Centaurea stoebe ssp. micranthos
- Centaurea stoebe ssp. micranthos is an herbaceous biennial or perennial plant that readily invades open areas. Its name is derived from the black margins of the flower bract tips which give the flower heads a spotted look.
- A basal rosette of deeply lobed leaves is produced the first year. Rosette leaves are deeply lobed, petiolate, and approximately 8 in. (20 cm) long. Flowering stems are 1-4 ft. (0.3-1.2 m) tall and branched. Stem leaves are alternate and may be slightly lobed or linear. Leaves become smaller and less lobed toward the apex.
- The small purple to pink flowers bloom in the early summer.
- Mature seeds are shiny black and produced in erect, slender green pods which turn pale brown when mature. The copious seeds are distributed by the wind and contaminated hay.
- Ecological Threat
- Centaurea stoebe ssp. micranthos invades a wide variety of habitats including pastures, open forests, prairies, meadows, old fields, and disturbed areas. It displaces native vegetation and reduces the forage potential for wildlife and livestock. Centaurea stoebe ssp. micranthos is native to Europe and western Asia. It was accidentally introduced into North America in contaminated alfalfa and clover seed in the late 1800s.
Spotted knapweed is a native of Europe and western Asia with its distribution from central and southeast Europe, northern Italy, and central Russia (Tutin et. al., 1976).
Introduction to North America
It was accidentally introduced into North America in 1890s in alfalfa seed from Asia Minor (Mauer et al., 1987). The seeds of spotted knapweed were also believed to be released into port in Victoria British Columbia in 1893 as contaminate in ship ballast. Spotted knapweed was first collected in Victoria British Columbia in 1893 (Groh, 1944). Spotted knapweed began to spread rapidly about 50 years ago (Eddleman & Romo, 1988). It has now spread into 45 of the 50 U.S. states and is found in Canada from Nova Scotia to British Columbia. It is listed as a noxious weed in 13 states (USDA, 2005). Spotted knapweed has been a serious invader of rangelands in the Rocky Mountain region and has invaded 1.9 million ha (4.7 million acres) in Montana alone (Alpher, J., 2004).
A native of Europe, C. maculosa was accidentally introduced to North America most likely in the 1890s in alfalfa seed from Asia Minor (Maddox 1979). Spotted knapweed was collected in Victoria, B.C. in 1893 (Moore and Frankton 1974). It is assumed that soil carried on ships as ballast and unloaded in the port transported knapweed seed to this site at that time (Roche et al. 1986). Although the earliest collections of C. maculosa are from coastal areas of British Columbia and Washington, evidence of observed densities and directions of spread suggest it has moved into Washington more rapidly from the east (Roche et al. 1986). This species was abundant in Montana before it became common in Washington (Roche et al. 1986).
Approximately 1.5 million ha of pasture and rangeland in Washington, Montana, Idaho, Oregon and California are infested with knapweed, and it threatens 10.7 million ha in western Canada (Harris and Cranston 1979). Inceptisol soils are susceptible to spotted knapweed invasion in western Canada (Harris and Cranston 1979). In 1988, Alberta reported 145 sites of scattered individuals (Ali 1988). British Columbia reported that 100,000 acres were presently occupied by Centaurea species, and 2.7 million acres could potentially be infested. Diffuse knapweed accounted for 75% of that total area infested, with spotted knapweed accounting for the second largest area (Cranston, 1988). In Montana spotted knapweed occupies 4.7 million acres (Lacey 1988), the largest area in one state or province. There it appears best adapted to well-drained, light-textured soils that receive summer rainfall, including habitats dominated by Ponderosa pine and Douglas fir, as well as foothill prairie habitats with bluebunch wheatgrass, needle-and-thread, and Idaho fescue (Chicoine 1984). In Washington, spotted knapweed rates third among the state's knapweeds, with four percent of the total acreage. It is reported in 19 counties, with a total area of 10,777 ha. Ninety-two percent of the spotted knapweed is found in three northeastern counties (Roche and Roche 1988). Thirty-nine percent (4,253 ha) grows on land classified as industrial, including gravel pits, stockpiles, power lines, grain elevators, railroad, and equipment yards. These are strategic seed distribution points (Roche and Roche 1988). Seventeen percent occurs on pasture, range, and timbered range, and sixty-eight percent of this pasture-range-timbered range total is on pasture (Roche and Roche 1988). In the counties that reported few infestations, the plants were almost exclusively along roads or in urban areas. In central Washington, it is often associated with irrigation, preferring areas of high available moisture, including areas of deep soil with threetip sagebrush/fescue and roadsides receiving runoff (Roche et al. 1986). C. maculosa occurs statewide as individuals or small colonies in North Dakota, and on at least 32 sites in 10 counties in Utah. In Oregon, it grows on 121,600 acres in 23 counties. Spotted knapweed seems to occur along the more mesic margins of the range of the more widely distributed diffuse knapweed (Centaurea diffusa) (personal communication, Larry Larson)
The plants prefer high sun exposure, but a preference for a specific soil type has not been found (Ochsmann, 2001). Spotted knapweed is most often found on disturbed sites such as roads; however, in the Pacific Northwest it is known to invade pastures where it causes the overgrazing of native grasses (Lacey et al., 1990; Tyser and Key, 1988). Watson and Renny (1974) found that level of disturbance was positively correlated to knapweed density.
