The weekend of April 14-16 brought, once again, cold temperatures that have potential to cause freeze injury to the 2018 wheat crop. Factors that influence the potential for freeze injury to wheat at any point in time include primarily:
- Growth stage of the crop
- Air temperatures
- Duration of cold temperatures
- Soil temperatures
- Snow cover
Other factors, such as position on the landscape (low lying areas are at higher risk) and presence of residue covering the soil surface, might also impact the extent of freeze damage within a field. The challenge is to integrate all these factors into a reasonable estimate of freeze injury.
Based on simple wheat development models and observations from K-State Extension personnel, the wheat growth stage around Kansas ranges from tillering to Feekes 5 in the northwest part of the state, to flag leaf emergence in the southeast with a few reports of early-sown fields approaching boot in the southeast region (Figure 1). Most of the crop in south central Kansas is at the first or second node, and the crop is less developed as we move north and west in the state. For fields that have not jointed yet, the crop generally withstands temperatures lower than 20 degrees F fairly well, especially if the growing point is still below ground. This is the condition for most of northwest and northern Kansas. If the growing point is already above ground (first joint visible), wheat can sustain temperatures down to about 24 degrees F for a few hours. Temperatures below 24 degrees F for extended periods of time increase the risk of crop injury. Information from the K-State Mesonet indicates that air temperatures dipped below this 24-degree F threshold for at least a few hours in most areas of the state. Many areas of the state experienced more than five hours with air temperatures below 24 degrees F, which could cause damage to fields at the first node of development or more advanced stages. More advanced fields, such as second node to flag leaf emergence (many fields in southeast Kansas), are more sensitive to freeze injury and temperatures near 25 degree F can cause damage. Temperatures below this threshold were measured in southeast Kansas where the crop is most advanced.
Figure 1. Estimated wheat growth stage as of April 16, 2018, for the state of Kansas. Growth stage is estimated for each county based on temperatures accumulated in the season and adjusted by observations of crop stage by K-State personnel. Local growth stage may vary with planting date and variety. Map created by Erick DeWolf, K-State Research and Extension.
While soil temperatures can help buffer freezing air temperatures if the growing point is below ground or near the soil surface. The buffering capacity of warm soils decreases as the crop develops and the growing point moves further from the soil surface. Thus, we can expect positive effect of the soil temperatures in northern and northwest Kansas where soil temperatures were sustained above 34 degrees during the entire weekend and the crop is still at tillering through Feekes 5 stages of development. Soil temperatures can be viewed via the Kansas Mesonet here: http://mesonet.ksu.edu/agriculture/soiltemp. However, the more advanced crop in south central and southeast Kansas likely did not benefit as much from the buffering influence of warm soil temperatures. There was also minimal snow cover across the state to help insulate plants against the cold air temperatures.
Based on these factors, we estimate that the north and northwest portions of Kansas are at a low risk of freeze injury from the cold temperatures on April 14-16 (Figure 2). The risk of freeze injury increases as we move down a northwest – southeast transect. The greatest risk of freeze damage appears to be in the south central and southeast portions of the state where the crop is most advanced and temperatures were below the threshold for freeze damage.
Figure 2. Estimated risk of freeze damage due to a combination of wheat growth stage sensitivity, lowest temperatures during April 14-16, 2018, number of hours below 24 degrees F during the same period, cumulative snowfall during the period, and soil temperatures at the 2-inch depth. Map created by Erick DeWolf, K-State Research and Extension.
Symptoms of freeze injury on foliage should occur over the next few days across the entire state. In most cases, however, this injury should not result in any long-term damage to the crop, especially if there is available moisture to help the crop recover the lost foliage. Freeze injury symptoms to the developing wheat head, such as a mushy, discolored/brown head, take slightly longer to be develop (10-14 days). Thus, growers with fields at advanced growth stages should check for potential injury to the developing head within this timeframe.
For detailed information on evaluating wheat for freeze damage, see the eUpdate article from Issue 683 on April 4, 2018, “Diagnosing freeze damage to wheat”.