For optimum crop establishment and to minimize winterkill approximately 8 weeks are needed for the seedlings to germinate and develop sufficient root reserves to survive. In south-central PA the first killing frost of the season can occur anytime after October 1. This places the ideal seeding time from August 1 to the 15th.
One advantage of summer seedings is that next spring yields from this seeding will almost equal yields from established stands. Another advantage is that weed pressure to the developing crop is significantly reduced, but not entirely eliminated. Insect pressure from potato leafhopper dramatically drops off by early August also reducing the potential for seedling injury. And finally an early August seeding window is probably a somewhat less busy time of the year compared to springtime.
A major management consideration for summer seedings is the availability of adequate soil moisture to allow for germination of the seeds and development of seedlings. If soil conditions are extremely droughty and frequency of rainfall is limited then it will be preferable to delay planting. Limited soil moistures will allow seed to germinate but without adequate soil moisture reserves the seedlings will perish.
For this reason alone the use of no till seeding is preferred for summer seedings. Whether no-tilled or conventionally sown, nurse crops should not be planted in summer seedings to minimize moisture competition.
Seeding can be delayed until late August if soil moistures are limiting but research at Penn State has shown that delaying establishment past September 1 can result in significant stand losses due to winter-kill. Agronomists have determined that this winter-kill is because seedling root systems and crowns are not able to develop completely and the stand becomes susceptible to heaving, especially on heavier soil sites.
Perhaps the best method for summer establishment of forage crops is no-tilled following a small grain crop. No-tilling maintains existing soil moisture and minimizes soil erosion risks. Working ground for summer seedings results in significant loss of soil moisture. Producers should pay attention to residue management and ensure straw is either adequately distributed behind the combine or baled and removed. Often the windrows of straw are not completely picked up by the baler resulting in heavy residue areas that can present challenges to ensure seed to soil contact.
Although there will be less weed pressure in a summer seeding winter annuals and volunteer small grain can provide competition to the alfalfa. Do not wait until next spring to apply control materials. Apply when weeds are small after the alfalfa has reached the 2 to 4 trifoliate leaf stage.
While there may be significant growth on the new seeding this fall wait until next year for first harvest. This enables the stand to maximize root reserves. Next spring allow the stand to reach full bud stage before harvesting to again ensure that root reserves are maximized.