One variation of early winter storms is sleet and freezing rain. This transition between rain and snow is often very rapid and seldom is considered to be more than a short-term travel hazard. Some ice storm events can persist for several hours and deposit a significant crust of ice. Should this be considered a risk to the successful overwintering of perennial crops, such as alfalfa or other forages?

The Iowa State University Extension publication, Evaluating Hay and Pasture Stands for Winter Injury, PM 1362, addresses ice sheeting briefly. PM 1362 is available in print--call Iowa State University Extension Distribution Center at (515) 294-5247; it is not available via the Iowa State University Extension online publications page.

Smothering of plants when covered by ice sheets

The roots and crowns of perennial forage plants continue to respire during winter dormancy. During respiration, they are using oxygen and accumulated carbohydrate stores and releasing carbon dioxide, ethylene, and other gaseous compounds. As long as there is open porosity in the soil, these gases exchange freely during the winter months. This exchange of gases is blocked by a persistent, continuous sheet of ice cover. This condition of anoxia is similar to the physiological injury caused by extended periods of saturated soils and water logging.

Smothering of alfalfa, due to ice cover, may cause noticeable winter injury with 1 to 3 weeks of cover, and death with 2 to 6 weeks of impaired gas exchange. Ice damage is somewhat unpredictable. Rather thick layers of ice can persist for weeks or months, if the ice is porous, or alternating layers of ice and snow. If the soil surface is warmer than freezing, ice will often melt sufficiently even though the air temperatures are colder.

One step toward prevention of ice sheet damage is the recommended practice of leaving 6 to 8 inches of vegetative stubble in the fall to help reduce the occurrence of solid ice sheet formation.

I am not aware of any research that has been done on attempts to alleviate damage from ice crusting. There are anecdotal reports of producers physically breaking ice crusts with chain harrows or rotary hoes. The success of these practices is unknown.

Ice can form in low or depression areas of fields where snow melt or winter rain ponds and freezes. Observations also have been made of lasting damage to alfalfa where wheel tracking on snow has caused persistent ice cover.

Birdsfoot trefoil, red clover, and white clover have tolerance to smothering that is similar to alfalfa, while ladino clover is more susceptible to injury. Grasses are more tolerant than legumes to smothering and can withstand injury for up to 10 to 14 weeks.

Cold exposure of root and crown tissue and the physical freeze/thaw cycle process called heaving are more frequent causes of winter injury in perennial forages.