Spring Freeze Damage in Alfalfa

Often in spring, alfalfa breaks its dormancy and begins growing, only to find below-freezing temperatures return to the forecast. It is important to remember that alfalfa is tolerant of cold temperatures and can withstand some time with temperatures that dip below freezing. Here are some general rules according to the University of Wisconsin:

1. Temperatures in the 25 to 30ºF range may cause some leaf deformation for leaves in early development stages but earlier and later leaves will not be affected.
2. Nighttime temperatures must fall to 24ºF or lower for four or more hours to freeze the alfalfa meristem at the top of the plant stem. This means that temperatures at freezing or just below (28 to 32ºF) will not damage the alfalfa.  In fact, we can actually have snow with no damage to growing alfalfa.

How can you tell if alfalfa is damaged from low temperatures? Assessment should happen 2 to 4 days after the cold event. Look for leaves that have a wilted or blackened appearance. If no damage is visible, there is no frost/freeze injury to the alfalfa. Remember that damage will occur mainly to the top of the plant since that is the part that's most exposed to the cold temperatures.

If you see damage, here are some recommendations:

• If leaf edges only are blackened or show signs of 'burn', damage is minimal with little to no yield loss and nothing should be done – wait to harvest at normal time.
• If only a few entire leaves are damaged but not the bud, yield loss will be minimal and nothing should be done – wait to harvest until normal time.
• If the entire stem top (some leaves and bud) is wilted and turns brown, then the growing point (bud) has been killed by frost and that stem will not grow any taller. However, axillary shoots that form may develop at leaf junctures on the stem.  In addition, new growth will occur from developing crown buds. When entire tops are frosted significant yield loss will occur.
• If the growth of frozen stems is too short to justify harvest, do nothing and new shoots will develop from crown and axillary buds. Yield will be reduced and harvest will be delayed while the new shoots develop.
• If the growth of frozen stems is sufficiently tall to be economic to harvest (20 inches or more), do so. There is no toxin in the frozen top growth and it will provide good high-quality forage. Mow immediately and harvest as normal. Regrowth will be slow and some total season yield loss will occur. After harvest ensure that soil fertility is adequate for good growth. Letting the next cutting grow to first flower will improve stand condition.