The biology and North American distribution of spotted knapweed are described in Reed and Hughes (1970), Moore (1972) and Watson and Renney (1974). Seeds germinate in fall and early spring. Thirty percent of seeds may be viable after eight years of burial (Davis and Fay 1991). Seedlings form rosettes which may produce 1-7 flowering stems the following spring. Marked plants in the Glacier National Park area in Montana have been observed to persist in the rosette stage for four years or longer before bolting (Tyser, personal observation, in Tyser and Key 1988). Plants may flower only once, or up to three years in succession, and perennial plants may have up to 20 flowering stems. Each plant produces 4-5 capitula in the first year (range 1 25), and 8-15 capitulas (range 1-89) in succeeding years. In central Oregon capitula are visible on the plants in late June; flowers open from mid/late July until mid August. Knapweeds are cross pollinated by insects, but are also self compatible (Lack 1982).
Estimates of the mean number of achenes (seeds) per capitula range from 9-37 in the literature (Watson and Renney 1974, Schirman 1981, Harris 1980b, Maurer et al in prep). Variations in numbers of stems, capitula, and seeds have been observed between sites and years, and were attributed to seasonal differences in precipitation (Schirman 1981). Up to 146,000 seeds per square meter have been reported using calculations based on seed capitula density and seed numbers (Schirman 1981). Dispersal is generally passive, occurring in late summer (but may continue throughout the fall, winter and spring), as seeds are shaken from drying capitula. The short pappus and weight of the seed (1.7 mg) keep dispersal distances relatively short; seeds generally fall within a 3-12 dm radius of the parent plant (Roche et al 1986). Existing populations spread outward at the perimeter and downwind (Roche et al 1986). Movement over greater distances requires transport by rodents, livestock, vehicles, or hay or commercial seed (Roche et al 1986).
Spotted knapweed seeds may germinate over a wide range of soil depths, soil moisture content and temperatures (Spears et al 1980, Watson and Renney 1974). Seed dormancy may be induced by exposure to light (Watson and Renney 1974) Seedlings emerging early in the season (April and May) have a high probability of survival and reproduction in the following year. Those emerging in June and July have a low survival rate and almost no stem production the following season (Schirman 1981). Schirman (1981) estimated that survival of only about 0.1% of seed production is required to maintain stands at observed plant densities in highly disturbed areas.
In seed sowing studies Roze et al (1984) found that rosettes and bolting plants appeared on plots sown at densities as low as 208 seeds per m2. Numbers of bolted plants were lower in plots with higher seed and rosette densities, possibly due to intraspecific competition. At low densities, the average number of capitula per plant tended to increase, although differences were not significant for the number of plots used in this study.
Lateral root sprouting in C. maculosa may result in rosettes that may remain attached to the parent for an indefinite length of time, but expansion of a colony is primarily dependent upon seed production (Tyser and Key 1988).
