Growers across the state are concerned about abnormally low temperatures that occurred several times over the past week. Figure 1 and 2 and Table 1 have the low temperatures for the early morning hours on May 21 and May 22 with many record low temperatures. Winter wheat is close to the normal growth stage at the present time but has been subjected to a number of stressors this growing season. Dry planting conditions and continued drought have reduced crop tillering and vegetative growth, and scattered hail has occurred in a handful of wheat-growing regions.

According to the USDA NASS May 23 Nebraska Crop Progress report, 27% of the wheat was headed, which is also the average. We recommend that growers wait four to five days and then scout intensively to assess damage and reduction in yields.

The current situation is similar to the major injury that occurred in western Nebraska May 9-10, 1981 and in late May 1992. In 1981, Red Willow County (McCook) suffered the highest losses for the state. Winter wheat had just headed when temperatures dropped to 28°F and slowly warmed to 37°F over four hours. Decatur County, Kansas, adjacent to Red Willow County (McCook), suffered the greatest losses in Kansas. In late May 1992, winter wheat and other crops also suffered major losses due to a freeze.

Figure 3 and Table 2 show how the effect of freezing temperature at various growth stages of wheat.

Injury does not always occur at the low air temperatures listed. Instead, injury and reduction of yield is influenced by both duration of the low temperature and growth stage of the wheat. If the head is not fully exposed, it benefits from the microclimate in the wheat canopy, and temperatures there may not be as low as the air temperatures reported. Once the wheat is headed, it is subject to the air temperature and injury described in Figure 3, and Table 2 is more likely to be observed.

With 73% of the wheat not headed yet, and while we can determine air temperature lows, it’s difficult to know the actual air temperatures in the wheat canopies of individual fields.

Since it takes several days of warm weather for freeze injury to become apparent, wait five days or so after a freeze and then assess plant damage.

Microclimate Factors Affecting Wheat Injury

While the air temperature may drop for several hours, the actual microclimate of the crop may be several degrees warmer and create a “cushion” of protection to help moderate temperature swings. Factors besides air temperature affecting the potential for damage include:

1. Crop condition — The current growth stage (not yet heading) and a fuller stand and dense canopy helps create a warmer microclimate.
2. Soil moisture — Generally, if the topsoil is moist, it helps mitigate against temperature changes. (Several years ago, when there was a hard freeze in the Republican Valley in June, the cultivated corn with dry topsoil suffered significantly more damage than the non-cultivated corn.)
3. Duration of the chill — Research indicates that temperatures would have to drop for more than two hours to damage wheat at its current growth stage.

The many factors influencing freeze injury to wheat — plant growth stage, plant moisture content and duration of exposure — often make it difficult to predict the extent of injury right after the freeze events. This is complicated further by differences in elevation and topography among wheat fields and between the fields and official weather stations across the state and even across a county. It is not unusual, for instance, for wheat growers to report markedly lower temperatures than are recorded at the nearest official weather station.

Areas that may have been particularly susceptible to the temperatures are low field areas, thin stands and dry soil.

Assessing Damage

Figure 4. After a hard freeze, wait for three or four warm days to assess wheat damage. Split the stem lengthwise with a sharp knife to view the head. Shown are a healthy growing point with a crisp, whitish-green appearance (left) and one that has freeze injury. A growing point that has been damaged loses its turgidity and greenish color within several days after a freeze and may turn white or brown and appear water-soaked. A hand lens may be needed to help detect subtle freeze damage symptoms.
Figure 5A. Discoloring and roughening of the lower stem are symptoms of spring freeze damage.	Figure 5B. The stem can split with severe freeze damage.

To check for head damage, wait for three or four warm days and then go out and collect stems from several places across the field. Split plant stems lengthwise with a sharp knife. A normal, uninjured head is bright yellow-green and turgid (firm), whereas freeze injury causes the head to become white or brown and water-soaked in appearance (Figure 4). This injury can occur even in plants that appear otherwise normal because the head is more sensitive to cold than other plant parts.

Stem growth stops immediately when the head is injured, but growth from later tillers may obscure damage. Partial injury at this stage may cause a mixture of normal tillers and late tillers and result in uneven maturity and some decrease in grain yield. It’s therefore also important to look at samples from primary and secondary tillers.

Freezing temperatures at this stage of development also can cause leaf injury, which is typically expressed as twisted leaves and a change in leaf color from dark green to light green or yellow. Leaf tips may become necrotic or "burned" by freezing temperatures a week or more after the freezing temperatures. Leaf injury alone does not usually result in significant yield losses, as new leaf and tiller growth resumes with warmer temperatures.

Injury to the lower stems in the form of discoloration, roughness, lesions, splitting, collapse of internodes and enlargement of nodes frequently occurs at the jointing stage and the following stages after freezing (Figures 4, 5A and 5B). Injured plants may break over at the affected areas of the lower stem so that one or two internodes are parallel to the soil surface.

Stem injury does not appear to seriously interfere with the ability of wheat plants to take up nutrients from the soil and translocate them to the developing grain. Lodging, or falling over, of plants is the most serious problem following stem injury. Wind or hard rain near maturity will easily lodge the plants, decreasing grain yield and slowing harvest.

Additional Resources

• Frost Identification Guide for Cereals
• Frost Identification Guide for Canola and Pulses

If the wheat is flowering or just past flowering when frost hits, wheat damage is not easily identified at this stage. The exposed anthers are most susceptible to the freeze. In general, it is only possible to evaluate seed fill by dissecting heads 10-14 days after the frost. If there are many blank glumes or kernels are not developing, damage is indicated. 

Sometimes we see stems that have ruptured during a freeze and the head turns white on the next hot dry day.

For more information on how to assess freeze injury in winter wheat, see Freeze Injury to Nebraska Wheat  (EC132) and CropWatch Wheat, or contact your local Crops and Water Extension educator or one of the authors listed here.