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''Centaurea stoebe'', the spotted knapweed or panicled knapweed, is a species of ''
Centaurea ''Centaurea'' () is a genus of over 700 species of herbaceous thistle-like flowering plants in the family Asteraceae. Members of the genus are found only north of the equator, mostly in the Eastern Hemisphere; the Middle East and surrounding reg ...
'' native to
eastern Europe Eastern Europe is a subregion of the European continent. As a largely ambiguous term, it has a wide range of geopolitical, geographical, ethnic, cultural, and socio-economic connotations. The vast majority of the region is covered by Russia, whic ...
, although it has spread to
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the C ...
, where it is considered an
invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
. It forms a
tumbleweed A tumbleweed is a structural part of the above-ground anatomy of a number of species of plants. It is a diaspore that, once mature and dry, detaches from its root or stem and rolls due to the force of the wind. In most such species, the tumbl ...
, helping to increase the species' reach, and the seeds are also enabled by a feathery pappus.


Description

''Centaurea stoebe'' is a
biennial Biennial means (an event) lasting for two years or occurring every two years. The related term biennium is used in reference to a period of two years. In particular, it can refer to: * Biennial plant, a plant which blooms in its second year and th ...
or short-lived
perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...
plant, and it usually has a stout taproot and pubescent stems when young. It has pale and deeply-lobed
leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...
covered in fine short hairs. First-year plants produce a basal rosette, alternate, up to long, deeply divided into lobes. It produces a stem in its second year of growth. Stem leaves are progressively less lobed, getting smaller toward the top. The stem is erect or ascending, slender, hairy and branching, and can grow up to tall. Protruding from black-tipped
sepals A sepal () is a part of the flower of angiosperms (flowering plants). Usually green, sepals typically function as protection for the flower in bud, and often as support for the petals when in bloom., p. 106 The term ''sepalum'' was coined ...
, the flower blooms from July to September. The
flower head A pseudanthium (Greek for "false flower"; ) is an inflorescence that resembles a flower. The word is sometimes used for other structures that are neither a true flower nor a true inflorescence. Examples of pseudanthia include flower heads, compos ...
is wide, with vibrant pink to lavender (or more rarely white) disc flowers around long; the
corolla Corolla may refer to: *Corolla (botany), the petals of a flower, considered as a unit *Toyota Corolla, an automobile model name *Corolla (headgear) A ''corolla'' is an ancient headdress in the form of a small circlet or crown. The fruit is an
achene An achene (; ), also sometimes called akene and occasionally achenium or achenocarp, is a type of simple dry fruit produced by many species of flowering plants. Achenes are monocarpellate (formed from one carpel) and indehiscent (they do not ope ...
about long with a short, bristly pappus. These are primarily dispersed by wind, sometimes assisted by the plant's
tumbleweed A tumbleweed is a structural part of the above-ground anatomy of a number of species of plants. It is a diaspore that, once mature and dry, detaches from its root or stem and rolls due to the force of the wind. In most such species, the tumbl ...
mode.


Taxonomy

''C. stoebe'' was formerly known as ''C. maculosa''. Two cytotypes of ''C. stoebe'' exist which have been considered as different species by some taxonomists. The diploid form of the plant is now called ''C. stoebe'' L. spp. ''stoebe'', while the tetraploid is known as ''C. stoebe'' L. spp. ''micranthos'' or by some
taxonomists In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...
as ''C. biebersteinii DC''. The names of ''C. stoebe'' and ''C. biebersteinii'' may have been accidentally switched early in the plant's taxonomic history. The
common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contrast ...
''spotted knapweed'' refers to the black-tipped sepals that look like spots.


Distribution and habitat

The species is native to
eastern Europe Eastern Europe is a subregion of the European continent. As a largely ambiguous term, it has a wide range of geopolitical, geographical, ethnic, cultural, and socio-economic connotations. The vast majority of the region is covered by Russia, whic ...
. It is also an
invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
in southern Canada, and northwestern
Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
, and nearly every state in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territo ...
; it has thrived in the western US in particular, much of which has a
dry climate A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most a ...
similar to the
Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...
. The plant grows on stream banks, pond shorelines, sand prairies, old fields and pastures, roadsides, along railroads, and in many open and disturbed areas. It is relatively tolerant of cold and reaches into alpine environments.


