Alfalfa Winter Injury

After last year’s damage and this year’s extreme cold, I had to dust off an article I wrote about Winterkill and get it out to everyone. What may be on most people’s mind is what the extreme cold temperatures we had this winter is going to do to the alfalfa?

Roots are sensitive to exposure from extreme cold temperatures in the soil. Two weeks or more of temperatures below 15 degrees is enough to do alfalfa in. Exposed ground would have seen that kind of temperature this year. The saving factor for most will be that there was a good amount of snow on the ground to insulate the soil from dropping into these low temperature ranges.

Alfalfa over-winters by storing nutrients in its roots and surviving off of the stores much as a bear over-winters by surviving on its own fat. During 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) cannot escape from the roots, then the plant can literally poison itself. This occurs when there are ice sheets covering the roots for extended periods of time. Unlike snow, ice will trap the waste gas given off by the roots and destroy the roots.

Sudden temperature changes can also be a problem. A sudden change in the fall 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 pattern in their animal health; animals don’t get sick when it is 0-10 degrees for a month, but can get sick when the weather swings quickly.

Sudden warm temperature shifts can also cause alfalfa to break its dormancy. This sends 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 heaving and thawing, which damage the roots by pulling them apart. In some cases it pushes them right out of the ground.

How you manage your alfalfa stand is also a big factor in your alfalfa surviving fierce winters. Make sure your fertility is correct, especially your potassium level. Do not 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.

As we move into spring this year, it is going to be important to keep an eye on several potential situations. The first is fields that did not have snow cover most of the winter. The second is areas that had ice sheeting, especially if the soil was saturated to start with. Check the roots. If you find that they are soggy and soft, then alfalfa is most likely not going to come back.

If you do have winter injury, your options are dictated by the age of your stand. With stands of two years or older you may have autotoxicity problems if you try to re-seed. This is caused by a toxic substance given off by the older plants, which 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.

April 2014 Earfull Newsletter
Written by Alex Renk

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