The competitive superiority of this species suggests preadaptation to disturbance (Roche et al 1986). The initial invasion of spotted knapweed, like other noxious weeds, is correlated highly to disturbed areas. Once a plant or colony is established though, it may invade areas that are relatively undisturbed or in good condition with gradual, broad, frontal expansion (Tyser and Key 1988, Lacey et al 1991). This invasion is associated with a decline in the frequency of some species and a decline in species richness overall (Tyser and Key 1988). Widespread invasion of spotted knapweed often results from overgrazing. It has a low palatability, as it contains a bitter compound cnicin (Roche 1990). As the native grasses and forbes are continually eaten, the food reserves of their roots are depleted, and they are less able to compete with the knapweed (Roche 1988). The knapweed is highly adept at capturing available moisture and nutrients, and it quickly spreads, choking out other vegetation (Roche et al 1986). As the network root system of the native species is lost, replaced by the taproot of the knapweed, the water storage capacity of the soil decreases (Roche 1988), and soil erosion increases (French and Lacey 1983, in Tyser and Key 1988). Lacey et al (1988) compared two elements of erosion on plots that were 90% bunchgrasses to plots that were 85% spotted knapweed. The average total runoff from the bunchgrass plots was 23%, and the average sediment yield was 39 pounds per acre. The total runoff from the knapweed plots averaged 36%, and the total sediment yield averaged 114 pounds per acre.
Although the quality of the land being invaded does not seem to be able to exclude spotted knapweed, it probably does effect the rate of spread of the infestation. In a study conducted in Glacier Nation Park, the front of a C. maculosa colony advanced by 10 meters in three years ((Tyser and Key, 1988). In another study, also conducted in Montana, the front of a colony advanced 14 meters in four years (Lacey et al 1991).
Spotted knapweed reproduces mostly by seed but may spread by some lateral root sprouting that produces new rosettes (Watson and Renney, 1974). Plants produce on average 1,000 seeds per plant or anywhere from 5,000-40,000 seeds/m2 (Sheley et al., 1998). The seeds are dispersed by vehicles and hay that is transported by farm equipment. Seeds are also dispersed by wind and animals. Jensen and Six (2006) have shown that the elaiosomes on the seeds of spotted knapweed may increase dispersal by ants. Upon maturation the seeds remain viable for eight years (Davis et al., 1993).
Spotted knapweed’s invasive behavior is also increased by the allelopathic chemical (-)- catechin. This chemical is secreted by the roots of spotted knapweed and has been shown to suppress germination and root and shoot differentiation of some grasses and row crops such as wheat (Bais et al., 2002). It has been shown in vitro that spotted knapweed begins to secrete lethal levels of (-)- catechin within 2-3 weeks of germination (Weir et al., 2003).
Recommended Removal Methods
Spotted knapweed can be removed by hand-pulling or digging with a spade in less dense areas (0 to 10 plants per m2) during volunteer work days. In denser areas (>10 plants per m2) repeated spot-burning with trained individuals is more effective and efficient.
Gloves should be worn when hand-pulling due to the plant’s allelopathic compounds that are thought to be carcinogenic in large quantities.
Swath or Spot-burning
Spot-burning methods follow those described for baby’s breath also on the website.
Swath burning is conducted with a six-wheeled Polaris® ATV that has a rear bed. The bed of the ATV has been retrofitted with a “weed burner,” constructed of two, 1.8 m (six ft) lengths of steel angle iron that are placed 1.2 m (four ft) apart, bolted to the ATV’s bed, and extend back from the bed of the ATV. Two metal conduit rectangles are bolted between the two arms of the angle iron and clips are attached to the long-side of the rectangles that hold up to eight propane torches, four on each rectangle. The torches are staggered and the rectangles are rotated and tightened to an angle that allows the greatest surface area to be covered. The torch bells are placed at a height of 0.46 m (1.5 ft) above the ground, not to exceed 0.61 m (2 ft) above. (The maximum flame temperature of 2,050°F+ occurs 15 to 31 cm (6 to 12 in) from torch bell tip.) The torches are connected to four 50 lb (23 kg) propane tanks that are secured in the bed of the ATV. The torches are ignited, and the ATV is driven slowly (5 mph or less) in a crop row pattern back and forth over the area. The first four torches pre-heat and burn the plants, while the second set cover any missed areas and prolong the duration and intensity of heat per unit area.
Everyone participating in swath-burning is outfitted in the safety gear described for spot-burning, including two-way radios.
Mop-up area completely after torching. Allow torches to cool down before disconnecting from propane tanks and putting them back into PVC safety cases. Follow disconnection, storage, and maintenance suggestions in the Red Dragon Torch Operating Instructions and Parts Manual.
