HomeMy WebLinkAboutElement 5 BMP-C120BMP C120: Temporary and Permanent Seeding
Purpose Seeding is intended to reduce erosion by stabilizing exposed soils. A
well-established vegetative cover is one of the most effective methods of
reducing erosion.
Conditions of Use x Seeding may be used throughout the project on disturbed areas that
have reached final grade or that will remain unworked for more than
30 days.
x Channels that will be vegetated should be installed before major
earthwork and hydroseeded with a Bonded Fiber Matrix. The
vegetation should be well established (i.e., 75 percent cover) before
water is allowed to flow in the ditch. With channels that will have
high flows, erosion control blankets should be installed over the
hydroseed. If vegetation cannot be established from seed before water
is allowed in the ditch, sod should be installed in the bottom of the
ditch over hydromulch and blankets.
x Retention/detention ponds should be seeded as required.
x Mulch is required at all times because it protects seeds from heat,
moisture loss, and transport due to runoff.
x All disturbed areas shall be reviewed in late August to early September
and all seeding should be completed by the end of September.
Otherwise, vegetation will not establish itself enough to provide more
than average protection.
x At final site stabilization, all disturbed areas not otherwise vegetated or
stabilized shall be seeded and mulched. Final stabilization means the
completion of all soil disturbing activities at the site and the
establishment of a permanent vegetative cover, or equivalent
permanent stabilization measures (such as pavement, riprap, gabions
or geotextiles) which will prevent erosion.
Design and
Installation
Specifications
x Seeding should be done during those seasons most conducive to
growth and will vary with the climate conditions of the region.
Local experience should be used to determine the appropriate
seeding periods.
x The optimum seeding windows for western Washington are April 1
through June 30 and September 1 through October 1. Seeding that
occurs between July 1 and August 30 will require irrigation until 75
percent grass cover is established. Seeding that occurs between
October 1 and March 30 will require a mulch or plastic cover until
75 percent grass cover is established.
x To prevent seed from being washed away, confirm that all required
surface water control measures have been installed.
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x The seedbed should be firm and rough. All soil should be roughened
no matter what the slope. If compaction is required for engineering
purposes, slopes must be track walked before seeding. Backblading or
smoothing of slopes greater than 4:1 is not allowed if they are to be
seeded.
x New and more effective restoration-based landscape practices rely on
deeper incorporation than that provided by a simple single-pass
rototilling treatment. Wherever practical the subgrade should be
initially ripped to improve long-term permeability, infiltration, and
water inflow qualities. At a minimum, permanent areas shall use soil
amendments to achieve organic matter and permeability performance
defined in engineered soil/landscape systems. For systems that are
deeper than 8 inches the rototilling process should be done in multiple
lifts, or the prepared soil system shall be prepared properly and then
placed to achieve the specified depth.
x Organic matter is the most appropriate form of “fertilizer” because it
provides nutrients (including nitrogen, phosphorus, and potassium) in
the least water-soluble form. A natural system typically releases 2-10
percent of its nutrients annually. Chemical fertilizers have since been
formulated to simulate what organic matter does naturally.
x In general, 10-4-6 N-P-K (nitrogen-phosphorus-potassium) fertilizer
can be used at a rate of 90 pounds per acre. Slow-release fertilizers
should always be used because they are more efficient and have fewer
environmental impacts. It is recommended that areas being seeded for
final landscaping conduct soil tests to determine the exact type and
quantity of fertilizer needed. This will prevent the over-application of
fertilizer. Fertilizer should not be added to the hydromulch machine
and agitated more than 20 minutes before it is to be used. If agitated
too much, the slow-release coating is destroyed.
x There are numerous products available on the market that take the
place of chemical fertilizers. These include several with seaweed
extracts that are beneficial to soil microbes and organisms. If 100
percent cottonseed meal is used as the mulch in hydroseed, chemical
fertilizer may not be necessary. Cottonseed meal is a good source of
long-term, slow-release, available nitrogen.
x Hydroseed applications shall include a minimum of 1,500 pounds per
acre of mulch with 3 percent tackifier. Mulch may be made up of 100
percent: cottonseed meal; fibers made of wood, recycled cellulose,
hemp, and kenaf; compost; or blends of these. Tackifier shall be plant-
based, such as guar or alpha plantago, or chemical-based such as
polyacrylamide or polymers. Any mulch or tackifier product used
shall be installed per manufacturer’s instructions. Generally, mulches
come in 40-50 pound bags. Seed and fertilizer are added at time of
application.
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