University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

April 6, 2007

Assessing Freeze Injury to Winter Wheat

Winter wheat is up to two weeks ahead of normal in many areas of Nebraska. At the University of Nebraska–Lincoln West Central Research and Extension Center Dryland Farm south of North Platte, this was the warmest March on record since recording weather data began in 1907. The average temperature for March from 1907 to 2007 is 36.2°F and this year the March average temperature was 45.8°F. The farm recorded no snowfall in March but had the fifth highest precipitation for the 1907-2007 period. These weather conditions are why crop development is ahead of normal.

Like wheat in the West Central District, wheat in the Panhandle is further along in development than normal, but little if any wheat in the Panhandle has jointed. This should minimize the injury caused by extended cold temperatures. If the unusually warm temperatures had continued another week, Panhandle wheat would have been more susceptible to injury than it is.

Figure 1. Temperatures that cause freeze injury to winter wheat at different growth stages. Winter wheat rapidly loses hardiness during spring growth and is easily injured by late frezes. (Graph, which links to a larger version, is from EC132, Freeze Injury to Nebraska Wheat)

Recent air temperatures and those forecasted for the near future are cold enough to cause freeze injury to winter wheat in this advanced growth stage (see Figure 1 and Table 1). Chances are that much of the winter wheat will not be seriously damaged because of the microclimate experienced by the crop. Moist soil cools and warms six times slower than dry soil and with the precipitation we received in many areas of the state, this reduces the temperature lows experienced by the wheat plant. Also, a good stand and dense canopy reduce the lowering of temperatures the plant is exposed to.

Table 1. Temperatures that cause injury to wheat at spring growth stages and symptoms and yield effect of spring freeze injury.
Growth Stage

Approximate
Injurious
Temperature
(Two Hours)

Primary Symptoms
Yield Effect
Tillering 12 F Leaf chlorosis; burning of leaf tips; silage odor; blue cast to fields Slight to moderate
Jointing 24 F Death of growing point; leaf yellowing or burning; lesions, splitting or bending of lower stem; odor Moderate to severe
Boot 28 F Floret sterility; head trapped in boot; damage to lower stem; leaf discoloration; odor Moderate to severe
Heading 30 F Floret sterility; white awns or white heads; damage to lower stem; leaf discoloration Severe
Flowering 28 F Floret sterility; white awns or white heads; damage to lower stem; leaf discoloration Severe
Milk 28 F White awns or white heads; damage to lower stems; leaf discoloration; shrunken, roughened or discolored kernels Moderate to severe
Dough   Shriveled, discolored kernels; poor germination Slight to moderate

In 2005, winter wheat fields that had the most damage from freeze injury were those that were low on soil water (under moisture stress), poor stand and/or poor canopy.

It will take a number of warm days (a week or more depending on temperatures) after freezing to determine the condition of the winter wheat crop, so don”t make any quick decisions. The main tillers may be killed or injured, but other tillers may survive and help compensate for some of the lost yield potential. See NebGuide G1429, Estimating Winter Wheat Grain Yields.

A number of items determine how much freeze injury there will be to the winter wheat crop. They include, as previously mentioned, air temperature, soil moisture, stand, canopy density and the winter wheat growth stage. Others include the length of time of the low temperature, wind speed and temperature gradient in the field. Not all parts of the field will have the same amount of damage. Cold air often settles in low spots in fields, although wheat canopies in low spots are frequently better as the result of increased soil moisture conditions.

Take time to thoroughly evaluate the winter wheat condition before making any decision to spray out the crop and seed or plant to another crop.

Following is information from EC132, Freeze Injury to Nebraska Wheat, to help determine injury in the tillering and jointed stage of winter wheat.

Figure 2. Burned and yellowing leaf tips are common spring freeze symptoms at the tillering stage. Figure 3. More severe freeze damage causes the entire leaf to turn yellowish-white and the plants to be limp or flacid. A silage odor may be detected after several days.

Tillering Stage

Figure 4. A yellow necrotic leaf emerging from the whorl indicates the growing point may be damaged. Figures 5a and 5b. A healthy growing point has a crisp whitish-green appearance (left). A growing point that has been damaged loses its turgidity and greenish color within several days after a freeze. A hand lens will help detect subtle freeze damage symptoms (right).
Spring tillering of wheat in Nebraska usually begins in March and continues through mid April. The growing point is just below the soil surface during this stage and is protected against injury. Most damage occurs to leaves, which become twisted and light green to yellow in color and are necrotic ("burned") at the tip within one or two days after freezing (Figures 2 and 3). A strong odor of dehydrating vegetation may be present after several days.

Injury at this stage slows growth and may reduce tiller numbers, but growth of new leaves and tillers usually resumes with warmer temperatures.

Jointing Stage

The jointing stage is when the internodes (stem segments between joints or nodes) are elongating in the wheat stem and the embryonic head is moving up through the stems. This usually occurs from early April through early May. Leaves of freeze injured plants show the same symptoms as the tillering stage (Figures 2 and 3), but the most serious injury occurs to the growing points (Figure 4).

The growing points can be located by splitting stems lengthwise with a sharp knife. A normal, uninjured growing point is bright yellow green and turgid; freeze injury causes it to become white or brown and water soaked in appearance (Figure 5). This injury can occur even in plants that appear otherwise normal because the growing point is more sensitive to cold than other plant parts. Stem growth stops immediately when the growing points are 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.

Figure 6. Discoloring and roughening of the lower stem are symptoms of spring freeze damage. Figure 7. Splitting of stem occurs with severe freeze damage.
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 (Figure 6 and 7). Injured plants often 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. Injured areas might become infected by microorganisms, however, which can cause further stem deterioration. Lodging or falling over, of plants is the most serious problem following stem injury. Wind or hard rain will easily lodge the plants, decreasing grain yields and slowing harvest.

Robert Klein
Extension Crops Specialist
West Central REC, North Platte
Drew Lyon
Extension Dryland Crops Specialist
Panhandle REC, Scottsbluff

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