The extent of possible winter damage to the developing wheat crop due to low temperatures will depend on several variables including:
- Crop development
- Extent and duration of low temperatures
- Soil temperature
- Soil moisture
- Snow cover
Minimum air temperatures and their duration are the leading factors in any possible winter injury. However, it is important to remember that the crown is protected by the soil during this stage, so factors other than air temperature also need to be considered. For instance, crown insulation by the soil (influenced by seed-to-soil contact at sowing and sowing depth), crown root development, above-ground crop development, soil temperature, soil moisture, snow residue, crop residue, and how well the crop acclimated during the fall, will all influence the crop’s response to below-freezing temperatures at this stage.
What level of cold can the wheat crop withstand at this stage?
Wheat needs at least 4-5 leaves and 1-2 tillers prior to winter dormancy for good cold tolerance, but for maximum cold tolerance, 3-5 well developed fall tillers is ideal. In this situation, wheat can withstand air temperatures of -5 to -10 degrees F for a couple hours without significant risk of winterkill (Figure 1) as long as temperatures at the crown level do not reach single digits.
Wheat that has fewer tillers and leaves will be more susceptible to winter kill, which unfortunately is the situation for the majority of the Kansas wheat crop during the 2017-18 season. During the fall of 2017, wheat sowing was delayed for about 60-80% of the Kansas crop due to early October precipitation. Therefore, the crop is behind in development as compared to the historical average. Many fields in north central Kansas had sowing delayed even further as producers had to finish a summer crop harvest prior to sowing wheat. The crop likely did not have enough time to tiller during the fall (Figure 2).
How cold did it get?
January has brought two very cold spells, the first on January 1 (for more information, please click here), and the second during January 13 – 16th. Minimum air temperatures reached very low levels on January 1 across Kansas (temperatures as low as negative 16 degrees F recorded in the north central and northeast portions of the state, Figure 3 upper panel), whereas the coldest temperatures during the second cold spell were recorded in northwest and parts of southwest and north central Kansas (minimum temperatures ranging between negative 7 and 10 degrees F, Figure 3 lower panel).
How long were these cold temperatures sustained?
As mentioned earlier, the risk of freeze damage to wheat is a function of the minimum temperature and duration of time spent potentially damaging temperatures. During the week ending on January 2, 2018, the number of hours below negative 5 degrees F varied according to geographical location within Kansas (Figure 4 upper panel). The majority of the wheat growing region had anywhere between 4 to 24 hours below -5 degrees F, with counties in the north central and northeast portions recording as many as 30 hours below the threshold. For the January 10-16th period, a total of approximately 2 hours below negative 5 degrees F was measured in southwest Kansas, but it was not as widespread as the January 1st event.
As freeze damage potential is a result of many interacting variables, evaluating only air temperatures may not completely reflect the conditions experienced by the wheat crop. Air temperatures might induce leaf damage, which might only be cosmetic. The wheat growing point at this developmental stage is located below ground, in the crown. Thus, damages to the crow are actually what might lead to winterkill. In this situation, soil temperatures can help determine the extent of the cold stress at crown level.
Ideally, temperatures at the crown level should be maintained above 10-15 degrees F, as temperatures below these thresholds can lead to damage to the crown. A timeline of air and 2-inch soil temperatures during December 20, 2017 – January 17, 2018 is shown for four selected Kansas locations in Figure 5. Air temperatures reached critical levels for foliar tissue damage and in the three selected locations during both cold spells (threshold being negative 5 degrees F). Meanwhile, soil temperatures at the 2-inch depth were sustained above 20 degrees F Garden City (southwest Kansas) and Hiawatha (northeast Kansas) the entire time period (Figure 5). However, in Scandia (north central Kansas), 2-inch soil temperatures reached single digits during both cold spells, especially the January 1st event. Therefore, for portions of the state where soil temperatures were sustained above single digits, they may have helped buffer the cold air temperatures and thus minimizing possible injury to the wheat crop. However, for north central Kansas, some winterkill might be expected. The effects of cold temperatures are worsened in this region due to this year’s late-sowing induced by early-October precipitation and double cropping wheat after soybeans, a typical practice in the region.
Soil moisture directly affects the capacity of the soil to buffer temperature changes. A soil with a higher moisture content will require more energy to increase or decrease temperatures, thus, has a greater buffer capacity. A dry soil, on the other hand, will more easily result in temperature changes. The entire wheat growing region in Kansas is now under some level of drought stress, ranging from abnormally dry in north central and parts of northwest Kansas, to extreme drought conditions in portions of southwest Kansas (Figure 6). The dry conditions currently being experienced might enhance any potential cold damage resulting from the cold air and soil temperatures from the past two cold events.
Another factor affecting the potential for winterkill to the wheat crop is the amount of snow cover when low temperatures occurred. Snow can act as a buffer to cold air temperatures. If a minimum of 1-2 inches of snow is present on top of the wheat canopy, but preferably 2 or more, temperatures at the soil level should be sustained close to 32 degrees F and mitigate potential winterkill. However, if less snow is present or if windy conditions removed the snow from the wheat fields into field corners or ditches, the buffering effect might not occur.
Figure 7 shows the total snowfall accumulated prior to January 1st (upper panel) and prior to or during January 10-16th (lower panel). Anywhere from 0 to 5 inches of snow accumulated in Kansas prior to the first cold spell, with the largest amounts measured in parts of north central and northwest Kansas (Figure 7 upper panel). During the second cold spell, the majority of the state received some amount of measurable snowfall, ranging between 0 and 7.7 inches (Figure 7 lower panel). The largest measured amounts were recorded in northeast Kansas, but the wheat growing region of the state received anywhere between 0 and 3.6 inches. Wheat fields that were covered by at least 1-2 inches of snow during the cold spell are likely safer than wheat fields that did not receive any snowfall.
Air temperatures measured during January 1st were cold enough to harm the wheat crop in many parts of the state, especially north central Kansas where temperatures where sustained below negative 10 degrees F for up to 10 hours. Meanwhile, the cold spell that happened between January 10 and 16th did not produce as cold temperatures.
The effects of the cold temperatures could be magnified by dry soil conditions and poor fall development due to late sowing across the state; thus, the potential for winterkill exists, especially in north central Kansas where very cold temperatures were sustained for a long period of time. In this region, 2-inch soil temperatures reached single digits and might induce winterkill. However, for the remaining regions of the state, soil temperatures appear to have been maintained above single digits at 2-inch depth, which will help the crop withstand the winter.
Snow cover as much as 5 inches in the first cold spell, and up to 7.7 inches in the second cold spell, may also have helped winter wheat survival. Fields where there was no snow cover were more exposed and may have sustained greater levels of damage.
It is difficult to truly assess the extent of the damage at this point. Thus, producers should not take any immediate action. While foliage damage will be apparent a few days after the cold event, the first apparent sign of freeze injury being leaf dieback and senescence, symptoms of winterkill will only be apparent at spring greenup. This is when the crop starts to take up water and nutrients for spring growth. Damaged leaves will appear burned back and dead, but that is not a problem as long as newly emerging leaves in the spring are green. Provided that the crown is not damaged, the wheat will recover from this foliar damage in the spring with possibly little yield loss. If damage to the crown occurred, the crop will not greenup in the spring or will greenup for a short period of time using existing resources, and perish shortly after. In any case, we will only be able to assess the true extent of the damage at spring greenup.
Romulo Lollato, Wheat and Forages Specialist
Mary Knapp, Weather Data Library
Christopher “Chip” Redmond, Kansas Mesonet Manager