Many areas of Kansas experienced freezing temperatures on April 3, 2020. Freeze events are common this time of year in this part of the country. In fact, April 3 is still well before the historical normal frost free date (i.e. date of last spring freeze). History has taught us, however, that freezing temperatures in April can potentially damage the yield potential of the Kansas wheat crop.
The main factors determining the potential for freeze damage are: 1) minimum temperatures achieved and duration of the cold conditions; and 2) the growth stage of the crop. The overlap or interaction of these factors strongly influences the risk of freeze injury. The greatest risk of damage occurs in regions where the crop is most vulnerable and temperatures were below freezing for a long time.
How cold did it get?
The weather event on April 3 had minimum air temperatures ranging from 16 to 36 oF (Figure 1). The number of hours that temperatures were below 24 oF depended on the region. The south central and southeast portions of Kansas did not record temperatures lower than 24 degrees. Central, southwest, and north central Kansas recorded as much as 11 hours below 24 degrees, and northwest Kansas recorded up to 21 hours (Figure 1).
Figure 1. Minimum air temperatures recorded during the recent freeze event across Kansas (upper panel) and number of hours below 24 degrees Fahrenheit (lower panel). Map generated by the Kansas Mesonet.
What is the growth stage of the wheat crop?
The most advanced wheat fields in Kansas are usually located in south central and southeast Kansas. In that region, our wheat development model (based on temperature accumulated since January 1st) indicates that the majority of the fields are about Feekes 6 (first node), but some more advanced early-planted fields might be getting to the second node (Fig. 2). The majority of the crop in the central and southwest portions of the state should be at about Feekes 4-5 (strongly erect leaves through first hollow stem – this would be valid for fields that emerged last fall), and the fields in northwest and north central Kansas are still going through the late stages of tillering.
The more advanced the wheat crop, the more sensitive it is to colder temperatures. Thus, from a development standpoint, the crop in south central KS would naturally be the most exposed to any potential damage from the freeze. While a crop that is tillering can handle temperatures as low as 12 degrees F without necessarily suffering injury, a crop that is jointed can suffer injury if temperatures of about 24 degrees F occur consecutively for up to two hours. This sensitivity increases so that a crop that is at the boot stage can handle temperatures of about 28 degrees for the same two hours.
Figure 2. Estimated wheat growth stage across Kansas based on accumulated temperatures. Map generated by the Kansas Mesonet.
Will there be freeze damage from weather on April 3?
Based on the three pieces of information above (minimum temperatures achieved, number of hours below critical thresholds, and stage of crop development), it is likely that the majority of the Kansas wheat crop will not be damaged by this cold spell. In general, it appears that the coldest temperatures occurred in regions where the crop was not at growth stages vulnerable to freeze injury. Some exceptions to this could apply to the most advanced fields in each of the different regions. For example, fields that were planted early, to an early-maturing variety, could possibly be more advanced than what our model suggests in Figure 2. Consequently, the more advanced fields in south central Kansas might already be at the second node, or perhaps jointing in central/north central Kansas. These fields would be more exposed to potential freeze damage and some tillers could be lost.
Other important factors determining freeze damage
Density of the stand and condition of the plants. If the stand is thick, that will tend to reduce the extent of freeze damage as the warmth of the soil will radiate up into the canopy. On the other hand, well-fertilized succulent wheat has often sustained more freeze injury than wheat that is not as well fertilized. Thin stands are at higher risk of injury because the air can penetrate the stand more easily. If the plants were wet before the freeze, this can result in a coat of ice on the plants that may protect the growing point to some extent. If temperatures get too low, however, the cold will go through the ice.
Residue. No-till fields often sustain more freeze damage because the residue acts as a blanket and doesn’t allow the heat from the soil to radiate up into the plant canopy.
Soil moisture. There is often less freeze injury at a given temperature when soils are wet than when dry. Wetter soils tend to buffer temperature changes better than dry soils. These warm, moist soils also often radiate a little more warmth than dry soils. On the other hand, drought-stressed plants tend to be more hardened against cold injury and their lower leaf water content tends to decrease the severity of the freeze injury.
Wind speed. Windy conditions during the nighttime hours when temperatures reach their lows will reduce the amount of warmth radiating from the soil and increase the chance of injury.
Temperature gradients within the field (position on the landscape). Low spots in the field are almost always the first to have freeze injury. The coldest air tends to settle in the low areas, especially under calm wind conditions.
Wheat variety. Although the sensitivity to freezing temperatures at a given growth stage is very similar across all varieties, varieties can differ in their release from winter dormancy by as much as three weeks. Wheat varieties that break dormancy (late-release varieties) may escape a freeze injury because they are less likely to be at vulnerable growth stages.
There are many possible scenarios after a freeze, and producers should not take any immediate decision following a freeze event. Several days of warm temperatures are needed to properly assess freeze damage to the wheat crop. Where wheat was at the jointing stage, producers should watch their fields closely over the next 7 to 10 days from the freeze event for the following:
Figure 3. Following an early freeze, crops at jointing might still develop healthy heads (left panel), but depending on minimum temperatures and duration of the freeze event, the developing head might be killed even if still within the stem killed (right panel). The dead head is whitish and flaccid while the healthy head is light green and turgid. Photos by Romulo Lollato, wheat and forage specialist, K-State Research and Extension.
The best thing producers can do for the first few days is simply walk the fields to observe lodging, crimped stems, and damaged leaves. Producers should not take any immediate actions as a result of the freeze, such as destroying the field for re-cropping. It will take several days of warm weather to accurately evaluate the extent of damage. After several days, producers should split open some stems and check the developing head.
Where stems and/or growing points were killed by the freeze, new tiller growth (coming from the crown area) will occur (Fig. 4). In many cases, new tiller growth can be observed even when the stems do not show any symptoms of freeze damage for some time. In those cases, the first sign that the tillers are dead is the sudden growth of new tillers at the base of the plant.
If secondary tillers may begin growing normally and fill out the stand, the wheat may look ragged because the main tillers are absent. Producers should scout for bird cherry oat aphids and other potential insect or disease problems on these late-developing tillers. Enough tillers may survive to produce good yields if spring growing conditions are favorable. If both the main and secondary tillers are injured, the field may eventually have large areas that have a yellowish cast and reduced yield potential.
Figure 4. Left: A stem that was split open by having ice form within the stem. This stem has died and a new tiller has begun to grow at the base. Right: Some of the tillers on this plant had freeze damage to the lower stems. These stems are dying, but the symptoms may not be immediately evident. The growth of new tillers from the base of the plant is a sure sign that the main tillers are dead or dying. Note the brown lesion on the stem with the two new tillers. Photos by Jim Shroyer, professor emeritus, K-State Research and Extension.
More information on freeze damage to wheat is available in Spring Freeze Injury to Kansas Wheat, K-State Research and Extension publication C646, available at: http://www.ksre.ksu.edu/bookstore/pubs/C646.pdf
Romulo Lollato, Wheat and Forages Specialist
Erick DeWolf, Wheat Disease Specialist
Mary Knapp, Weather Data Library
Chip Redmond, Weather Data Library