Overview

Brief Summary

History in the United States

Spotted knapweed was introduced to North America from Eurasia as a contaminant in alfalfa and possibly clover seed, and through discarded soil used as ship ballast. It was first recorded in Victoria, British Columbia in 1883 and spread further in domestic alfalfa seeds and hay before it was recognized as a serious problem. 

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History in the United States

Spotted knapweed was accidentally introduced into North America in the late 1800s in contaminated alfalfa and clover seed and in soil used for ship ballast. In North America, plants generally live 3 to 7 years but can live up to nine years or longer and regrow from buds on the root crown. Reproduction is by seed. Individual plants are capable of producing an estimated 500-4,000 seeds per square foot per year. Most of the seed is viable at the time of dispersal and can remain viable in the soil for 5-8 years. Most seed is dispersed near the parent plant but can be moved great distances by people, livestock, wildlife, and vehicles and in soil, crop seed, and contaminated hay.

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Comprehensive Description

Description

This adventive plant is a short-lived perennial about 2-3' tall. It branches occasionally to frequently, becoming broader toward the flowering stems. The stems are ribbed and pubescent with a stiff woody texture. The alternate leaves are up to 3½" long and 1½" across; they are sparsely distributed along the stems. They are pubescent and pinnately lobed; these lobes are narrow, and the terminal lobe of each leaf is usually the longest. If the basal leaves haven't withered away, they are somewhat larger than the cauline leaves and deeply pinnatifid. Both the stems and the foliage are whitish or greyish green, although the base of the central stem often turns brown with age. The upper stems terminate in flowerheads about ¾" across. Each flowerhead consists of numerous ray florets that are pink to purplish pink, overlapping floral bracts that are greyish green, and no disk florets. Each floral bract is ovate, tapering to a black tip with coarse short bristles that are dull white to brownish black; there are several dark green veins toward the base of each bract. These bracts are appressed together. The ray florets are slender, thread-like, and branching; the outer florets are the longest and sterile. The blooming period occurs from mid-summer to early fall and lasts about 1-2 months. Each floret is replaced by an oblong achene that has a crown of short bristles on top; these bristles may fall off. The root system consists of a taproot. It often forms colonies at favorable sites, and spreads by reseeding itself. The achenes are spread to a limited extent by the wind.
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Distribution

Range and Habitat in Illinois

Spotted Knapweed is common in NE and east-central Illinois, and occasional to absent elsewhere within the state. Official records undoubtedly underestimate the distribution of this plant; it is probably still spreading. Habitats include gravelly areas along roadsides and railroads, dry waste areas and eroding slopes, pastures and abandoned fields, and vacant lots in cities. This plant occurs in disturbed areas primarily; sometimes it is found along the edges of prairie remnants near railroads. In some of the Western states, Spotted Knapweed has become a serious weed, but it is less of a problem in Illinois. This species is adventive from Eastern Europe and the Ukraine, and first appeared in the United States and Canada during the 1890's.
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Distribution in the United States

Spotted knapweed is a widely distributed species reported to occur throughout Canada and in every state in the U.S. except Alaska, Georgia, Mississippi, Oklahoma and Texas (see map). It has been designated as a noxious weed in Arizona, California, Colorado, Idaho, Minnesota, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming.

It has been identified as invasive in natural areas by eighteen organizations in twenty-six states (Arizona, California, Colorado, Connecticut, Delaware, Kentucky, Idaho, Illinois, Massachusetts, Maryland, Michigan, Minnesota, Montana, North Carolina, New Jersey, New York, Oregon, Pennsylvania, South Dakota, Tennessee, Utah, Virginia, Washington, Wisconsin, West Virginia, and Wyoming). Fifteen national parks also identify spotted knapweed as an invasive plant and a threat to natural habitats.

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Native Range

Central Europe, east to central Russia, Caucasia, and western Siberia
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Distribution and Habitat in the United States

Spotted knapweed is widely distributed in the U.S. and is reported to occur in every state in the Lower 48 except Texas, Oklahoma and Georgia. Over two dozen states and seventeen large national parks across the country recognize it as a significant invasive plant of natural areas. It invades open habitats, preferring full sun and can tolerate nutrient poor soils and harsh dry conditions.

