Tom's Blog

Friday, January 31, 2014

Overseeding prairie and savanna remnants

This post is concerned not with new plantings, but with planting remnant prairies or savannas that have become heavily degraded with invasive shrubs. The first two phases in the restoration of such sites are:

  1. Clearing the brush by cutting and treating
  2. Burning after clearing, preferably annually for at least 5 years. Either spring or fall burns can be done.

Those two phases should never be reversed. Burning before clearing will top-kill brush, but will not eradicate it, and the dormant root-stock will send up one or many resprouts.

In the growing season after the first burn the understory flora of the site should be monitored carefully and a species list developed. Since the site has been degraded, there will likely be a rather impoverished flora. This is why overseeding (sometimes called interseeding) is recommended. Planting should be done soon after the site has been burned. Fall burns are preferable because many species require a cold, moist treatment in order to germinate. However, if the site must be burned in the spring, planting can still be done, provided the burn has taken place early enough.

The seed mix should reflect the native flora of the area being restored. The mix should contain as many species as can be obtained, either by collecting from nearby sites (recommended), or purchased. At least 50 species should be used, including both forbs and grasses. NRCS recommends a very dense seeding rate. "For instance, if the seed used was hand-collected from a local source, the NRCS specifies a minimum seeding rate of 50 seeds per square foot. An additional provision is that the seeding rate should be adjusted so that no single species amounts to more than 20% of the seeds in each square foot. NRCS also specifies that at least 25 seeds per square foot must be native grasses or sedges and at least 5 species of grasses and 15 species of forbs and legumes must be seeded. These criteria are applied by NRCS for both prairie and oak savanna restorations.

Note that the number of species given in the previous paragraph is a minimum and great effort should be expended to plant more species. If only small amounts of hard-to-get species are available, then they should not be placed in the general seed mix, but hand planted separately in suitable parts of the savanna." Quoted from

Since these recommendations are for prairie plantings on bare ground, they are higher than needed for overseeding, However, the higher the seeding rate, the better is the chance of success,

Details on planting oak savannas, including seed mixes suitable for Midwest habitats, can be found at the following link:

Because many species take quite a few years to become established from seed, don't expect great results the first year or two. In fact, some species may not sppear as flowering plants for 5-7 years. Also, warm-season grasses  usually take three years to become established. Because of this, repeating the overseeding several years running is a good idea.

Several factors will affect the success of overseeding. The germination rate of hand-collected seed can vary widely from species to species and for the same species from year to year. Also, growing conditions will be strongly affected by the weather, especially by temperature and rainfall. If planting is done in a drought  year, it must be repeated the following year.

One of the interesting things about overseeding is that species will sort themselves out with time. This is not surprising, because most natural area sites are heterogeneous, with dry, dry-mesic, mesic, and wet-mesic parts. Thus, if Echinacea pallida is planted uniformly over the whole site, four or five years down the line it may be found primarily in the dry-mesic areas, because this is where it competes best with the rest of the flora.


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