Small Grain Damage from Frost Dependent on Many Factors
Frost damage to small grains is highly dependent on the stage of the crop, the actual sub-freezing temperature reached and duration of the low temperature, and environmental conditions that existed before the frost occurred.
"Small grains are among the most frost tolerant of crops," says Terry Gregoire, NDSU Extension Agronomist. "From early developmental stages to jointing, small grains can easily withstand temperatures of 25 F."
After jointing, the growing point moves above the soil surface and is more susceptible to frost damage. From boot through flowering, small grains are most sensitive to frost and will not tolerate temperatures below 28 F, Gregoire notes. But when temperatures are below 50 F for several days before the frost, small grain crops go through a hardening process and develop more frost tolerance. Drought stress prior to frost can also cause hardening of small grains and increase frost tolerance. Prior to jointing, hardened small grains have been shown to withstand temperatures as low as 14 F.
Once a plant's growing point is above ground it is more likely to be injured by frost, but Gregoire says small grains in the jointing stage have been shown to withstand temperatures of 25 F without damage to the growing point, although injury is more likely than at earlier stages.
Small grains are most susceptible to frost from the late boot through flowering, the stage when reproductive tissue is developing. Anthers, the plant's pollen-producing structures, are easily damaged by frost, which prevents or severely reduces pollen production. Frost injury occurs during boot through flowering when temperatures fall below 28 F. Barley is more susceptible to frost injury than wheat during the boot stage because it flowers while still in the boot. The longer low temperatures persist, the greater the potential for injury, Gregoire says.
Small grain plants can avoid injury through a process call super cooling, particularly when temperatures cool slowly, explains Gregoire. Super cooling occurs when water from within plant cells moves out--the effect is to lower the temperature at which water within the cells will freeze. This reduces the potential for ice-crystal formation and physical damage to cell membranes.
Before evaluating frost damage, producers should wait two or three days so injury is more easily observable. Most winter wheat fields presently are in the boot to flowering stage and are at the greatest risk for frost injury. Evaluating these crops for damage involves dissecting flowers and looking at the anthers, says Gregoire. Darkening with a watery appearance indicates frost injury. Damaged anthers will not shed pollen. If pollen shed has already occurred, florets need to be evaluated for kernel development, which may require several days.
During early development, the growing point of a small grain plant is below the soil surface, thereby making it less susceptible to injury. With this protection, plants can suffer loss of above-ground foliage without dying. Again, Gregoire advises producers to wait several days to make an accurate determination of injury. At this point, new growth on plants should have begun.
"If no regrowth is observable, the stem of the plant may be split to inspect the growing point," explains Gregoire. "The growing point should be white or cream colored. Darkening or softening, with a watery appearance of the growing point, indicates injury and usually precedes plant death."