Spotted Knapweed

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Spotted Knapweed (Centaurea Maculosa) Eradication Project: Spotted knapweed, an "A-rated" pest, is a biennial or short-lived perennial weed of very limited distribution in eastern Fresno County. It has been detected in the Sierra Mountain range in the eastern portion of Fresno County. Spotted knapweed grows and spreads very rapidly in disturbed areas. There are some indications that it releases chemical substances that inhibit the growth of adjacent plants. Spotted knapweed can reach 1 to 3 feet tall. It has solitary flower heads at the end of branches. The pinkish-purple flower heads have stiff bracts at the base with a dark comblike fringe giving it a spotted appearance. Flowering occurs June to October.

Spotted Knapweed Flower Head With Dark Tip Bract Rosette Stage

Spotted Knapweed Flower Head With Dark Tip Bract (Left) Rosette Stage (Right)

Spotted napweed was introduced from Eurasia. It was detected in Fresno County on August 1, 1986, along Highway 168, east of the Big Creek turnoff at an elevation of approximately 6,000 feet. The infestation was treated and subsequent surveys were negative. On August 7, 2001, spotted knapweed was detected by U.S. Forest Service (USFS) personnel on Dinkey Creek Road east of Glen Meadow at an elevation of approximately 6,000 feet. Five adult plants in full bloom were hand pulled. Surveys since 2001 have been negative. The latest find was on August 22, 2005. The infestation site was approximately 1/2 acre at an elevation of 5,664 feet. All stages of growth were present. On August 23, blooming plants were hand pulled. The remaining rosettes were treated. Follow-up surveys in 2006 and 2007 found single flowering plants, which were hand pulled. Subsequent surveys in 2008 through 2010 were negative.

Shaver Lake infestation (August 22nd, 2005)

Spotted knapweed grows and spreads very rapidly in disturbed areas. Once established spotted knapweed produces a very stout, and deep taproot and becomes much difficult to control. There are some indications that it releases chemical substances that inhibit the growth of adjacent plants.

Spotted knapweed has the potential to become the yellow starthistle of the high Sierra's. Native vegetation, and associated wildlife, would be negatively impacted. Timber harvest activities would be delayed or curtailed entirely. Infestations of meadows and grazing areas would have a negative impact the cattle industry. Due to the limited distribution of spotted knapweed early detection and treatment is critical.

Each year, survey for spotted knapweed will begin the last week of July in the Shaver Lake area.