by Margaret Smith, PhD, Forage Agronomist
The below-freezing temperatures across Minnesota, Wisconsin, the Dakotas, and parts of Iowa may have damaged seedling or established alfalfa and red clover stands. Don’t react too quickly, though, to the temperatures reported at your nearby official weather station, as these are recorded five feet above the ground.
Several factors influence the temperatures experienced by forage legumes within their canopy, much closer to ground level, and whether or not growing points may have been damaged. Factors include:
- Plant Growth Stage: Just after emergence, seedling alfalfa and red clover are fairly tolerant of temperatures in the mid to low twenties (20°F – 25°F). After they reach the second trifoliate stage, they are more vulnerable to somewhat higher temperatures.
- Air Temperature at Growing Point Level: Soils warmer than air temperatures radiate heat upward during the night.
- Duration of Low Temperatures: Plants have the ability to withstand short drops in critical temperatures, but they can be damaged with extended periods at these same temperatures.
- Weather Preceding Sub-32°F Temps: Cold weather for a week or more before temperatures drop below 32°F cause plants to gradually shift their enzyme complement and their soluble sugar levels, which acts as ‘antifreeze’, lowering the temperatures at which damage may occur.
- Overnight Wind During Cold Snap: Wind increases evaporation from the leaf surfaces and can contribute to cold damage. Light, intermittent winds, though, contribute to mixing of night air and can keep temperatures at the leaf surface warmer than when cold air is still.
- Colder air is denser than warmer air and, when winds are still, will settle in lower areas of the landscape.
- Night temperatures tend to be lower on east and north slopes, due to less soil warming during the day.
- Companion Cover Crop with Seeding or Hay Stand: New seedings with a cover crop, such as oats, will have a different microclimate within the seeding canopy than straight-seeded alfalfa. There may be an insulating effect of the mixed species seeding.
Even if you could predict the exact impact of these factors affecting your fields, the only way to really assess cold damage is with a visual assessment. After checking fields immediately following these recent cold night temperatures, wait three to five days for a thorough evaluation before making a decision about your stand. Management decisions and criteria differ for new seedings compared to those for established stands.
Summary of Management for Frosted Alfalfa*
- Newly emerged alfalfa seedlings have good cold tolerance down to the low twenties
- Cold tolerance decreases when plants reach the second trifoliate stage
- After that growth stage, four hours of 26°F can kill seedlings
- Evaluate stands after 3 to 5 days before reseeding or interseeding
- Seedlings frozen to the point of all their trifoliate leave discolored and dying will not regrow
- Damage will likely be variable across a field
Post-Frost Plant Seedling Stand Counts
- 20+ plants per ft2: (in the Upper Midwest) keep stand
- 15-19 plants per ft2: consider overseeding additional alfalfa
- < 15 plants per ft2: overseed alfalfa at a rate of 10-12 lbs/A
Light Frost – 26-31°F
- Alfalfa growing points and buds not likely damaged
- May observe loss of some trifoliate leaves and twisting of stems near top of canopy
- Plants will outgrow damage: no changes in management required
Moderate Frost – 26-27°F
- Significant injury to topmost growing points, but entire stem is NOT dead
- Axillary buds at each base of each leaf and crown buds will regrow
- Stems in lower canopy will grow normally
- Harvest base on maturity of undamaged stems to optimize forage quality
- Avoid cutting these stands early; delaying harvest allow for maximum carbohydrate storage in the crown and roots
Severe Frost – temperatures below 26°F
- Stems will be killed and won’t regrow
- Regrowth will come from crown buds
- If growth is over 12” tall, harvest the forage
- If growth is less than 12” tall, leave or graze the stand
- If grazing, wait three to five days to allow forage to dry and to avoid the possibility of bloat. Nitrate levels may increase, but rarely to hazardous levels. Allow grazing livestock access to free-choice, high-quality hay when grazing frost-damaged alfalfa or clover.
- Leaving dead material may slow regrowth
Temperatures for an extended period in the low twenties (20°F – 25°F) may kill established alfalfa or red clover. If fewer than five live plants per square foot remain following a freeze, consider interseeding forage grasses or rotating to another crop in that field.
* Summary adapted from Managing Frost-Damaged Alfalfa Stands, University of Kentucky.
Albert Lea Seed can provide both conventional and organic alfalfa and red clover seed and a variety of forages grasses for overseeding or interseeding into damaged stands. Seed for replanting or supplementing 2020 seedings is available at half-price.
- Assessing Alfalfa Post Freeze, University of Nebraska
- Frosted Alfalfa?, University of Minnesota
- Cold Injury to Alfalfa, Iowa State University
- Cold Comfort Farm: the acclimation of plants to freezing temperatures
- Freeze Damage to Alfalfa, University of Nebraska
- Frost Damage in Alfalfa: What to do after it? (05/19/16), North Dakota State University
- Frost Damage to Alfalfa, University of Wisconsin-Madison
- How Alfalfa Responds to Frost, University of Nebraska
- Managing Frost-Damaged Alfalfa Stands, University of Kentucky