Spring Freeze Resources

Comprehensive Guide to Soft Red Winter Wheat Management in Kentucky

Materials originated by the Kentucky Cooperative Extension Service are copyrighted © 1997 by the University of Kentucky Cooperative Extension Service. This publication may be reproduced in portions or its entirety for educational or non-profit purposes only. Permitted users shall give credit to the author(s) and include this copyright notice. Issued 10-97.

From the Cultural Practices chapter by
Morris Bitzer, James Herbek and David Van Sanford

Winterkill and Freeze Injury

Wheat is subjected to adverse weather conditions during much of its growth period. As long as wheat is seeded in the fall close to the recommended planting dates, you can expect little freeze damage in the fall. Autumn frosts and cool temperatures actually help by hardening plants for the months of cold winter weather ahead. Expect winterkill on poorly drained soils, with extreme temperature fluctuations, where poor fall root development occurred, and with sustained low temperatures (particularly with no snow cover). Extremely cold winters tend to cause more winterkill in varieties developed in more southerly locations because they have less winter hardiness. Heaving is a major cause of late winter or early spring damage to small plants due to extreme temperature fluctuations, especially on poorly drained soils.

Spring freeze injury can occur when low temperatures coincide with sensitive plant growth stages. Injury can occur across large areas of the field but usually is most severe in low areas or depressions in the field where cold air settles. The risk of spring freeze injury is greater when wheat initiates spring growth early due to higher than average temperatures and advances through its developmental stages more quickly than normal or when an unusually late freeze occurs after the wheat is further advanced. A late spring freeze can reduce yield because of damage to the head and stem. Head and stem damage usually is not visible for a week to ten days after the freeze. The stem will likely be damaged close to the ground. Weakened stems will likely break over or lodge as the plant matures. To check for damage to an unemerged head, cut into the stem to find the growing point (developing head). An undamaged spike (or head) normally appears light green, glossy, and turgid, whereas a killed head is pale white or tan, limp, shrunken, and not developing in size. Growing tissue of plants that have been frozen is dry, bleached, and shrunken. Table 3-3 gives a summary of injury-causing temperatures, symptoms, and yield effects of freeze injury at various stages of growth.

Table 3-3. Freeze injury in wheat.

	Bent elbow (bending of the stem at the lowest node forming an elbow) is another symptom of freeze damage to the stem. Bent stems usually resume an upright position but lodging can be a problem with the added weight of the grain as heads begin to fill.
	Growing point damage. After the jointing stage, the growing point (developing wheat head) is susceptible to freeze damage. To check for damage, cut into the stem. A normal, uninjured head (two plants on the left) is glossy, turgid, and yellow green. A freeze damaged (killed) head (three plants on the right) is pale white or tan, limp, shrunken, and not developed in size. Plants were collected 13 days after the freeze.
	The pollination (flowering) stage is the most freeze-sensitive stage. Exposure to temperatures of 30 degrees F at pollination can kill the male parts (anthers) of the flower and cause sterility without any symptoms appearing on plant vegetative parts. The head on the right is sterile due to freeze damage at pollination. No kernels developed in the florets because the anthers and pollen were killed. The undamaged head on the left has a developing kernel in each floret.
	Color banding (purple or yellow banding on young leaves at emergence) is an environmental problem caused by warm days and cool nights. Seedlings will become cold-tolerant and symptoms will fade.
	A healthy wheat head has green anthers and white stigmas, as in the lower left floret. The yellow anthers of the middle floret indicate pollen has recently been shed. The developing kernel in the upper floret indicates fertilization has occurred and there has been no freeze damage.
	Varietal differences in spring freeze injury have been observed, but differences are mostly caused by variations in plant growth stages when freezes occur. The variety on the left has more leaf burn than the variety on the right.
	Leaf burn is a symptom of freeze damage to wheat that has broken dormancy and has had prolonged exposure to low temperatures (24 degrees or lower) during lush, rapid growth.
	Stem freeze damage can occur after the jointing stage and usually occurs to the lower stem. Symptoms include discoloration, lesions, splitting, collapse of internodes, and enlargement of nodes. Damaged stems usually deteriorate further by breaking or rotting. The four stems on the left show freeze damage.
	Freeze injury to the head at the boot, heading, or flowering stage can result in death of the heads or floret sterility. The most obvious symptom is a white head color. Due to the differences in maturity of the florets along the length of the head at the time the freeze occurred, there is a range in the location of the injury to the head. The center or one or both ends of the head may be sterile. In some cases, the whole head may be sterile.
	Heaving results in plants being uplifted from the soil with the crowns above the soil surface and only a few roots attached into the soil. This is a common problem with frequent, alternate freezing and thawing of the soil, particularly with shallow plantings. As wheat begins its spring green-up, some plants begin to die.

Materials originated by the Kentucky Cooperative Extension Service are copyrighted © 1997 by the University of Kentucky Cooperative Extension Service. This publication may be reproduced in portions or its entirety for educational or non-profit purposes only. Permitted users shall give credit to the author(s) and include this copyright notice. Issued 10-97.