Here's How Alfalfa Prepares for Winter

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

        Alfalfa plants prepare for winter through a process called "hardening".  This process puts the plant in a better position to withstand both drought and freezing injury.  When day lengths become shorter and temperatures begin to fall, alfalfa alters both its morphological (structural) and physiological processes. The degree to which these changes take place can vary between varieties and thus we see differences in performance relating to cold tolerance and stand survival.

Structural Changes:

1.   Reduced Herbage Production:  During the fall, alfalfa plants produce fewer stems and leaves while increasing the amount of plant tissue in the root and crown. This is associated with the accumulation of more "food" reserves in the below ground plant parts.

2.   Prostrate Top Growth:  Alfalfa varieties that are more dormant will tend to produce shoots that grow in a prostrate (horizontal) manner. This type of growth habit has a good, but no where close to perfect, association with a variety's winter survival ability.

3.   Crown Bud Development:  During the fall, buds at the plant's crown, which are the sites of next year's spring growth, develop and partially elongate. These are very cold tolerant structures and partial elongation ensures rapid regrowth in the spring.  

Alfalfa Greenup in the Spring (if all goes well!)

Physiological Changes:

1.   Decrease in Total and Free Water Content: Lowering the water content reduces the probability of ice crystal formation within the plant cell.  When ice crystals form, cell walls can be broken and death of the plant tissue results.

2.   Increase in Total Root Carbohydrates:  Alfalfa plants translocate large quantities of carbohydrates (manufactured in leaves and stems from photosynthesis) to the roots.

3.   Conversion of Starch to Sugar:  Much of the carbohydrate in a plant root is starch.  This starch is not used for growth, but rather converted to sugar (primarily sucrose).  This sugar has an "antifreeze" effect within the plant cell, lowering the freezing point.  An untimely fall harvest will result in the utilization of starch for regrowth rather than conversion to sugar.  Thus, the likelihood of winterkill increases.