Fall Frost Damage to Field Crops
A killing frost is a temperature which kills the plant tissue. If your crops have frost (low temperature) damage it is the result of tissue death.
CRITICAL STAGES AND TEMPERATURES FOR VARIOUS MAJOR CROPS ARE DESCRIBED AS FOLLOWS:
WHEAT, DURUM AND OTHER SMALL GRAINS: Temperatures below 32 degrees will cause sterile spiklets if the plant is in the late boot through the flowering stage. In the milk stage it will create shriveled kernels. Frozen immature spikes will turn white. After mid-dough stage, temperatures as low as 25 degrees will result in bran damage, some kernel shriveling and possible germination reduction. The bran damage will change test weight and probably will be discounted in the market. Grain that may be saved for conditioning to planting seed should be tested for germination and vigor.
FLAX: is most susceptible during flowering and early boll stage. Immature seeds can be killed by temperatures from 28-32 degrees. After flax reaches dough stage it is more resistant to frost.
SUNFLOWER: is most susceptible at bud and flowering. Temperatures of 28 degrees to 30 degrees can result in damaged buds and sterile sections or rings in the flowering head. After pollination and petal drop sunflower can withstand temperatures as low as 25 degrees with only minor damage. Twenty-five degree temperatures at the bud stage will often damage stalk below the bud and seeds will not develop.
SOYBEAN: Easily damaged by light frosts in the 28-32 degree range. Beans that are still green and soft will shrivel. Stalks rapidly turn dark green to brown and will not recover. Beans in pods that have turned yellow will mature normally. Some green beans will turn yellow after 30-40 days of storage.
PINTO & NAVY BEANS: are very sensitive to frost (30-32 degree range). Earlier pods with yellow to brown color are sufficiently mature to escape damage. Late green pods or flowers are easily damaged by frost. Green beans will shrivel but should be left in field until dry in order to separate from mature beans.
CORN: Usually damaged by temperatures in 28 degree range or less. Corn is usually physiologically mature 50-55 days after the 50 percent silking date. Colder temperatures will kill entire stalk. If only leaves above the ear are frosted, kernel development will continue. If entire stalk and leaves are frozen kernel development will cease and soft shriveled corn will result. If corn is at around 35 percent moisture or if a black layer has formed at the base of the kernel the plant is physiologically mature and kernels will develop normally despite frost. Frosted immature corn is best used for silage or fodder.
OTHER CROPS: Most other crops such as buckwheat and proso millet are easily damaged in flower to milk stage. Buckwheat is reported by the Canadians to be very sensitive to frost prior to the mid-dough stage.
POTATO TOPS: will turn black but tubers are not usually damaged by light frosts.
SUGARBEETS: are very resistant to frost.
ALFALFA: can withstand light damage to tops but if frozen to ground level will not recover and should be harvested as soon as possible.
Remember temperatures of 32 degrees at weather stations or farmsteads may result in temperatures of 28 to 29 degrees in low lying areas of fields. Also, time of exposure to freezing temperatures will influence degree of damage done. Two other factors which may influence critical frost temperatures are soil moisture and wind velocity.