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Origin

Europe and western Asia

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National Distribution

Canada

Origin: Exotic

Regularity: Regularly occurring

Currently: Unknown/Undetermined

Confidence: Confident

United States

Origin: Exotic

Regularity: Regularly occurring

Currently: Unknown/Undetermined

Confidence: Confident

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Physical Description

Morphology

Description

Spotted knapweed, previously known as Centaurea biebersteinii, is a biennial or short-lived perennial. Its name is derived from the spots formed by black margins on the flower bract tips. Spotted knapweed typically forms a basal rosette of leaves in its first year and flowers in subsequent years. Rosette leaves are approximately 8 inches long by 2 inches wide, borne on short stalks, and deeply lobed once or twice on both sides of the center vein, with lobes oblong and wider toward the tip. The taproot is stout and deep. Flowering stems are erect, 8 to 50 inches tall, branched above the middle, and sparsely to densely hairy. Stem leaves alternate along the stem, are unstalked, and may be slightly lobed, or linear and unlobed. Leaf size decreases towards the tip of the stem.

Flowers are purple to pink, rarely white, with 25 to 35 flowers per head. Plants bloom from June to October, and flower heads usually remain on the plant. Flower heads are oblong or oval shaped, ¼ inch wide and ½ inch across, and are single or borne in clusters of two or three at the branch ends. Leaf like bracts surrounding the base of the flower head are oval and yellow green, becoming brown near the base. The margins of these bracts have a soft spine like fringe, with the center spine being shorter than the lateral spines. The brown, oval seeds are 1/16 to 1/8 inch long, with pale longitudinal lines and a short fringe on one end.

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Description and Biology

  • Plant: herbaceous biennial or perennial plant.
  • Leaves: a basal rosette of deeply lobed leaves is produced the first year; rosette leaves are deeply lobed, stalked and about 8 in. long; stem leaves are alternate and may be slightly lobed or linear; leaves become smaller and less lobed toward the apex.
  • Flowers, fruits and seeds: flowering stems are 2/3- 4 ft. tall and branched; flowers are produced early summer after the first year and resemble tiny pineapples topped with a showy ring of pink to light purple, highly dissected petals; the bract tips of the base (“pineapple”) are dark, giving the spotted look for which the plant is named.
  • Spreads: by wind-dispersed seed.
  • Look-alikes: other knapweeds and thistles including Canada thistle (Cirsium arvense) which occurs in similar habitats in the mid-Atlantic region.

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Ecology

Habitat

Range and Habitat in Illinois

Spotted Knapweed is common in NE and east-central Illinois, and occasional to absent elsewhere within the state. Official records undoubtedly underestimate the distribution of this plant; it is probably still spreading. Habitats include gravelly areas along roadsides and railroads, dry waste areas and eroding slopes, pastures and abandoned fields, and vacant lots in cities. This plant occurs in disturbed areas primarily; sometimes it is found along the edges of prairie remnants near railroads. In some of the Western states, Spotted Knapweed has become a serious weed, but it is less of a problem in Illinois. This species is adventive from Eastern Europe and the Ukraine, and first appeared in the United States and Canada during the 1890's.
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Source: Illinois Wildflowers

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Habitat in the United States

Spotted knapweed is found at elevations up to and over 10,000 feet and in precipitation zones receiving 8 to 80 inches of rain annually. Spotted knapweed prefers well-drained, light-textured soils that receive summer rainfall, including open forests dominated by ponderosa pine and Douglas fir, and prairie habitats dominated by Idaho fescue, bluebunch wheatgrass, and needle-and-thread grass. Disturbance allows for rapid establishment and spread; however, spotted knapweed is capable of invading well managed rangelands. Spotted knapweed does not compete well with vigorously growing grass in moist areas. In seasonally dry areas, spotted knapweed's taproot allows it to access water from deep in the soil, beyond the reach of more shallowly rooted species.