Mechanical removal of spotted knapweed involves the use of a Weed Popper®. This tool consists of a row of spikes at the end of a spring-loaded pedal. The user inserts the spikes into the ground at the base of the plant, steps on the square foot pad on the top of the pedal and pushes down lightly (the same movement as when using a shovel). In one motion, the spikes move forward and up, thus uprooting the plant, and a plate moves forward that pushes the plant off of the spikes.
When removing knapweed mechanically, individuals generally spread out in a line and walk through an area to ensure complete coverage.
Zouhar, Kris. (2001, July). Centaurea maculosa. Fire Effects Information System, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory 
King County Noxious Weed Control Program 
University of California, Jepson Flora Project 
University of Alaska Anchorage 
USDA NRCS PLANTS 
- Alpher, J. (2004). Wicked weed of the west. Smithsonian. 2005.
- Bais, H. P., Walker, T. S., Stermitz, F. R., Hufbauer, R. A., and Vivanco, J. M. (2002). Enantiomeric-dependent phtotoxic and antimicrobial activity of (+/-) catechin. A rhizosecreted racemix mixture from spotted knapweed. Plant Physiology. 128:1173-1179.
- Boggs, K. W., and Story, J. M. (1987). The population age structure of spotted knapweed (Centaurea maculosa) in Montana. Weed Science. 35:194-198.
- Davis, E. S., Fay, P. K., Chicoine, T. K., and Lacey, C. A. (1993). Persistence of spotted knapweed (Centaurea maculosa) seed in soil. Weed Science. 41:57-61.
- Eddleman, L. E., and Romo, J. T. (1988). Spotted knapweed germination response to stratification, temperature, and water stress. Canadian Journal of Botany. 66:653-657.
- Groh, H. (1944). Canadian weed survey. 2nd annual report. Ottawa, ON.
- Jensen, J. M., and Six, D. L. (2006). Myrmecochory of the exotic plant, Centaurea maculosa; A potential mechanism enhancing invasiveness. Environmental Entomology. 35(2): 326-331.
- Lacey, J., Husby, P., and Handl, G. (1990). Observations on spotted knapweed and diffuse knapweed invasion into ungrazed bunchgrass communities in western Montana. Rangelands. 12: 30-32.
- Ochsmann, J. (2001). On the taxonomy of spotted knapweed (Centaurea stoebe L.). Proceedings from The First International Knapweed Symposium of the Twenty-First Century. March 15-16, 2001. Coer d' Alene, Idaho. 33-41.
- Sheley, R. L., Jacobs, J. S., and Carpinelli, M. F. (1998). Distribution, biology, and management of diffuse (Centaurea diffusa) and spotted knapweed (Centaurea maculosa). Weed Technology. 12: 353-362.
- Tutin, T. G., Heywood, N. A., Burges, D. M., Moore, D. M., Valentine, D. H., and Walters, S. M. (1976). In Webb D. A. (Ed.), Flora Europea 4. Plantaginaceae to Compositae (and Rubiaceae). Cambridge. Cambridge University Press.
- Tyser, R. W., and Key, C. H. (1988). Spotted knapweed in natural area fescue grasslands: An ecological assessment. Northwest Science. 62(4):151-159.
- United States Department of Agriculture. (2007). Plants Database. Centaurea stoebe subsp. micranthos. Retrieved 8/15, 2007, from http://plants.usda.gov/java/profile?symbol=CESTM
- Watson, A. K., and Renney, A. J. (1974). The biology of Canadian weeds. Centaurea diffusa and C. maculosa. Canadian Journal of Plant Science. 54: 687-701.
- Weir, T. L., Bais, H. P., and Vivanco, J. M. (2003). Intraspecific and interspecific interactions mediated by a phytotoxin, (-) catechin, secreated by the roots of Centaurea maculosa (spotted knapweed). Journal of Chemical Ecology. 29(11): 2397-2410.
For Habitat and Reproduction sections: Element Stewardship Abstract for Centaurea maculosa; Teresa Mauer, Mary J Russo (Revision), Margaret Evans (Revision)
For Recommended Removal Methods section: Recommended Removal Methods for Spotted Knapweed (Centaurea maculosa); Jack McGowan-Stinski, 2005.
Cminteer 16:35, 23 January 2008 (EST)
Images from Bugwood.org