What is a Killing Freeze?
What is a killing freeze for alfalfa? Interesting question, but in its simplest form a killing freeze is when temperatures get cold enough to kill all the top growth on the alfalfa plant. We see the plant wilt, turn tannish color, and leaves fall off.
However, 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 still see green alfalfa today in many areas even though several hard freezes have occurred. In fact, only rarely to we get a freeze that actually kills alfalfa tops suddenly. So instead of worrying about a killing freeze, I think it might be wiser to think about the reasons why we look for a killing freeze. Once alfalfa tops die, yield no longer increases and winterizing ends. Thus, a killing freeze can signal when we can harvest in the fall without increasing the risk of winter injury. So what we really need to consider is when fall harvests are safe.
Experience in our region shows us that alfalfa that has had at least six weeks of regrowth in mid-October since the previous cutting will have developed enough winterhardiness for all but the most severe winters. And, 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.
You may not can a true killing freeze very often, but you still can take advantage of what it can cause by monitoring the calendar and your harvest timing.
[November 21st, 2007]
Dr. Bruce Anderson, Professor of Agronomy
Agronomy & Horticulture, University of Nebraska - Lincoln, Lincoln, NE