By Dave Wilson and Zachary Larson
Although we often think of “winter-kill” occurring with winter annual legumes during the middle of winter, in certain years it occurs as we transition from winter to spring as the legume cover crop breaks dormancy.
Winter-kill of winter-annual (Hairy Vetch, Austrian Winter, Crimson Clover) and perennial legumes (alfalfa, red clover white clover) can occur for several reasons. Some of the common weather-related causes of winter injury or winter-kill include extremely low or fluctuating temperatures, ice sheeting and lack of snow cover. An overwintering legume cover crop with a snow covering is protected buy subfreezing temperatures from what can be though of as an Igloo affect, where the snow insulates the cover crop. The temperature of the snow remains around 32° Fahrenheit and the cover crop underneath the snow is not exposed to subfreezing air temperatures during winter.
Although we often don’t fertilize cover crops, maintaining soil test potassium levels in the optimum range also helps legume cover crops defend against the cold. Adequate levels of Potassium and sugars in plant cells helps to lower the cell sap’s freezing point, acting as an ‘anti-freeze’ agent in preventing frost damage and limiting winter-kill.
In Pennsylvania we have certainly had our share of snowfall this year but now that the snow is melting, we may experience some fluctuating temperatures as we transition from winter into spring. As temperatures increase the overwintering legume cover crops will break dormancy. Usually when the plant is dormant it has a certain amount of protection against the cold because its metabolism is very low in the dormant state. In the fall as days get shorter and cooler the plant goes through a process of cold acclimation and there are several physiological, biochemical and molecular changes that take place in the plant which are associated with the plants’ development of freezing tolerance for the winter. Come springtime these processes are reversed once dormancy is broken and then the plant is more vulnerable, especially to extreme cold temperatures. A sudden cold snap can then inflict frost damage on plant tissue or even cause winter-kill depending on the extremity and duration of the cold temperature. So, although we may be out of the depths of winter, we may not be out of the woods from the effect of cold temperature winter-kill on our legumes if we were to experience an unexpected cold snap.
In southeastern Pennsylvania we have already had a few days in the 50s and even hit 60° while there was still snow on the ground. We did experience another cold blast with temperatures down to 18 or so. Now as the temperature continues to warm some of the legume cover crops will start to break dormancy. As dormancy is broken and the metabolism of the plant increases its vulnerability to cold is greater if extreme cold temperatures should occur.
In the northern tier counties, the snow remained a bit longer and with cooler temperatures, legumes will not break dormancy as quickly. Looking at the extended forecast in the southeast, things look like we may have a warmer and wetter spring. Hopefully, there are no unexpected cold snaps that can cause freezing of the legumes.
Source : psu.edu