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Associations

Flower-Visiting Insects of Spotted Knapweed in Illinois

Centaurea biebersteinii (Spotted Knapweed)
(also known as Centaurea maculosa; the butterfly sucks nectar, while beetle activity is unspecified; information is limited; observations are from Herms, MacRae, and Swengel & Swengel)

Butterflies
Lycaenidae: Lycaeides melissa samuelis fq (Hm, Sw)

Beetles
Buprestidae: Acmaeodera tubulus (McR)

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Faunal Associations

The nectar of the flowers attracts various long-tongued bees, butterflies, and skippers. Various insects from abroad have been introduced to control this species (and other Knapweeds), including Cyphocleonus achates (Knapweed Root Weevil), Agapeta zoegana (Yellow-Winged Knapweed Moth), Sphenoptera jugoslavica (Yugoslavian Root Beetle), and two Urophora spp. (Seedhead Flies). It is possible that some granivorous birds and small rodents eat the seeds to a limited extent. Mammalian herbivores normally shun this plant because of the bitter foliage; but they will eat it if little else is available, as occasionally occurs in overgrazed pastures. Some of the seeds of Spotted Knapweed can pass through the digestive tracts of livestock and remain viable; thus, these animals may help to distribute the seeds. People also help to spread Spotted Knapweed by means of their motor vehicles (the seedheads can become caught on the under-carriage) and through the transportation of baled hay or contaminated seed stock.
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Life History and Behavior

Reproduction

Biology and Spread

Spotted knapweed plants in North America generally live 3 to 7 years but can live up to nine years or longer. Plants regrow from buds on the root crown. Reproduction is by seed, and plants are capable of producing 500- 4,000 seeds per square foot per year. About 90% of the seeds are viable at the time of dispersal, and they can remain viable in the soil for 5-8 years. Most seeds are dispersed near the parent plant but can be transported by people, wildlife, livestock, vehicles, and in soil, crop seed, and contaminated hay. Gravel pits, soil stockpiles, powerlines, grain elevators, railroad and equipment yards are important seed distribution points. 

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage: Centaurea biebersteinii

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 0
Specimens with Barcodes: 2
Species With Barcodes: 1
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Source: Barcode of Life Data Systems (BOLD)

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Conservation

Conservation Status

National NatureServe Conservation Status

Canada

Rounded National Status Rank: NNA - Not Applicable

United States

Rounded National Status Rank: NNA - Not Applicable

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NatureServe Conservation Status

Rounded Global Status Rank: GNR - Not Yet Ranked

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Management

Prevention and Control

The most cost effective management strategy for spotted knapweed is to prevent its spread to non-infested areas. Spread by seed can be minimized by avoiding travel through infested areas by: 1) cleaning footwear, clothing, backpacks, and other items after hiking through infested areas; 2) not grazing livestock when ripe seeds are present in the flower heads; and 3) using certified weed-free hay. Individual plants can be pulled by hand when the soil is moist as long as the entire crown and taproot are removed, using a shovel or weed-popper type tool if necessary. Control of spotted knapweed infestations is very difficult and may require large investment of time, labor and materials to remove using manual and mechanical means or repeated applications of herbicides often at higher rates.

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Relevance to Humans and Ecosystems

Benefits

Cultivation

The preference is full sun, mesic to dry conditions, and a poor soil that contains gravel or clay; alkaline soil is tolerated quite well. Both the foliage and roots of this species exude a powerful toxin that can destroy the root systems of other plants. This is one of the reasons it often forms colonies.
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Risks

Ecological Threat in the United States

Spotted knapweed infests a variety of natural and semi-natural habitats including barrens, fields, forests, prairies, meadows, pastures, and rangelands. It outcompetes native plant species, reduces native plant and animal biodiversity, and decreases forage production for livestock and wildlife. Spotted knapweed may degrade soil and water resources by increasing erosion, surface runoff, and stream sedimentation. It has increased at an estimated rate of 27% per year since 1920 and has the potential to invade about half of all the rangeland (35 million acres) in Montana alone. 

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Ecological Threat in the United States

Spotted knapweed invades a wide variety of habitats including open forests, shale, serpentine and other barrens, meadows, prairies, old fields, and disturbed areas. It forms deep taproots allowing it to capture moisture and nutrients and spreads rapidly, displacing native vegetation and reducing the forage potential for wildlife and livestock.

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Names and Taxonomy

Taxonomy

Comments: Kartesz (1994) gives Centaurea maculosa auct. non Lam. as a synonym of C. biebersteinii. Hickman (1993) accepts the name C. maculosa Lam. and states that is noxious weed in disturbed places and native to Europe. Name C. bierbersteinii followed here, with C. maculosa as a synonym. Native to Europe, exotic in North America, according to Jepson Manual.

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