Central Europe, east to central Russia, Caucasia, and western
knapweed, previously known as Centaurea biebersteinii, is a biennial or short-lived perennial. Its name is derived from
the spots formed by black margins on the flower bract tips. Spotted knapweed
typically forms a basal rosette of leaves in its first year and flowers in
subsequent years. Rosette leaves are approximately 8 inches long by 2 inches
wide, borne on short stalks, and deeply lobed once or twice on both sides
of the center vein, with lobes oblong and wider toward the tip. The taproot
is stout and deep. Flowering stems are erect, 8 to 50 inches tall, branched
above the middle, and sparsely to densely hairy. Stem leaves alternate along
the stem, are unstalked, and may be slightly lobed, or linear and unlobed.
Leaf size decreases towards the tip of the stem.
Flowers are purple to pink, rarely
white, with 25 to 35 flowers per head. Plants bloom from June to October,
and flower heads usually remain on the plant. Flower heads are oblong or
oval shaped, ¼ inch wide and ½ inch across, and are single
or borne in clusters of two or three at the branch ends. Leaf like bracts
surrounding the base of the flower head are oval and yellow green, becoming
brown near the base. The margins of these bracts have a soft spine like fringe,
with the center spine being shorter than the lateral spines. The brown, oval
seeds are 1/16 to 1/8 inch long, with pale longitudinal lines and a short
fringe on one end.
Spotted knapweed infests
a variety of natural and semi-natural habitats including barrens, fields,
forests, prairies, meadows, pastures, and rangelands. It outcompetes native
plant species, reduces native plant and animal biodiversity, and decreases
forage production for livestock and wildlife. Spotted knapweed may degrade
soil and water resources by increasing erosion, surface runoff, and stream
sedimentation. It has increased at an estimated rate of 27% per year since
1920 and has the potential to invade about half of all the rangeland (35
million acres) in Montana alone.
IN THE UNITED STATES
Spotted knapweed is a widely distributed species
reported to occur throughout Canada and in every state in the U.S. except
Alaska, Georgia, Mississippi, Oklahoma and Texas (see map). It has been
designated as a noxious weed in Arizona, California, Colorado, Idaho, Minnesota,
Montana, Nebraska, Nevada, New Mexico, North Dakota, Oregon, South Dakota,
Utah, Washington, and Wyoming.
It has been identified as invasive in natural
areas by eighteen organizations in twenty-six states (Arizona, California,
Colorado, Connecticut, Delaware, Kentucky, Idaho, Illinois, Massachusetts,
Maryland, Michigan, Minnesota, Montana, North Carolina, New Jersey, New York,
Oregon, Pennsylvania, South Dakota, Tennessee, Utah, Virginia, Washington,
Wisconsin, West Virginia, and Wyoming). Fifteen national parks also identify
spotted knapweed as an invasive plant and a threat to natural habitats.
HABITAT IN THE UNITED STATES
knapweed is found at elevations up to and over 10,000 feet and in precipitation
zones receiving 8 to 80 inches of rain annually. Spotted knapweed prefers
well-drained, light-textured soils that receive summer rainfall, including
open forests dominated by ponderosa pine and Douglas fir, and prairie habitats
dominated by Idaho fescue, bluebunch wheatgrass, and needle-and-thread grass.
Disturbance allows for rapid establishment and spread; however, spotted knapweed
is capable of invading well managed rangelands. Spotted knapweed does not
compete well with vigorously growing grass in moist areas. In seasonally
dry areas, spotted knapweed's taproot allows it to access water from deep
in the soil, beyond the reach of more shallowly rooted species.
Spotted knapweed was introduced
to North America from Eurasia as a contaminant in alfalfa and possibly clover
seed, and through discarded soil used as ship ballast. It was first recorded
in Victoria, British Columbia in 1883 and spread further in domestic alfalfa
seeds and hay before it was recognized as a serious problem.
BIOLOGY & SPREAD
plants in North America generally live 3 to 7 years but can live up to nine
years or longer. Plants regrow from buds on the root crown. Reproduction
is by seed, and plants are capable of producing 500- 4,000 seeds per square
foot per year. About 90% of the seeds are viable at the time of dispersal,
and they can remain viable in the soil for 5-8 years. Most seeds are dispersed
near the parent plant but can be transported by people, wildlife, livestock,
vehicles, and in soil, crop seed, and contaminated hay. Gravel pits, soil
stockpiles, powerlines, grain elevators, railroad and equipment yards are
important seed distribution points.
The most cost effective
management strategy for spotted knapweed is to prevent its spread to non-infested
areas. Spread by seed can be minimized by avoiding travel through infested
areas; by cleaning footwear, clothing, backpacks, and other items after hiking
through infested areas; by not grazing livestock when ripe seeds are present
in the flower heads; and by using weed free hay.
Manual and Mechanical
infestations of spotted knapweed can be controlled by persistent hand-pulling
done prior to seed set. Gloves should be worn because of the possibility
of skin irritation. Because spotted knapweed can regrow from the base, care
must be taken to remove the entire crown and taproot.
of natural enemies are used as biological control agents for large infestations
of spotted knapweed. Most biocontrol techniques use insect larvae to damage
the root, stem, leaf, or flower. Two species of seed head flies, Urophora
affinis and U. quadrifasciata, are well-established on spotted
knapweed. The larvae of these species reduce seed production by as much as
50% by feeding on spotted knapweed seed heads and causing the plant to form
galls. Three moth species (Agapeta zoegana, Pelochrista medullana,
and Pterolonche inspersa) and a weevil (Cyphocleonus achates)
that feed on spotted knapweed roots have also been released.
The collective stress on the plant
caused by these insects reduces seed production and may lead to reduced competitiveness.
Biological control agents may be more effective when combined with other
control methods such as herbicides, grazing, and revegetation with desirable,
Control of spotted
knapweed infestations using three chemical herbicides (2,4-D, clopyralid,
and picloram) has been reported but is problematic. Existing plants can be
killed with 2,4-D but it needs to be reapplied yearly to control new plants
germinating from seed stored in the soil. Picloram is a more persistent herbicide
and has controlled knapweed for three to five years when applied at 0.25
lb/acre at any stage of plant growth; or with clopyralid (0.24 lb/acre) or
clopyralid (0.2 lb/acre) plus 2,4-D (1 lb./acre) applied during bolt or bud
growth stage. In the absence of desirable native grasses, longevity of control
may be increased by revegetating with competitive grasses and forbs. Picloram
may pose a risk of groundwater contamination where soils are permeable, particularly
where the water table is shallow.
grazing by sheep and goats has been found to control spotted knapweed. Burning,
cultivation, and fertilization typically are not effective on spotted knapweed
unless combined with other methods of control.
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.
For more information on
the management of spotted knapweed, please contact:
Michael Carpinelli, USDA-ARS, Burns,
Jil M. Swearingen, National Park
Service, National Capital Region, Natural Resources and Science, Center for
Urban Ecology, Washington, DC
Ruth Douglas, Charlottesville, VA
Karen Enyedy, Austin, TX
Jim Jacobs, Montana State University, Bozeman, MT
Lori Makarick, National Park Service, Grand Canyon National Park, Grand Canyon,
Phillip Moore, AHTD, AR
Monica Pokorny, Montana State University, Bozeman, MT
Leila M. Shultz, Harvard University
Herbaria, Cambridge, MA
Ed Shadrick, V3 Consultants, Woodridge, IL
Washington State Noxious Weed Control
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