Centaurea jacea

Brown knapweed is named for the brown bracts at the base of the flower.

Photo: Cindy Roche, Bugwood.org

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Description

Brown knapweed is a member of the Aster family (Asteraceae).
Native to Europe, brown knapweed is a short-lived perennial belonging to the daisy family. The species forms a basal rosette of unlobed leaves which eventually produce multiple flowering spikes 1-4 feet tall displaying summer-blooming, thistle-like flowers lavender-pink in color. (2)

Leaves
Leaves are alternate, lance-shaped soft-hairy, and unlobed with occasionally wavy margins. (1)

Flowers
Flowers bloom towards midsummer in the Hudson Valley, and are approximately an inch across. Thistle-like in shape, the flowers are subtended by brown or black dryish bracts. (2)

Introduction History

Although no sources are certain when brown knapweed was first introduced to North America, the species was likely a contaminant of forage seed, or introduced deliberately as a forage plant itself.

Ecology and Habitat

Brown 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. Brown 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. (8, 9)

Reproduction and Phenology

Brown knapweed 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 likely varies greatly per plant and within different climatic zones. Little information is available on this species’ reproductive potential but the closely related spotted knapweed is capable of producing anywhere from 65- 4,000+ seeds. (8) Brown knapweed’s seed bank is long lived and persists for at least 7 years. (9) Vectors include wind, water, mammals, birds and human activity (i.e. ATVs, hiking). (3)

Impacts of this species

An aggressive, short-lived perennial, brown 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. This species is commonly misidentified as spotted knapweed, and vice versa. (8)

Management Methods

Biological Control
The larvae of the tephritid fly, Urophora quadrifasciata, is currently in use against brown knotweed. (6)

Manual or Mechanical Control
Pulling / Digging Up: Consistent pulling by hand is effective against brown 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. (3)

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. (3)

Girdling: Not applicable

Prescribed Fire: The hotter the controlled burn, the more effective it will be at controlling brown knapweed. The plant’s deep taproot does render the species somewhat resistant to moderate blazes. (8, 9)

Torching: This is a somewhat effective, but time consuming, method of control and therefore acceptable for smaller populations of brown knapweed. (9)

Prescribed Grazing: Grazing by sheep and goats has proven effective at reducing the number of stems of brown 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. (4, 8)

Soil Tilling: Although brown 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. (4)

Mulching: Mulching with black plastic is effective at controlling small areas of brown knapweed. (8)

Solarization: See above. This method may kill seedlings and reduce the seed bank due to high summer time temperatures beneath mulch. (5)

Hot Foam Spray: Not applicable

Chemical Control
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 brown 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 brown knapweed but is not selective: the slender form of brown 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. (7, 8)

Cut Stump: Not applicable

Basal Bark: Not applicable

Hack-And-Squirt

Stem Injection: Not applicable

Pre-Emergent Spray: Not applicable

Summary of Best Managment Practices

General management overview and recommendation
Sequential herbicide treatments will provide the highest rate of control of brown knapweed. Two applications may be necessary during the first year of control, and all managed sites will need to be retreated in the following season— possibly for retreatment of mature plants and to target newly emerged seedlings. As an alternative, managers may choose to employ grazers and mowing to reduce seed set and stem count before applying herbicides. For very small infestations, hand pulling will eventually achieve eradication.

Post treatment monitoring
Depending on the management method employed, controlled populations should be revisited throughout the growing season for repeat management activities in approximately 2-3 weeks post treatment. Due to the species long-lived seed bank, all managed sites should be visited for at least 5 years to prevent germination of viable seed.

Disposal Methods
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.

Additional Information

REFERENCES

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