County. Agriculture is his specialty.