Minimum air temperatures across Kansas reached very low levels during April 3-4. Cold temperatures occurred from the very southwest border of Kansas all the way east to Columbus and north to Cheyenne County. The minimum temperature observed across the state was 11 degrees F reported at both the K-State Mesonet station in Scandia and the station in Meade County.
Different stages of wheat development vary in their sensitivity to cold temperatures. This year, wheat development is delayed relative to the past two years in Kansas. For the southern portion of the state, the wheat has passed first hollow stem and therefore the developing head is above ground. 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 2-3 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 extreme south central and southeast Kansas to as much as 10.3 hours near St. Francis, in northwest Kansas. In southwest Kansas, the number of hours below 24 degrees F averaged 5.8 hours. While it is late in the year for these observed temperatures, in most of the state the wheat crop is less advanced than normal due to a relatively cool winter. Producers who have jointed wheat might be concerned with possible damage to their crop.
Figure 1. Number degrees F hours below 24 degrees F experienced during April 3 – 4, 2018.
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 cold stress at the crown and lower canopy levels.
Figure 2 shows the 2-inch depth soil temperatures measured on April 4 at 8:36 a.m. While air temperatures reached critical levels for damage to the developing wheat head (if the head is above ground), soil temperatures at the 2-inch depth were above 30 degrees F 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 early 5), little to no substantive damage should be expected from the cold temperatures on April 3-4. 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.
Figure 2. Current 2-inch soil temperature reported as degrees F at 8:36 (CDT) on April 4, 2018.
You can monitor temperatures throughout the weekend on the K-State Freeze Monitor page, part of the Kansas Mesonet web site, at: http://mesonet.k-state.edu/weather/freeze/. There is a column for temperatures below 24 degrees and a link at the bottom for a map showing the number of hours below 24 degrees. Soil temperatures can be found at http://mesonet.k-state.edu/agriculture/soiltemp/.
For more information on freeze damage to wheat, please see the accompanying article in this special mid-week issue of the eUpdate or the 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