By field agronomists Brian Lang, Meaghan Anderson and Rebecca Vittetoe

Late summer can provide a window of opportunity to seed perennial forage legumes and grasses, whether you want to establish a new forage crop or need to fill in bare and thin spots in an existing forage stand. To help improve the chances for a successful late summer seeding of forages, consider the following.

Field Preparation Prior to Seeding

  • It is suggested to take soil samples and fertilize based on fertility needs of the field. Testing is the only way to really know the fertility levels and needs in a field.
  • Have problematic weeds under control.
  • Check herbicides used previously in the field as many can have residual soil activity that could prevent establishment of new forage seedings if the crop rotation restriction intervals are not observed. A good resource to check herbicide labels is

Timing of Seeding and Environmental Conditions

  • Ideally, we want 6 to 8 weeks of growth after emergence before a killing frost in the fall; therefore, the recommended window for late summer forage seedings ranges from early August to early September, but it varies slightly depending upon location in the state as listed below.
    • Northern Iowa: Early to mid-August
    • Central Iowa: Mid-August to late August
    • Southern Iowa: Late August to early September
  • One of the biggest challenges with late summer seedings is having adequate moisture available for germination and seedling establishment. This is especially a concern for western Iowa this year. If conditions are dry, a late summer seeding is not recommended.

Seedbed Preparation

  • Loose seedbeds dry out very quickly. Deep tillage should be completed several weeks ahead of seeding so rains can settle the soil before final seedbed preparation. A cultipacker or roller is an excellent last-pass tillage tool. The soil should be firm enough for a footprint to sink no deeper than 3/8 to 1/2-inch.
  • If moisture is a concern, interseeding and no-till forage seeding can help conserve moisture, provided weeds are controlled prior to planting.
  • Seeding depth is important since most forage species are small-seeded. Final seed placement should be no deeper than ½-inch for heavier soils and ¾-inch for lighter soils. If seeding with a drill, it is recommended to set the drill at the ¼-inch depth. You should see approximately 10% of the seed visible on the soil surface. If you are seeing a smaller amount, the seed is being placed too deep, and you need to adjust your seeding depth.

Other Considerations

  • Thickening up alfalfa stands with more alfalfa is only recommended within 12 to 15 months of the original planting date due to autotoxicity.
  • If seeding a legume, make sure the legume seed has fresh inoculum of the proper rhizobium.
  • Do not harvest late summer perennial forage seedings this fall. It is best to let them establish and develop winterhardiness.

Late summer can be an excellent opportunity to thicken up forage stands or start new seedings; however, use the above tips to help ensure success. For more information on late summer forage seeding or to get specific questions answered, please reach out to your local Iowa State University Extension and Outreach field agronomist.