A good time to plant alfalfa is in early spring or late summer. The exact dates vary with location. The rule of thumb is to make spring seedings after the occurrence of the late freezes and make late summer seedings six to eight weeks before the first hard freeze.
Two of the major considerations when planting alfalfa are soil moisture and temperature. Alfalfa roots grow best at soil temperatures of 69 to 76 degrees Fahrenheit, which occur in mid September to early October and late April to early May. If the soil doesn't have enough moisture, as often is the case in late summer planting, prepare the seedbed but wait until there's an inch of rainfall. If possible, irrigate the soil until the rains take over. Comparing the different planting times, studies show that planting alfalfa in mid September results in the best yield among all other times of the year.