Spotted knapweed
(Centaurea stoebe)
Biological Category
Plants
NY Legal Status
Prohibited
Species Type
Other Herbaceous
DESCRIPTION
Spotted knapweed is a member of the Aster family (Asteraceae).
Native to eastern Europe, spotted knapweed is a short-lived perennial belonging to the daisy family. The species forms a basal rosette of deeply divided blueish-green leaves which eventually produce a flowering spike displaying summer-blooming, thistle-like flowers lavender-purple in color.
Note: Research for these pages was generated from citations listed. References for specific statements available upon request.
Leaves: Leaves are hairy, covered in conspicuous shiny specks, alternately arranged, and deeply lobed, particularly within the plant’s basal rosette.
Flowers: Flowers bloom towards midsummer in the Hudson Valley, and are approximately an inch across. Thistle-like in shape, the flowers are subtended by green, dryish bracts, each with a black tip, from which the species draws its name.
First recorded in 1893, spotted knapweed was likely a contaminant of alfalfa seed, transported to North America via its native range in eastern Europe.
Knapweed will invade any open, disturbed habitat, from parking lots to old fields to roadsides. Although the species requires moisture at germination, it does not tolerate consistently moist conditions well and is a much better competitor on dry sites. Spotted knapweed will not colonize closed canopy environments, having high light requirements, although it can germinate in deep shade and will exploit and persist in canopy gaps and forest openings.
Spotted knapweed generally becomes productive around 5-7 years of age. Flowers are obligate out-crossers, limiting the fecundity of isolated populations but potentially increasing the competitiveness of the species as a whole due to increased genetic diversity. Seed production varies greatly per plant and within different climatic zones: a single plant is capable of producing anywhere from 65- 2,000+ seeds. The external pericarp of spotted knapweed seed is extremely thick and tough: evidence suggests the seedbank of this species can remain dormant but viable for at least five years. Vectors include wind, water, mammals, birds and human activity (such as ATVs and hiking).
A highly aggressive, short-lived perennial, spotted knapweed is a conspicuous invader of old fields and waste places in the Hudson valley, quickly outcompeting native species and homogenizing the vegetative community. The early formation of deep taproots may help the species acquire nutrients more rapidly and increase the aridity of soils, quickly altering habitat characteristics for other species present. Additionally, spotted knapweed is allelopathic and interferes with the germination and establishment of other plants.
As of this writing at least 13 insects from spotted knapweed’s native Eurasian range are in use against this invasive species. Several attack the species’ flower heads, the rest, its root system. These insects are most effective when feeding in tandem on a population of spotted knapweed.
Pulling / Digging Up: Consistent pulling by hand is effective against spotted knapweed, so long as all the species’ extensive taproot is removed. This is a very time and labor-intensive effort and useful only for small populations.
Mowing: Although consistent mowing prior to or at flowering onset does diminish flowering and seed set somewhat it is not a very effective method of control.
Girdling: Not applicable
Prescribed Fire: The hotter the controlled burn, the more effective it will be at controlling spotted knapweed. The plant’s deep taproot does render the species somewhat resistant to moderate blazes.
Prescribed Grazing: Grazing by sheep and goats has proven effective at reducing the number of stems of spotted knapweed, particularly seedlings and young plants. This is especially effective when combined with the use of other biological controls such as the knapweed root or flower weevil.
Soil Tilling: Although spotted knapweed will not survive consistent annual tiling, this is not an acceptable method of management in natural areas. It can be utilized effectively on a small scale in cultivated fields or gardens.
Mulching: Mulching with black plastic is effective at controlling small areas of spotted knapweed.
Solarization; See above. This method may kill seedlings and reduce the seed bank due to high summer time temperatures beneath mulch.
Hot Foam Spray: Not applicable
The pesticide application rates and usage herein are recommendations based on research and interviews with land managers. When considering the use of pesticides, it is your responsibility to fully understand the laws, regulations and best practices required to apply pesticides in a responsible manner. At times, the pest you seek to treat may not be on a pesticide label, requiring a 2ee exemption from NYSDEC. Always thoroughly read the label of any pesticide and consult the NYSDEC or a licensed pesticide applicator with questions.
Foliar Spray: 2-4D, Clopyarlid, Dicamba and picloram are all effective against spotted knapweed when applied during active growth. Dicamba and 2-4D provide inconsistent control and must be applied for at least two seasons to control mature plants. Glyphosate is effective against spotted knapweed but is not selective: the slender form of spotted knapweed makes it very difficult to avoid non-target plants when performing management via foliar spray. Repeat applications of all herbicides will be necessary to control emerging seedlings for at least five years after initial treatment. Always read and follow all directions on the label. Follow-up treatment will be necessary the following season.
Cut Stump: Not applicable
Basal Bark: Not applicable
Hack-And-Squirt
Stem Injection: Not applicable
Pre-Emergent Spray: Not applicable
Waste material must be thoroughly dried and can then be composted, burned, or disposed of in a landfill. Taproots have the potential to re-sprout. All viable seed must be bagged and disposed of.
The pesticide application rates and usage herein are recommendations based on research and interviews with land managers. When considering the use of pesticides, it is your responsibility to fully understand the laws, regulations and best practices required to apply pesticides in a responsible manner. At times, the pest you seek to treat may not be on a pesticide label, requiring a 2ee exemption from NYSDEC. Always thoroughly read the label of any pesticide and consult the NYSDEC or a licensed pesticide applicator with questions.
Foliar Spray: 2-4D, Clopyarlid, Dicamba and picloram are all effective against spotted knapweed when applied during active growth. Dicamba and 2-4D provide inconsistent control and must be applied for at least two seasons to control mature plants. Glyphosate is effective against spotted knapweed but is not selective: the slender form of spotted knapweed makes it very difficult to avoid non-target plants when performing management via foliar spray. Repeat applications of all herbicides will be necessary to control emerging seedlings for at least five years after initial treatment. Always lead and follow all directions on the label. Follow-up treatment will be necessary the following season.
Cut Stump: Not applicable
Basal Bark: Not applicable
Hack-And-Squirt
Stem Injection: Not applicable
Pre-Emergent Spray: Not applicable
Minnesota Department of Agriculture
Davis, Edward S.; Fay, Peter K. 1989. The longevity of spotted knapweed seed in Montana. In: Fay, Peter K.; Lacey, John R., eds. Proceedings: knapweed symposium; 1989 April 4-5; Bozeman, MT. Bozeman, MT: Montana State University: 67-72. [37797]
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