SLOPE STABILIZATION WITH CROWN VETCH
A. J. Powell, Jr.
Department of Agronomy
Advantages and Uses of Crown vetch
Once established, crown vetch provides
an excellent, almost maintenance-free, cover for soil stabilization and
slope beautification. It grows approximately two feet tall and blooms from
June through September, producing pink to pinkish-purple flowers. Crown
vetch is strongly rhizomatous (underground root stalks) and grows well
on clay textured and shallow soils. It is highly recommended for low maintenance
areas such as slopes around commercial or public buildings, highway and/or
driveway slopes, lawn areas too steep for safe mowing and slopes associated
with many recreational facilities or parks.
•The foliage of crown vetch will die
back after the first few freezes and remain brown until dormancy breaks
the following spring. In Kentucky, this is normally December through March.
•Crown vetch is slow to establish,
requiring from two to five years.
•Because Kentucky is located in the
southern-most area of crown vetch adaptation, it performs best and establishes
quicker on north facing (cooler) slopes.
•Crown vetch will be seriously injured
if mowed more than once or twice per year.
•Occasionally certain broadleaf weeds
such as wild lettuce and thistles must be rogued from crown vetch beds
to improve its aesthetics.
Seedbed Preparation & Fertilization
It is useless to attempt crown vetch
establishment without proper soil amendments and preparation. Crown vetch
is a legume that will not establish on unlimed acid soils. Contact your
local agricultural Extension agent to obtain soil test information and
recommendations. From two to four tons per acre of ground agricultural
limestone may be required to reduce the acidity and adjust soil pH to a
Because of its slow seedling vigor,
fast germinating grasses must be seeded with crown vetch in order to reduce
the immediate erosion hazard. In order to get good establishment of the
companion grass, it is necessary to apply some nitrogen. In most cases,
nutrient needs for establishment can be provided by applying approximately
50 lbs/acre of actual nitrogen, 100 lbs/acre of P205
and 100 lbs/acre of K20. This is equivalent to applying 1000
pounds of 5-10-10 fertilizer per acre (25 lbs/1000 sq ft), or 500 pounds
per acre of a 10-20-20 fertilizer (12 lbs/1000 sq ft). A soil test is needed
to determine the exact needs.
For best establishment, lime and fertilizer
should be worked into the top four to six inches of soil. Do not prepare
a fine, fluffy seedbed. Since many sloping areas will not likely be mowed,
leave the seedbed as rough and non-uniform as possible. The presence of
rocks, large clods or even stumps will help stabilize the soil and improve
the microenvironment for better crown vetch establishment. However, it
is important to loosen the soil in order to get good soil-seed contact.
On steep slopes where the incorporation of lime and fertilizer is unfeasible,
shallow furrows or grooves (about 2 inches deep and 2 feet apart) should
be made laterally along the slope. This rough seedbed and/or furrows will
help hold additional moisture and fertilizer to give quicker establishment.
Most crown vetch is established by
seeding. However, crown vetch plugs or peat-potted plants are often available.
These are usually more expensive, require much hand labor to set the plugs
(usually planted on 2 feet centers), and seldom increase the rapidity of
establishment. Also some irrigation may be required to insure survival
of the small crowns. Similar soil preparation is needed with either planting
Penngift, Chemung and Emerald are varieties
of crown vetch commercially available.
When to Seed
The best time to establish crown vetch
is from mid-February through March. The second best establishment time
is from mid-August to mid-September. Seedings made in late-spring through
early-summer are seldom successful due to heat, drought and weed competition.
Young plants from seedings made after mid-September are often not hardy
enough for good winter survival.
Use from 15-20 lbs/acre (1/2 lb/1000
sq ft) of crown vetch seed. This seed is usually very expensive because
it is difficult to produce and harvest. Also it has an extremely hard seed
coat that must be scarified.
In order to stabilize the soil while
crown vetch is becoming established, it is necessary to also include 40-60
lbs/acre (1 lb/1000 sq ft) of KY 31 tall fescue. If extreme erosion potential
exists, add 5-10 lbs/acre (1/4 lb/1000 sq ft) of annual ryegrass.
The inoculant is a specially grown
and very specific microorganism. Once in the soil, this organism fixes
itself to crown vetch roots, captures atmospheric nitrogen and after degradation,
supplies this essential nutrient to the plant. Most commercial sources
of crown vetch seed also provide a small plastic bag of inoculant with
directions for proper use. When using the hydro-mulching method of establishment,
it is important to super-inoculate by using about five times the specified
amount. Because the inoculant is perishable, it should be kept cool and
dry before using and applied to the seed (or tank) just prior to application.
Do not use inoculant labeled for hairy vetch.
Conventional vs. Hydromulching
The seeding mixture should be broadcast
evenly over the soil surface. This can be done by hand or with conventional
Because of the severity of certain
slopes and the need to purchase commercial mulch materials, the hydromulcher
is often used in seeding crown vetch. It is possible to include the woodfiber
mulch, fertilizer, lime, seed and inoculant in the hydromulcher tank to
be applied as a slurry. However, it is usually more successful if the seed,
inoculant and mulch are applied after the fertilizer and lime have been
worked into the surface soil.
Establishment is hastened and soil
erosion is lessened if an appropriate mulch can be applied. For small areas,
evenly spread one to two bales of clean straw per 1000 sq ft. This straw
can be held in place by use of string and stakes laced back and forth across
the slope. For larger areas, it is necessary to use a straw blower and
apply a tacking agent such as asphalt. This requires 50-60 bales (3000
lbs) of straw per acre. When woodfiber mulch is applied with the hydromulcher,
use approximately 1500-2000 pounds per acre.
Additional sources of mulch would include
cheesecloth or used tobacco canvas, woodbark or woodchips (50-75 cu yds/acre),
Seeding Into Existing Vegetation
Crown vetch may be desired on slopes
where some existing vegetation is already established. Because of the possibility
of severe erosion when the soil is disturbed, it is usually unfeasible
to kill existing vegetation and completely re-establish. If the existing
vegetation is thin, crown vetch can often be established successfully with
minimum soil preparation by seeding in late February or early March while
the soil is honeycombed from freezing and thawing. It is helpful if additional
soil scarification can be obtained by using a rake, hoe or open-disk. Good
soil-seed contact is necessary. When using this method, seed with the recommended
rate of crown vetch and apply the appropriate inoculant, lime and phosphate-potash
fertilizer. Do not use the tail fescue and nitrogen fertilizer if enough
vegetation is present for soil stabilization.
Crown vetch Maintenance
Very little maintenance is required.
Soil tests should be taken every five to ten years to determine if additional
lime, phosphate or potash is needed.
In the preparation of this publication,
the author wishes to acknowledge the professional consultation received
from Mr. Carl Wells and staff of the Department of Transportation, Frankfort.