NATIVE
RANGE
Iran to Japan (according to Hortus 3rd. ed.)
DESCRIPTION
Silk tree, also known as
mimosa, or silky acacia, is a small to medium-sized tree that can grow up
to 20-40 feet tall. The bark is light brown, nearly smooth, and generally
thin with lens shaped areas along the stem. The attractive fern-like
leaves of mimosa are finely divided, 5-8 inches long by about 3-4 inches
wide, and alternate along the stems. Silk tree has showy and fragrant pink
flowers, about 1½ inches long, that resemble pom-poms and are arranged
in panicles at the ends of branches. Fruits are flat, straw-colored
pods about 6 inches long containing light brown oval-shaped seeds about ½ inch
in length. Pods ripen in August to September and begin to disintegrate
soon after, but remain on the trees into winter.
ECOLOGICAL THREAT
Because silk tree
can grow in a variety of soils, produce large seed crops, and resprout when
damaged, it is a strong competitor to native trees and shrubs in open areas
or forest edges. Dense stands of mimosa severely reduce the sunlight
and nutrients available for other plants.
DISTRIBUTION
IN THE UNITED STATES
Silk tree is naturalized from New Jersey to Louisiana
and in California.
HABITAT IN THE UNITED STATES
Silk tree
takes advantage of disturbed areas, often spreading by seed from nearby ornamentals
or from contaminated fill dirt. It prefers full sun and is often seen
along roadsides and open vacant lots in urban/suburban areas. Silk
tree can tolerate partial shade but is seldom found in forests with full
canopy cover, or at higher elevations (above 900 m or 3,000 ft), where cold
hardiness is a limiting factor. It can, however, become a serious problem
along riparian areas, where it becomes established along scoured shores and
where its seeds are easily transported in water. Like many successful
exotics, it is capable of growing in a wide range of soil conditions.
BACKGROUND
Silk tree was introduced
to the U.S. in 1745. Silk tree continues to be a popular ornamental
because of its fragrant and showy flowers.
BIOLOGY & SPREAD
Silk tree reproduces
both vegetatively and by seed. Silk tree seeds have impermeable seed
coats that allow them to remain dormant for many years. One study showed
that 90% of the seeds were viable after five years and, for another species
of mimosa, a third of its seeds germinated after 50 years in open storage. Seeds
are mostly dispersed below or around the parent plant, but can be dispersed
further by water. Silk trees grow rapidly under good conditions but
are short-lived and have weak, brittle wood. If cut or top-killed,
trees resprout quickly and sprouts can grow over three feet in a season.
MANAGEMENT OPTIONS
Silk tree can be
controlled using a variety of mechanical and chemical controls.
Mechanical
Trees can be cut at ground
level with power or manual saws. Cutting is most effective when trees
have begun to flower to prevent seed production. Because mimosa spreads
by suckering, resprouts are common after treatment. Cutting is an initial
control measure and will require either an herbicidal control or repeated
cutting for resprouts.
Girdling is effective on large trees where the
use of herbicides is impractical. Using a hatchet, make a cut through
the bark encircling the base of the tree, approximately six inches above
the ground. Be sure that the cut goes well below the bark. This
method will kill the top of the tree but resprouts are common and may require
a follow-up treatment with a foliar herbicide. Hand pulling will effectively
control young seedlings. Plants should be pulled as soon as they are
large enough to grasp, but before they are old enough to flower. Seedlings
are best pulled after a rain when the soil is loose. The entire root
must be removed since broken fragments may resprout.
Chemical
Silk tree seedlings and small
trees can be controlled by applying a 2% solution of glyphosate (e.g., Roundup®)
or triclopyr (e.g., Garlon) and water plus a 0.5% non-ionic surfactant to
thoroughly wet all leaves. Systemic herbicides such as glyphosate and
triclopyr can kill entire plants because the chemicals travel through a plant
from the leaves and stems to the actively growing roots, where they prevent
further cell growth. Use a low pressure and a coarse spray pattern
to reduce damage from spray drift on non-target species. Use
caution when applying these products, as glyphosate is a non-selective herbicide
that may kill non-target plants that are only partially contacted. Triclopyr
is a selective herbicide for many broadleaf plant species. Triclopyr is a
selective herbicide for many broad-leaved plant species and should be considered
for sites where native or other desirable grasses are meant to be conserved.
