Minimum air temperatures in southwest Kansas reached very low levels during March 16-17. Cold temperatures occurred from the very southwest border of Kansas all the way east to St. John and north to Sherman County, encompassing most of the southwest quarter of Kansas. The minimum temperature observed across the state was 14.6 degrees F at the K-State Mesonet station in Hamilton County.
Different stages of wheat development vary in their sensitivity to cold temperatures. Where the developing head is already above ground (jointing or later stages), cold temperatures can damage the developing wheat head. The threshold below which economic damage can occur when wheat is jointed is approximately 24 degrees F. Additionally, temperatures need to be sustained at levels below 24 degrees F for a minimum of two-to-three hours to be potentially damaging to the developing head.
Figure 1 shows the number of hours minimum temperatures were below 24 degrees F across Kansas. The number of hours below 24 degrees F ranged from 0 hours in most of central and eastern Kansas to as much as 9.6 hours near Tribune, in western Kansas. In southwest Kansas, the number of hours below 24 degrees F averaged 5.3 hours. While these observed temperatures are not uncommon for this time of the year, in most of the state the wheat crop is well advanced due to a relatively warm winter. Producers who have jointed wheat might be concerned with possible damage to their crop.
While this is a tangible concern for jointed wheat in southwest Kansas, it is important to consider that air temperatures reported by meteorological monitoring stations are often measured 5 feet aboveground, and do not fully reflect the microclimate to which the wheat canopy is actually exposed. For instance, a lush wheat canopy will tend to reduce the extent of freeze damage as the warmth of the soil will radiate up into the canopy. In addition, moist soil buffers temperature changes better than dry soils and therefore there is often less freeze injury at a given temperature when soils are wet. Crop residue (or lack thereof) will influence how much heat will radiate out of the soil up into the plant canopy. Windy conditions will also increase the chance of injury.
As a result of so many interacting variables, evaluating solely air temperatures may not completely reflect the conditions experienced by the wheat crop. In this situation, soil temperatures can help determining the extent of the cold stress at crown and lower canopy levels.
Figure 2 shows the minimum 2” depth soil temperatures measured March 17. While air temperatures reached critical levels for damage to the developing wheat head (if the head is above ground), soil temperatures at the 2” depth were above 30 degrees F all across western Kansas, and well above 40 degrees F in southwest Kansas. Higher soil temperatures may have helped buffer the cold air temperatures experienced, minimizing possible injury to the wheat crop.
Where the developing head is still below ground and therefore insulated from cold air temperatures (Feekes 3, 4, or 5), little to no substantive damage should be expected from the cold temperatures of March 16-17. For wheat that has already jointed, it is still too early to define what possible yield losses the wheat crop may have experienced, if any.
Another update on minimum temperatures observed across the state will be released in the Agronomy eUpdate Monday, March 21st, to reflect temperatures measured during the weekend.
You can check temperatures through the weekend on the K-State Freeze Monitor page, part of the Kansas Mesonet web site, at: mesonet.ksu.edu/freeze/ A column for temperatures below 24 degrees and a link at the bottom for a map showing the number of hours below 24 degrees have been added to the page.
For more information on freeze damage to wheat, please see accompanying publication, Spring Freeze Injury to Kansas Wheat, K-State Research and Extension publication C646, at: http://www.ksre.ksu.edu/bookstore/pubs/C646.pdf
Chip Redmond, Weather Data Library
Mary Knapp, Weather Data Library
Romulo Lollato, Wheat and Forages Specialist