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Permanent Seeding

General

Permanent seeding stabilizes disturbed areas with perennial vegetation. Dense, established vegetation protects the soil from raindrop impact, reduces flow velocities, increases infiltration and reduces soil loss from the site and is the most effective erosion control practice available. In addition, seeding is economical and adaptable to most any area, improving aesthetics and reducing dust and mud problems that are common on many construction sites.

Permanent seeding is applicable on any area of the site on which final grading has been performed or on areas that will remain undisturbed for over 1 year. Permanent seeding can be performed until September 15th, while dormant seeding should be completed after November 1st. Seeding outside these dates greatly increases the failure rate of the practice and may require seeding to be repeated. To prevent erosion during the establishment period, additional management practices are often required, such as mulching, erosion matting or temporary seeding.

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

Permanent vegetation provides effective erosion control only once densely established. “Dense” is defined as a stand of 6-8 inch vegetation that uniformly covers at least 70% of a representative 1 square meter plot. As a result, until vegetation is firmly established (generally 60 days after it has been planted) it cannot be relied upon to prevent soil loss from the site.

The species of vegetation selected will vary greatly depending upon the characteristics of the site and the long-term 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 establishment is more difficult, companion vegetation may be used in addition to or as an alternative to mulching. It provides cover and stabilizes the soil on steep slopes, during late planting schedules, and to give slower growing plants an opportunity to become established. Relatively non-competitive, annual species of vegetation, such as those listed in the Temporary Seeding section of this appendix, may be used for this purpose, provided the seeding rate is cut in half.

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 3-4 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.

Application

Seed 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 and to 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.

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 after planting 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 spraying may be necessary to control weed growth
  • Water seeded ares regularly until they become established

References