The cut-stump and basal bark herbicidal methods
should be considered when treating individual trees or where the presence
of desirable species preclude foliar application. Stump treatments
can be used as long as the ground is not frozen. Horizontally cut stems
at or near ground level. Immediately apply a 25% solution of glyphosate
or triclopyr and water to the cut stump making sure to cover the outer 20%
of the stump. Basal bark applications are effective throughout the
year as long as the ground is not frozen. Apply a mixture of 25% triclopyr
and 75% horticultural oil to the base of the tree trunk to a height of 12-15
inches from the ground. Thorough wetting is necessary for good control;
spray until run-off is noticeable at the ground line.
USE PESTICIDES WISELY: ALWAYS READ THE ENTIRE PESTICIDE LABEL CAREFULLY, FOLLOW ALL MIXING AND APPLICATION INSTRUCTIONS AND WEAR ALL RECOMMENDED PERSONAL PROTECTIVE GEAR AND CLOTHING. CONTACT YOUR STATE DEPARTMENT OF AGRICULTURE FOR ANY ADDITIONAL PESTICIDE USE REQUIREMENTS, RESTRICTIONS OR RECOMMENDATIONS.
NOTICE: MENTION OF PESTICIDE PRODUCTS ON THIS WEB SITE DOES NOT CONSTITUTE ENDORSEMENT OF ANY MATERIAL.
CONTACTS
For more information on
the management of silk tree, please contact:
- Kris Johnson, Great Smoky Mountains National
Park, Gatlinburg, TN
SUGGESTED ALTERNATIVE PLANTS
Many
small to medium-sized trees make excellent alternatives to silk tree. A
few examples include serviceberry (Amelanchier arborea), redbud
(Cercis canadensis), flowering dogwood (Cornus florida),
river birch (Betula nigra), fringe tree (Chionanthus virginicus),
American holly (Ilex opaca), and sweetgum (Liquidambar styraciflua). Check
with the native plant society in your state for plant recommendations for
your particular area.
- OTHER LINKS
AUTHORS
Tom Remaley, Great Smoky Mountains National
Park, Gatlinburg, TN
EDITORS
Jil M. Swearingen, National Park Service,
Washington, DC
Larry Morse, The Nature Conservancy, Arlington,
VA
PHOTOGRAPHS
Tom Remaley, Great Smoky Mountains National
Park, Gatlinburg, TN
REFERENCES
Bonner, F.T., M.S. Hooda, D.P. Singh. 1992. Moisture
determination on seeds of honey locust and mimosa. Tree Plant Note, USDA
Forest Service 43(3):72-75.
Bransby, D.I., S.E. Sladden, G.E. Aiken. 1992. Mimosa
as a forage plant: a preliminary evaluation. Proceedings of the Forage Grasslands
Conference. Georgetown, Texas; American Forage and Grassland Council 1:28-31.
Gleason, H.A., A. Cronquist. 1991. Manual
of vascular plants of northeastern United States and adjacent Canada. 2nd
ed. The New York Botanical Garden, 910.
Hartel, P.G., B.L. Haines. 1992. Effects
of potential plant CO2 emissions of bacterial growth in the rhizosphere.
Journal of Soil Biological Biochemistry and Science 24(3).
Panizzi, A.R., F. Slansky, Jr. 1991. Suitability
of selected legumes and the effect of nymphal and adult nutrition in the
southern green stink bug (Hemiptera: Heteroptera: Pentatomidea). Journal
of Econ-Entomology 84(1):103-113.
Plant Conservation
Alliance, Alien Plant Working Group.
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