Manage Alfalfa For Winter Survival

By Dan Undersander

Fall management of alfalfa can greatly affect winter survival and/or yield the next year. Let’s review the biology of alfalfa winter survival, then consider management steps to be taken.

First, alfalfa needs a good level of root carbohydrates going into winter. This allows plant roots to respire, so they produce compounds over winter to enhance survival. It’s managed by taking the last cutting either early enough so regrowth and root replenishment can occur, or so late minimal regrowth and root carbohydrate use occur. This is illustrated in the chart.

Secondly, the plant must “harden” for winter. This means that cell membranes change form to become more resistant to freezing. Over summer, membranes are saturated fatty acids that allow sugars, other organic compounds and minerals to easily pass through, but turn solid near freezing, which would kill the plant. In the hardening process, these membranes become unsaturated, which keeps them liquid at lower temperatures.

Hardening begins in fall with shorter days and when the high temperature is 60° F or less. It is increased by fluctuating temperatures with highs around 50° and lows near freezing for two or more weeks. If the year suddenly changes from a warm summer to winter with a short hardening period, the plant will be more susceptible to winterkill and injury.

Hardening is lost when the air temperature reaches 60° for three to four days for fall dormancy 3 or 4 varieties, and two days for fall dormancy 5. Thus, less-dormant varieties are more susceptible to winter injury or kill if a warm period occurs over winter.

In addition to cell membrane changes, healthy plants also pump sugars into the cells so they supercool or resist freezing to temperatures below 32°. It’s the same principle as Diet Coke freezing before the more sugary, non-diet version. It’s also the reason alfalfa and all clovers need a high level of root carbohydrates for winter survival.

Note that more winterhardy alfalfa varieties, with winter survival scores of 2 rather than 3 or 4, will harden to a great extent, survive winter better and suffer less yield loss to winter injury in spring.

Alfalfa topgrowth insulates the crown area and captures snow, which further insulates the crown area and keeps it from falling below 13°, which will kill alfalfa. So, whether or not a late-fall cutting is taken should depend on the potential for snow cover over winter. If snow cover is unlikely or uncertain, leave topgrowth; if snow is likely, topgrowth can be harvested in late fall.

We saw the insulating value of snow last winter in Wisconsin during an early January cold spell. The average high air temperature was -10°, and after a week, the soil temperature of bare ground at 4” below the surface was about 7°, which would kill alfalfa. But, under 10-12” of snow, the soil temperature was 32°. Snow insulated against 40° or more of air temperature.

Other important management tools: soil pH and fertility. Alfalfa with less than the optimum 6.8 soil pH will suffer more winterkill. Additionally, potassium is particularly important to winter survival. If the ground isn’t frozen, it’s not too late to fertilize to replace the potassium removed this year – 55 lbs potash per ton of hay harvested.