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Seeding

General

Seeding is one of the most effective erosion control practices available. It stabilizes disturbed areas with vegetation, which protects the soil from raindrop impact, reduces flow velocities, increases infiltration, and reduces soil loss from the site. Seeding is economical and adaptable to most any area, improving aesthetics and reducing dust and mud problems that are common on many construction sites. Temporary seeding stabilizes disturbed areas with fast growing annual grasses, small grains, or legumes until permanent seeding can occur and to establish perennial vegetation.

Permanent seeding is applicable on any area of the site that has reached final grade or on areas that will remain undisturbed for over 1 year. To prevent erosion during the establishment period, additional management practices are often required, such as mulching or erosion matting, or temporary seeding. Permanent seeding of turf grass can be performed until September 15th, while dormant seeding should be completed after November 1st, but may not be applied on top of snow. Dormant seeding must be mulched and should not germinate until the following spring. If seed fails to germinate and thrive in spring, additional seeding may be necessary. Seeding of turf grass between September 15th and November 1st greatly increases the failure rate of the practice (due to early seed germination) and may require seeding to be repeated. Permanent seeding using native seed mixtures have different requirements than turf grass. Seeding dates differ, as well as management needed to successfully establish vegetation. Consult your native seed supplier for specific information on establishing native seed mixes.

Temporary seeding is applicable on any area of the site that will remain inactive for at least 14 days, but less than 1 year. It is often used to prevent erosion between construction activities and during the winter months if established early enough. Due to its short-term nature, temporary seeding may be ineffective on its own and should be used in conjunction with other management practices.

Recommended dates for temporary and permanent seeding of different species are shown below.

Advantages

  • Established vegetation can reduce the erosion rates by up to 99%
  • Cost-effective
  • Easy to apply
  • Requires little maintenance
  • Increases infiltration and water retention
  • Reduces dust

Disadvantages

  • Use limited by the growing season
  • Requires the addition of fertilizer on infertile soils

Design

Vegetation

Vegetation provides effective erosion control once densely established, generally 60 days after it has been seeded. “Dense” is defined as a stand of 6-8 inch vegetation that uniformly covers at least 70% of the disturbed area. All sites must have dense perennial vegetation before the erosion control permit can be closed and perimeter control removed.

The species of vegetation selected will vary greatly depending upon the characteristics of the site and the maintenance requirements of the species. Soil type, pH, slope, site use, maintenance, growth rate, use of native or non-native species, and the time of year it is planted are all factors that must be weighed when selecting vegetation. Native species require a longer period of time to establish and are generally more costly and more difficult to establish than non-native vegetation. However, they offer many advantages over non-native vegetation. Careful species selection is crucial to avoid planting exotic or invasive species, as they may upset the local ecosystem’s balance.

In situations where permanent establishment is more difficult, temporary seeding may be used in addition to or as an alternative to mulching. It provides cover, stabilizes the soil on steep slopes and gives slower growing plants an opportunity to become established. Relatively non-competitive, annual (temporary) species of vegetation, such as those listed in Table 1, may be used for this purpose by halving their recommended seeding rates.

Seedbed Preparation

To be successful, permanent seeding requires a properly prepared seedbed. Areas that are limited by poorly drained soils, steep slopes, or that allow concentrated flow to develop should not be used for seedbeds unless amendments to correct the situation are made.

Soils should be tested for nutrient content and pH to determine the amount, if any, of fertilizer or lime required. Over-application of these soil amendments is costly, ineffective, and may cause serious pollution problems. As a result, lime and slow releasing fertilizers should be applied only as needed and shall be incorporated into the soil to keep them on site and in the root zone.

The organic content of the soil is also an important consideration when preparing the seedbed. Soils rich in organic matter possess high levels of nutrients and microorganisms, which improve the growth rate and require less fertilizer to be applied and increase the porosity of the soils. To improve the organic content of the soil, organic compost may be incorporated into the top ten inches of soil.

A minimum of 3-6 inches of topsoil is required for permanent vegetation. It should be loose, uniform, and well pulverized to promote rapid growth. Compacted soils should be loosened to a depth of at least 6-8 inches by using a chisel plow or similar implement to ensure adequate pore space.

Temporary seeding does not require the same level of seedbed preparation as permanent seeding, but does require at least 2 inches of loose soil for successful establishment.

Application

Permanent seeding should be applied uniformly following the supplier’s recommendations by broadcast seeding, hydroseeding, or drill seeding. Broadcast seeding involves scattering the seeds on the soil surface by hand or mechanical means and is best utilized on smaller areas and for patching applications. After application, the site should be raked and firmed with a roller or cultipacker. Seeded areas should then be mulched to provide protection for the seed, retain soil moisture and reduce erosion before the vegetation becomes established.

Hydroseeding and drill seeding are more costly than broadcast seeding and are used on larger sites to maximize the application’s cost effectiveness. Hydroseeding, a method that mixes the seed and water together into a slurry, is applied on areas that may be difficult to seed with alternative means. Other amendments, such as tackifiers, polymers, fertilizers, and/or fiber mulch are often added to the slurry, which is sprayed on, to protect the seed and to promote its growth. Drill seeding utilizes a drill or cultipacker seeder to inject the seeds beneath the soil surface. Seeding depth is set based upon the supplier’s specifications, but generally is ¼ - ½ inch deep for grasses and legumes. Drilling, while more costly than broadcast or hydroseeding, is generally very effective when performed properly because the seed is protected from wind, water, and wildlife. Temporary seeding may be applied with any of the methods described above.

Table 1: Temporary Seeding Species and Application Rates

SpeciesLbs/AcreSeason
Oats131Spring and Summer
Cereal Rye131Fall
Winter Wheat131Fall
Annual Ryegrass80Spring and Fall

Construction

  • All grading and tracking shall be completed before permanent seeding begins
  • All management practices should be installed and online before seeding
  • Seedbed should be adequately prepared before seeding begins
  • To promote growth, seeding should not be performed during excessively wet conditions, as soils may become excessively compacted

Maintenance

  • Inspect seeded areas weekly following germination to ensure that vegetation is adequately established - reseed as necessary
  • Seeded areas shall be inspected after each rainfall event to check for evidence of erosion and bare spots
  • Add fertilizer as necessary at proper rates
  • Mowing and/or weeding is necessary for successful establishment of desired vegetation.
  • Water seeded areas regularly until they become established

Practice Efficiency

The efficiency of this practice is derived from reducing the amount of time that the site is left bare and exposed. To determine the efficiency for this practice, use the new, shortened exposure time and replace the pre-existing one in the USLE and recalculate. The difference between the two equations is the efficiency for the practice.

References