Assessing Freeze Injury to Wheat

Assessing Freeze Injury to Wheat

Winter wheat development is about the same as last year in Nebraska. At the University of Nebraska–Lincoln West Central Research and Extension Center Dryland Farm south of North Platte, the average temperature since January 1, 2020, was 34.6° F. This compares to an average temperature of 33.7°F during the 1907-2020 period. In 2012, when winter wheat was as much as two weeks ahead of normal growth stage, the average temperature from January 1 to this point was 40.2°F. The growth stage this year is about normal.

The April 20 USDA NASS Nebraska Crop Progress and Condition News Release reported 2% of the winter wheat was rated very poor, 8% poor, 21% fair, 61% good and 8% excellent.

Low temperature reports for the early morning hours on April 13 include 4 degrees for Harrison. Across Nebraska freezing temperatures affected many wheat fields. Table 1 is a 6 day summary for the period April 12 to April 17 at a number of Nebraska sites.

Table 1. 6 Day Summary of Low Temperatures April 12 to April 17, 2020. Data courtesy of the Nebraska State Climate Mesonet.
Ave. Norm. Dep. Hi/Day Lo/Day Last Day Miss
AINSWORTH 2NE 19.9 34.3 -14.4 26./ 5 13./ 2 24.8 0
CHAMPION 5SE 12.9 33.7 -20.8 18./ 4 10./ 3 10.7 0
HARRISON 4NW 10.2 32.2 -22 17./ 4 4./ 2 12.2 0
HOLDREGE 5N 18.3 35.8 -17.6 28./ 5 13./ 3 16.6 0
LEXINGTON 4S 18.7 35.3 -16.6 26./ 5 14./ 3 17.3 0
ITHACA 3E 21.7 38.4 -16.7 30./ 5 12./ 4 22.4 0
NEBRASKA CITY 3NW 25.6 38.5 -12.9 31./ 5 20./ 4 29.3 0
NORTHPLATTE 3SW BETA 16.4 33.8 -17.4 19./ 5 15./ 2 15.2 0
SCOTTSBLUFF 2NW 14.9 32.3 -17.4 24./ 4 8./ 3 10 0
SIDNEY 2NW 10.1 32.5 -22.5 23./ 4 5./ 6 4.6 0

Remember, it takes several warm days (a week or more, depending on temperatures) after a freeze before an accurate determination of injury can be made.

Most of Nebraska’s winter wheat is in the tillering to jointing growth stage, the following points relative to low temperatures should be considered.

#1 Tillering wheat can be damaged with temperatures below 12 degrees for 2 hours.

#2 Jointing wheat can be damaged with temperatures 24 degrees and below for 2 hours.

Remember that temperatures in low-lying areas are even lower that the recorded temperatures from higher spots (Figure 1).

Field has a yellow cast.
A field in excellent condition this spring.
Figure 1 - Winter Wheat Fields in Southwest and West Central Nebraska
Left Photo April 5, next April 20 after low temperatures and close up of plant.

Additional information on assessing freeze injury follows. Figure 2 shows the temperatures that cause freeze injury to winter wheat at different growth stages. Table 2 lists the temperatures that cause injury to wheat at spring growth stages, symptoms and yield effect of spring freeze injury.

freeze injury to winter wheat
Figure 2. 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 freezes. (Graph, which links to a larger version, is from EC132, Freeze Injury to Nebraska Wheat)
Table 2. 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  28°F Shriveled, discolored kernels; poor germination Slight to moderate

The following section provides information on how freeze injury occurs at various growth stages of winter wheat and what the implications are for the crop.

Winter wheat is protected by a microclimate. Moist soil cools and warms six times slower than dry soil. With the precipitation (snow and rain) we received in many areas of the state, this reduces the temperature lows experienced by the wheat plant. Also, good stands and dense canopies reduce the lowering of temperatures to which the plant is exposed.

Most of the 2020 Nebraska winter wheat crop has good stands with the exception of portions of southwest Nebraska where dry conditions limited growth, resulting in thin stands.

In 2005, winter wheat fields experiencing the most freeze injury were those low on soil water (under moisture stress) and which had poor stands and/or a poor canopy.

It takes 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 after a freeze. 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 factors determine the degree of freeze injury to a winter wheat crop. They include, as previously mentioned:

  • air temperature,
  • soil moisture,
  • stand,
  • canopy density, and
  • the winter wheat growth stage.

Other factors include the length of time of the low temperature, wind speed and temperature gradient across 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.

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. Always consider that winter wheat crop residue provides a good foundation for succeeding crops.

Before destroying any crop or making changes, check with your crop insurance provider, Farm Service Agency, and any other representatives who need to be informed.

Assessing Freeze Injury by Crop Growth Stage

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

Tillering Stage

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 3 and 4). 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.

Burned and yellowing leaf tips
Figure 3. Burned and yellowing leaf tips are common spring freeze symptoms at the tillering stage.
leaf yellowed from freeze damage
Figure 4. More severe freeze damage causes the entire leaf to turn yellowish-white and the plants to be limp or flaccid. A silage odor may be detected after several days.

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 3 and 4), but the most serious injury occurs to the growing points (Figure 5).

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 6). 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.

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 7 and 8). 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.

A yellow necrotic leaf
Figure 5. A yellow necrotic leaf emerging from the whorl indicates the growing point may be damaged.
Figure 6a and 6b. 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).
Discoloring and roughening of the lower stem
Figure 7. Discoloring and roughening of the lower stem are symptoms of spring freeze damage.
Split stem
Figure 8. The stem can split with severe freeze damage.