Centaurea stoebe or Centaurea maculosa
Spotted knapweeds first North American appearance was in Victoria, British Columbia in the 1990s. It was thought to be transported in contaminated ballast water, or through seeds of alfalfa from its native range in Eurasia. It is a member of the sunflower family, Asteraceae. Knapweed is a biennial or short-lived perennial, that can either self- or cross-pollinate. It is allelopathic, releasing toxins into the soil to prevent the growth of native or competing plants. This plant is a threat to the rangelands in the Western United States.
Spotted Knapweed threatens dry prairie, oak and pine barrens, dunes, and sand ridges. It spreads rapidly in disturbed and man-made (anthropogenic) corridors, gravel pits, agricultural fields, and overgrazed pastures. The rangelands in the western United States have a severe risk of reduced diversity due to spotted knapweeds preference for well-drained soils. This plant has been seen to reduce native plant diversity, limit forage, and crop production, and in large populations, increase surface runoff and sedimentation. It is able to out-compete native species by releasing toxins that prevent them from colonizing.
Spotted knapweed prefers heavily disturbed sites with full sun and well-drained or gravelly soils. It is shade intolerant and does not grow in constant moisture. It is distributed throughout portions of the US.
Stem: Upright and branched with one or a few stems per plant.With a height 4-5 feet tall.
Leaves: This plant is biennial, which refers to its two different growth stages. The first year the plant produces rosettes that grow up to 8 inches long and 2 inches wide with deep lobes. The second-year it produces bolting stems with alternate leaves that are pinnately divided (feather-like) that may be slightly hairy. The leaves are of decreasing size as they get closer to the top with a grayish-green color.
Flowers: are thistle-like with a pinkish-purple color, rarely appearing white. There are 25 to 26 flowers per head that will bloom in June through October. The bracts of the flowers have black tips which help distinguish from other knapweed species.
Seed: Seeds are brown with pale longitudinal lines with an oval shape. They are approximately 1/16 to 1/18 of an inch long and are dispersed by rodents, livestock, and the transport of commercial hay. Each plant can produce up to 140,000 seeds per year that are viable for 7 years.
Control and Management:
Control and management are difficult in established infestations. This plant is easily spread through contaminated hay or plant fragments that get caught in car undercarriages. It is stressed that individuals using hay from road ditches exercise caution when transporting it.
Some studies have shown that a combination of livestock, such as goats and sheep, grazing and herbicide treatments are the most effective treatment. Livestock will graze on spotted knapweed despite the bitter taste. This management is most effective when the plant is in the rosette form.
Mechanical: Cutting or pulling the plant before it flowers is effective in small populations. This must be repeated for several years and may not be cost-effective for some individuals. When pulling, you should remove as much of the root as possible to decrease the chance of the plants re-sprouting. Mowing can be effective to stop the plant from seeding. Prescribed burns are another possibility for control, but it must burn hot and will damage native plants.
Warning Spotted knotweed can cause irritation when in contact with bare skin.
Chemical: Herbicides such as Aminopyralid, Picloram, and Clopyralid have shown to be effective in controlling spotted knapweed. Picloram is effective for 3 to 5 years after spraying, while Clopyralid is only effective for one. Clopyralid must be sprayed during bud growth, typically early June, for the best results.
Biological: The biological control of spotted knapweed has occurred since the 1960’s and is one of the oldest classical biological control programs in the United States and Canada. It began with seedhead flies, kapweed banded gall fly (Urophora affinis) and the UV knapweed seedhead fly (U. quadrifasiata). Other seedhead feeders include the green clearwing fly (Terellia virens) and Knapweed peacock fly (Chaetorellia acrolophia), the moth Spotted knapweed seedhead moth (Metzneria paucipunctella), and the beetles Lesser knapweed flower weevil (Larinus minutus), Blunt knapweed seedhead moth (L. obtusus) and Broad-nosed knapweed seedhead weevil (Bangasternus fausti). Root borers used in the control of spotted knapweed include the moths Sulfur knapweed root moth (Agapeta zoegana), Brown-winged knapweed root moth (Pterolonche inspersa), and Gray-winged knapweed root moth (Pelochrista medullana), and the beetles Knapweed root weevil (Cyphocleonus achates) and Bronze knapweed root borer (Sphenoptera jugoslavica).
Below are two photos of biological control agents used to manage spotted knapweed: the top photo is of a seedhead weevil; the bottom photo is of a knapweed root weevil.
Title photo: Matt Lavin (mtweed.org). Habitat/Distribution map: (www.nps.gov). Stem photo: (gobotany.org). Leaves photo: Jeremy snell (thelifeofyourtime.wordpress.com). Flower photo: Matt Lavin (flicker.com). Seed photo: Jake Lehmann (mtwow.org). Seedhead and root weevil photos: Minnesota Department of Agriculture (mda.state.mn.us).
Great Lakes Restoration https://www.youtube.com/watch?v=RZj5cSbNFlQ
University of Wisconsin Extension https://www.youtube.com/watch?v=iEQ_vo10_aQ
“Centaurea Stoebe L.” Centaurea Stoebe (spotted Knapweed): Go Botany. Go Botany, n.d. Web. 28 July 2015. <https://gobotany.newenglandwild.org/species/centaurea/stoebe/>.
“Invasive Species: Plants – Spotted Knapweed (Centaurea Stoebe).” Invasive Species: Plants – Spotted Knapweed (Centaurea Stoebe). United States Department of Agriculture, n.d. Web. 28 July 2015. <http://www.invasivespeciesinfo.gov/plants/spotknapweed.shtml>.
“Spotted Knapweed.” (n.d.): n. pag. Spotted Knapweed. Alberta Invasive Species Council, Jan. 2014. Web. 28 July 2015. <https://www.abinvasives.ca/factsheets/140610-fs- spottedknapweed.pdf>.
“Spotted Knapweed.” Weed of the Week (2007): n. pag. Spotted Knapweed. U.S. Department of Agriculture: Forest Service, 15 Mar. 2006. Web. 28 July 2015. <http://na.fs.fed.us/fhp/invasive_plants/weeds/spotted-knapweed.pdf>.
Wilson, Linda M., and Carol Bell Randall. “Biology and Biological Control of Knapweed.” Forest Health Technology Enterprise Team 3rd ser. 20001.7 (2005): 15-49. U.S. Department of Agriculture, U.S. Department of Agriculture: Forest Service, Forest Health Technology Enterprise Team, and University of Idaho, Apr. 2005. Web. 28 July 2015. <http://www.invasive.org/weeds/knapweedbook.pdf>.