Spotted Knapweed

(Centaurea stoebe)

Family: Asteraceae

Life cycle: Biennial of short lived perennial

Roots: Tap root

Stems: Several branched, upright stems

Flowers: are purple to pinkish with black tipped bracts

Spotted knapweed seeds can remain viable for 7-10 years

Note: This weed has allelochemicals or allelopathies that inhibit other plant species from growing within their vicinity.  When hand-pulling make sure to wear gloves.

Effective control methods: 

Cultivation/ hand-pulling, herbicide, bio-controls

Hand pulling small infestation works very well if the majority of the taproot is removed.

Using a selective herbicide will greatly decrease knapweed and increase grasses, which will help out-compete knapweed infestation.

Cultivation  works well in controlling small infestations of spotted knapweed

Mowing will reduce seed production very well for that year but will not kill the plant.  The plant will adapt to the height of the mower blade and resume seed production at that height.

Bio-controls:  Root weevils (Cyphocleonus achates), seed head weevils (Larinus minutes) and root bores (Agapeta zoegana) work very well in conjunction with one another and on their own to control dense patches of knapweed.  Control may take some time to see results but this is a very affordable, hands off, environmentally friendly method.  (Contact Weedbusters at www.weedbustersbiocontrol.com. ) Satellite infestations should be treated with herbicide and infestation needs to be monitored in cases where *successional degradation may occur, in this case, most often, cheatgrass Bromus tectorum.

*successional degradation – replaces one unwanted species with another.