Sept. 29, 2011

In its simplest form a killing freeze is when temperatures get cold enough to kill all the top growth on the alfalfa plant. The plant wilts, turns tan, and its leaves fall off.

Alfalfa tops don’t die at any set temperature. In fact, as we get later and later into the fall without a killing freeze, it takes colder and colder temperatures to actually kill alfalfa tops. That’s why we often see green alfalfa even after several hard freezes have occurred. Only rarely do we get a freeze that fully kills alfalfa tops suddenly.

Instead of worrying about a killing freeze, it might be wiser to think about why we look for a killing freeze. Once alfalfa tops die, yield no longer increases and winterizing ends. Most importantly, this means we can take another harvest without increasing the risk of winter injury.

Experience has shown us that alfalfa that has had at least six weeks of regrowth since the previous cutting will have developed enough winterhardiness for all but the most severe winters. By mid-October alfalfa begins to go dormant naturally because of shorter days and cooler temperatures. As a result, harvest in mid-October or later is not likely to jeopardize stand persistence.

Bruce Anderson
Extension Forage Specialist