Uncontrollable Factors Fall hardening - Periods of cool temperatures are required in the fall for alfalfa to develop resistance to cold temperatures. Sudden drops in temperature to unseasonable levels in the early fall will cause problems. Snow cover of 6 inches or more protects alfalfa plants from severe cold. Cold temperatures - Even hardy varieties can be injured by two weeks of temperatures below 5 to 15 degrees. Midwinter thaws cause alfalfa to break dormancy and result in less resistance to freezing.
Cold resistance is lost much faster than it is achieved.
Severe temperature fluctuations above and below freezing can also result in plant heaving. Excessively wet soil in the fall reduces hardening and predisposes alfalfa to winter injury. Controllable Factors Varieties developed for winter hardiness and disease resistance will better tolerate late fall cuttings. Cutting management - The more cuttings that have been harvested, the more plant reserves are reduced and the potential for winter injury is increased. Alfalfa needs at least 30 days of re-growth prior to a killing frost to establish root reserves.