Impact of March 19 Temperatures on Wheat

Carrie Knott

Across Kentucky almost all of the wheat crop has jointed (Feekes 6) and much of it has developed at least two nodes (Feekes 7). When temperatures dropped into the teens and low 20’s (°F) overnight March 19 (Table 1), we once again find ourselves asking the question: Will the low temperatures a couple of nights ago damage this year’s wheat crop?

For wheat fields that are Feekes 5 or less advanced, these temperatures should not harm the wheat. The growing point was still below the soil surface and well protected by the soil temperatures.

For wheat fields that were at Feekes 6 or later, damage may be possible. The national rule of thumb is that wheat at this growth stage is damaged when temperatures are 24°F or less for 2 or more hours. Although there are certainly more factors that contribute to severity of freeze damage than simply duration of a threshold temperature (many of which we are investigating), this is still the most widely accepted condition to ‘trigger’ a need to scout fields for damage.

When scouting for freeze damage in wheat, it is important to remember that a minimum of four to five days of good growing conditions (high temperatures exceeding 40°F) are needed before damage becomes visible. In reality, waiting a full week to ten days generally makes it easiest to see freeze damage. Therefore, Tuesday March 26 would be the ideal time to begin scouting fields for freeze damage.

Even if freeze damage is found, this does not immediately indicate that final grain yield will be impacted. Wheat has a tremendous ability to redistribute its resources to living tillers and therefore compensate for primary stems and tillers that may be lost in the freeze. This can result in little to no yield impact. If damage is found, refer to Figure 1 and AGR-253: Identifying Damage and Estimating Yield Reductions following a Spring Freeze in Winter Wheat to help determine potential yield reductions based upon estimated damage observed in the field.

For a visual guide to identify freeze damage refer to AGR-253: Identifying Damage and Estimating Yield Reductions following a Spring Freeze in Winter Wheat.

There are also videos demonstrating how to assess freeze damage at different growth stages: jointing (https://www.youtube.com/watch?app=desktop&v=oaPiOU-s-Ro), flowering (https://youtu.be/u0DUgEa23bE) and during grain fill (https://youtu.be/OhcqjeiIE8s).

Table 1: Minimum air temperatures, relative humidity at minimum air temperature, average relative humidity when air temperature 24°F or less, and duration that air temperatures were 24°F or less and 32°F or less for all available KY Mesonet sites overnight March 18 to early morning hours of March 19. Data obtained from KY Mesonet http://www.kymesonet.org/.