By Andrew Frankenfield
Cold injury risk is reduced where vegetative growth or cover is protecting the new seedlings or forage growth lower in the plant canopy. Air temperature, a few feet above a bare or grass covered soil surface, is what is measured and reported. Plant tissue temperature is influenced by leaf surface color, density of the plant canopy, air movement within the canopy, the temperature of the soil, and likely more subtle conditions. The air within the forage canopy is likely 'layered', meaning the temperature at the top of the canopy is colder than the temperature at the soil surface, and below the soil surface in the taproot and crown area. Simple statements about the influence of the reported temperature can be misleading. To complicate things a bit more, tolerance of leaves to frost varies somewhat among varieties and individual plants, and is not always related to winter hardiness of the variety.
New Forage Seedings
At emergence, alfalfa and most winter hardy forage grass and legume seedlings have good cold tolerance. But, spring cold snaps can hurt new seedings too. I tend to agree with the article from Oregon that states; "For alfalfa, at second trifoliate leaf stage (and older) seedlings become more susceptible to cold injury and may be killed by four or more hours at 26 F or lower temperatures. Alfalfa seeded with a companion crop survives lower temperatures and longer exposure times before showing frost damage."
Established Stands
Well established, developing forage plants have lost their winter cold hardiness. Exposed tissue is susceptible to cold temperature injury. Several hours of 25-27 F temperature, or lower, will damage leaf tissue and may seriously damage buds and growing points. Upper 20s F air temperatures will likely damage one to several sets of trifoliate leaves that were exposed at the top of the canopy. The buds and growing stem tips are somewhat more protected and often continue to grow normally.
Where does that leave us? There will likely be leaf tissue damage in some parts of the state where overnight temperatures go 25 F to 27 F or lower for several hours. Slope position, soil temperature, companion crop of oats, wind, snow cover, all will influence what occurs in a particular field or part of a field.
Management Suggestions
The only management suggestion at the moment is to wait a week or so to see what the damage is.
New seedings
Seedlings that were frozen so that all trifoliate leaves are discolored and dying will not regrow. If new seedings were permanently damaged, consider re-seeding as soon as possible. Keep the good areas and drill into thin or damaged areas. Tillage may not be necessary. If you think that a cereal grain companion crop, still present, will be too competitive or will impede the reseeding, then tillage may be required.