Spotted Knapweed (Centaurea maculosa) is a biennial or short-lived perennial that is commonly found throughout the northeast and central-north of the US. This weed typical grows on dry, low-fertility soils, and is a problem in New York pastures and disturbed areas. It is considered a widespread invasive species across New York.
Identification
Seedlings: Cotytledons are 1″ (25 mm) long, with a narrow base and rounded tip. Early leaves have a rounded tip and taper toward a short petiole. The leaves form a rosette in early plants, with multiple stems bearing alternate leaves on mature plants.
Spotted knapweed seedlings
photo by the Ohio State Weed Lab of The Ohio State University via Bugwood.org
Leaves: By the first year, the leaves of the mature plant are 15 cm long and deeply lobed. They remain arranged in a basal rosette. The mature pinnate plant leaves are gray-green and alternate. Upper leaf surfaces have tiny hairs and leaf margins have rough bristles, but leaf undersides are distinctly hairless. Leaves close to the flowers are smaller, narrow, and either less lobed or unlobed.
Mature spotted knapweed leaves on stem
photo by Bruce Ackley of The Ohio State University via Bugwood.org
Close-up of mature spotted knapweed leaf
photo by Bruce Ackley of The Ohio State University via Bugwood.org
Mature plant: Stems are slender with rough surfaces. The plant can grow to be 2-4′ (0.6-1.4 m) tall, with one or more stems. Lower leaves are largest, with leaves becoming smaller farther up the stems. The dark tips of the small, scale-like structures underneath the flower (bracts) are helpful to distinguish spotted knapweed from other knapweed species.
Close-up of mature spotted knapweed stem
photo by Rob Routledge of Sault College via Bugwood.org
Mature spotted knapweed plant
phptp by Bruce Ackley of The Ohio State University via Bugwood.org
Flowers/Fruits: Flowers emerge in mid to late summer. Each spotted knapweed plant can have as many as 200 flower heads. Each flower head is 1/4-1/2″ (9-15 mm) long and wide, and each one stands alone at the ends of the main and the other stems. All flowers are slightly flattened with the marginal flowers enlarged. Pedals of the flower are pink to purple. Although rare, some flower heads may be white. The plant’s bracts, or phyllaries, are 10-13 mm long, with fringed black tips and tiny white, hair-like features on the side.
The fruit (achene), which holds its single seed on the inside, is olive green to dark brown and 2.5-4 mm long by 1.1-1.5 mm wide. The tip of the achene is noticeably notched at one end. The end opposite the notched tip has a short bristly pappus which serves to help with seed dispersal.
Top view of spotted knapweed flower
photo by user Alan Vernon via flickr.com
Side view of spotted knapweed flower head
photo by user Matt Lavin via flickr.com
Management
Chemical control
For management in New York agricultural situations, please refer to the Cornell Crop and Pest Management Guidelines. You can also visit Cornell’s Turf Weed Identification site for chemical control suggestions.
Non-chemical control
Spotted knapweed has a vigorous taproot that can resprout if not completely removed; when pulling or digging, try to get as much of the taproot as possible and revisit the area to check for sprouts. Mowing a stand before flowering can prevent seed production, but repeat mowing will be required from late summer through fall to prevent the plant from forming additional flowers. For detailed management information, please visit the Minnesota Department of Natural Resources’ spotted knapweed profile and King County, Washington’s spotted knapweed profile.
References
Uva R H, Neal J C, DiTomaso J M. 1997. Weeds of the Northeast. Book published by Cornell University, Ithaca NY. The go-to book for weed ID in the Northeast; look for a new edition in 2022.
Cornell University’s Turfgrass and Landscape Weed ID app. Identification and control options for weeds common to turf, agriculture, and gardens in New York; uses a very simple decision tree to identify your weed.
Webpage on information for various varieties of knapweeds from North Dakota State University: Know Your Knapweeds. Provides identification information on spotted, diffuse, and Russian knapweed.
Webpage on spotted knapweed identification and control from King County, Washington.
Minnesota Department of Natural Resources’ spotted knapweed information page.