Pests Under Eradication: Russian & Spotted Knapweed

RUSSIAN KNAPWEED
Centaurea repens l.

SPOTTED KNAPWEED
Centaurea maculosa Lam.

There are many varieties of knapweed considered to be noxious perennial weeds. Some knapweeds are poisonous and others just aggressively invasive, displacing native plants and desirable forage. In San Bernardino County, Russian knapweed and Spotted knapweed have been found in a few locations and are being eradicated. Both of these weeds are widely distributed in other areas and frequently are a problem in cultivated fields, roadsides, over-grazed rangelands and many other sites. The plants tolerate a wide range of conditions but do poorly in shaded areas. They are invasive but not poisonous.

Both Russian and Spotted knapweed reproduce by seed as well as sprouting from the roots. Russian knapweed has a creeping root system (rhizome) which is its primary means of increasing an existing infestation. Spotted knapweed has a single strong taproot that sends up additional flowering stalks. Each plant flowers extensively and a single Spotted knapweed plant can produce up to 18,000 seeds.

Russian knapweed is an erect plant, 1 to 3 feet high, and has many branches and various shapes of leaves. The stems are covered with grayish-white soft hairs. Sometimes the hairs shed, leaving small areas of the stems hairless. All branches are leafy and terminate in flower heads. The flower heads are small with rose-colored or lilac-colored showy flowers. Each head produces from 15 to 20 seeds each. It is unpalatable to livestock and therefore, objectionable in hay fields.

The Department has eradicated populations of Russian knapweed since October 1970 in order to prevent the spread of this noxious weed. There are currently only four locations left in the County and are located in the high desert. Treatments currently consist of manual removal of mature plants and chemical treatment of immature plants with Roundup Pro herbicide.

Spotted knapweed can have 1 or more stems that are branched and grow from 1 to 3 feet tall. Flowering heads are solitary borne at the end of the branches. The majority of the stem growth occurs in June, flowering July through August. Flowers are thistle-like, lavender to purple, sometimes white with 25 to 35 flowers per head. Each flower head has stiff bracts, marked with dark triangular spots, which give the flower head a spotted appearance.

The Department has been working on eradicating Spotted knapweed at the Big Bear airport since 1997 in order to prevent the spread of this noxious weed. Treatment currently consists of using contact herbicides.