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NOXIOUS WEED
CO, NM, UT

Centaurea diffusa
Centaurea diffusa

Rhaponticum repens.  Synonyms: Centaurea repens, Acroptilon repens. (Russian Knapweed)    
Asteraceae (Sunflower Family)

Foothills, montane.  Meadows, disturbed areas.  Summer.
Disappointment Valley, June 26, 2011.

Rhaponticum repens spreads widely from black roots and forms dense colonies. The plant is highly poisonous to horses and is generally avoided by grazing animals, thus allowing it to spread even more widely. When Rhaponticum repens is in small patches, it can be controlled fairly well simply by pulling the mature plants when the ground is fairly moist. 

Although it may be noxious, Rhaponticum repens is very attractive with numerous bright flowers in plants 10-36" tall  The plant is a long-lived perennial introduced into the U.S. at the end of the 19th century. Rhaponticum repens occurs in cultivated fields, pastures, and along roadsides and other disturbed areas from the lowest altitudes to the low foothills.

According to the Flora of North America,

Acroptilon repens is a serious weed pest, especially in the western United States. It is a strong competitor in infested areas, often forming dense colonies, and has allelopathic effects on other plants growing nearby. It is very difficult to control or eradicate once it becomes established. It reproduces vigorously from seed and spreads from adventitious buds borne on deep-seated runner roots. Root fragments readily regenerate as new individuals after cultivation. In addition, Russian Knapweed is very poisonous to horses, causing neurological symptoms. Because of its bitter taste, it is usually avoided by grazing animals, and consequently it tends to spread  when more palatable plants are consumed.

Linnaeus named this species Centaurea repens in 1753, De Candolle renamed it Acroptilon repens in 1838, and Hidalgo recently renamed it Rhaponticum repens.

Centaurea diffusa

Rhaponticum repens.  Synonyms: Centaurea repens, Acroptilon repens. (Russian Knapweed)    
Asteraceae (Sunflower Family)

Foothills, montane.  Meadows, disturbed areas.  Summer.
Disappointment Valley, June 26, 2011.

Rhaponticum repens has no ray flowers, only disk flowers.

Outer phyllaries are broad and mostly green but with a papery white margin. Inner phyllaries are long and narrow often with a fringed top.

Range map © John Kartesz,
Floristic Synthesis of North America

State Color Key

Species present in state and native
Species present in state and exotic
Species not present in state

County Color Key

Species present and not rare
Species present and rare
Species extirpated (historic)
Species extinct
Species noxious
Species exotic and present
Native species, but adventive in state
Eradicated
Questionable presence

Rhaponticum repens

Range map for Rhaponticum repens