Alfalfa can suffer winter injury in numerous ways. None of these events can a producer control but recognizing what could happen and following some basic management practices can give some protection to this problem.
Alfalfa over-winters by storing nutrients in its roots and surviving off the stores much as a bear over-winters surviving on its fat to make it through until spring. During the winter the plant is still active, although at a much-reduced level and respires or "breathes" small amounts of waste gas into the atmosphere. If for some reason this gas (along with other gases in the soil) can not escape from the roots then the plant can literally poison itself. This occurs when there are ice sheets covering the roots for extended period of times. Ice, unlike snow, will trap the waste gas given off by the roots and destroy the roots.
Roots are also sensitive to exposure to extreme cold temperatures. Two weeks or more of temperatures below 5 to 15 degrees is enough to do alfalfa in. In situations like this snow accumulation can help. 6" of snow or more will help insulate the roots and keep them from freezing to death.
Sudden temperature changes can also be a big problem. In the fall, a sudden change from warm to cold may not give the roots enough time to adapt to their winter activity level and leave them unprepared for winter. Anyone with livestock would see this same type of pattern in their animal health; animals don’t get sick when it is 0-10 degrees for a month but get sick when the weather swings quickly.
Sudden warm temperature shifts can also cause alfalfa to break its dormancy, sending the plant into its higher activity level which causes the roots to use up their reserves, leaving little for when spring comes.
Temperature swings in the late winter can result also in excessive soil heaving which damage the roots by pulling them apart and in some cases pushing them right out of the ground.
While all alfalfa is subject to winter injury, some definitely do better against it than others. If you're in areas where you don’t have a lot of snow cover this should be your number one criteria in selecting an alfalfa.
When selecting a winter-hardy alfalfa it is not good enough to go strictly on the winterhardiness rating. The rating is based on one year's worth of testing and is no measure of survival over the life of the stand. A better method is to look at Wisconsin-Minnesota yield to winter survival test run by the extensions of the two states. Varieties with good winter survival and yields over time are varieties that should be on your list when you start your selection process. Unfortunately, not all varieties are on this list every year.
How you manage your alfalfa stand is a big factor in your alfalfa surviving fierce winters. Make sure your fertility is correct, especially your potassium level. Also don’t take a cut six weeks before the first killing frost (September 1st to October 15th in the Midwest) and have at least 6 inches of re-growth left for the winter.
If you do have winter injury your options are dictated by the age of your stand. A stand two years or older will have autotoxicity problems if you try to re-seed. This is caused by a toxic substance given off by the older plants that prohibits new seedlings from establishing themselves. If you have two-year or older stands it is best to plow the field down and put in another crop or survive with what you have if the damage is not severe. If you have a young stand you may re-seed damaged areas. If you have less then 40 stems (not plants) per foot then reseeding should be considered.