Ecology

''Centaurea stoebe'' has been introduced to
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the C ...
, where it is considered an invasive species in much of the western US and Canada. In 2000, ''C. stoebe'' occupied more than in the US. Spotted knapweed is a
pioneer species Pioneer species are hardy species that are the first to colonize barren environments or previously biodiverse steady-state ecosystems that have been disrupted, such as by wildfire. Pioneer flora Some lichens grow on rocks without soil, so ...
found in recently disturbed sites or openings, growing in dense stands. As such, human disturbance is a major cause of infestations. It readily establishes itself and quickly expands in places of human disturbance such as industrial sites, along roadsides, and along sandy riverbanks. Once established, it also has the potential to spread into undisturbed natural areas. Because cattle prefer the native bunchgrass over knapweed,
overgrazing Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, game reserves, or nature res ...
can often increase the density and range of knapweed infestations. This species is believed to have several traits that contribute to its extreme competitive ability: # A tap root that sucks up water faster than the root systems of its neighbors. # Rapid dispersal through high seed production. # Low palatability, making it less likely to be eaten. # Its purported
allelopathy Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have ben ...
allows it to thrive by stunting the growth of neighboring plants.


History in North America

Spotted knapweed likely spread to North America in an
alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, and silage, as we ...
shipment. It was first recorded in Bingen,
Klickitat County, Washington Klickitat County is a county located in the U.S. state of Washington. As of the 2020 census, the population was 22,735. The county seat and largest city is Goldendale. The county is named after the Klickitat tribe. History Klickitat Coun ...
, in the late 1800s. By 1980, it had spread to 26 counties in the
Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though ...
. In the year 2000, it was reported in 45 of the 50 states of the US. Spotted knapweed primarily affects rangelands of the northwest US and Canada. A 1996 study estimated the direct plus secondary economic impact of spotted knapweed in Montana to be approximately $42 million annually. When spotted knapweed replaces native grasses, soil erosion and surface runoff are increased, depleting precious soil resources. In 2015, a
Missoula, Montana Missoula ( ; fla, label= Séliš, Nłʔay, lit=Place of the Small Bull Trout, script=Latn; kut, Tuhuⱡnana, script=Latn) is a city in the U.S. state of Montana; it is the county seat of Missoula County. It is located along the Clark Fork ...
, beekeeper whose bees rely on local knapweed stated that "knapweed produces great honey ... people should consider planting native wildflowers instead of just taking out weeds."


Catechin controversy

The roots of ''Centaurea stoebe'' exude (-)-
catechin Catechin is a flavan-3-ol, a type of secondary metabolite providing antioxidant roles in plants. It belongs to the subgroup of polyphenols called flavonoids. The name of the catechin chemical family derives from '' catechu'', which is the tanni ...
, which has been proposed to function as a natural
herbicide Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...
that may inhibit competition by a wide range of other plant species. While this
phytotoxic Phytotoxins are substances that are poisonous or toxic to the growth of plants. Phytotoxic substances may result from human activity, as with herbicides, or they may be produced by plants, by microorganisms, or by naturally occurring chemical react ...
compound can inhibit seed germination and growth at high concentrations, it is debated whether concentrations in field soils are high enough to affect competition with neighboring plants. Several high-profile papers arguing for the importance of catechin as an allelochemical were retracted after it was found that they contained fabricated data showing unnaturally high levels of catechin in soils surrounding ''C. stoebe''. Subsequent studies from the original lab have not been able to replicate the results from these retracted studies, nor have most independent studies conducted in other laboratories. Thus, it is doubtful whether the levels of (-)-catechin found in soils are high enough to affect competition with neighboring plants. The proposed mechanism of action (acidification of the cytoplasm through oxidative damage) has also been criticized, on the basis that (-)-catechin is actually an antioxidant.


Control

An 8-year study in
Michigan Michigan () is a state in the Great Lakes region of the upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the 10th-largest state by population, the 11th-largest by area, and the ...
found that restoring native plant communities in knapweed-infested sites requires multi-faceted and multi-year approaches. This includes an initial site preparation by mowing and an optional application of a
clopyralid Clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) is a selective herbicide used for control of broadleaf weeds, especially thistles and clovers. Clopyralid is in the picolinic acid family of herbicides, which also includes aminopyralid, piclo ...
or
glyphosate Glyphosate (IUPAC name: ''N''-(phosphonomethyl)glycine) is a broad-spectrum systemic herbicide and crop desiccant. It is an organophosphorus compound, specifically a phosphonate, which acts by inhibiting the plant enzyme 5-enolpyruvylshik ...
herbicide followed by reseeding with the desired plant communities. Yearly hand pulling of ''C. stoebe'' over the course of the study virtually eradicated the infestation. Burning treatments of infested sites reduced the labour needs for pulling and encouraged native plant community establishment. Another study over 3 years comparing the effectiveness of different combinations of annual spring, summer, and fall mowing treatments recommends an annual fall mowing during the flowering or seed-production stage in controlling ''C. stoebe'' infestations.


Biocontrol

Thirteen
biological pest control Biological control or biocontrol is a method of controlling pests, such as insects, mites, weeds, and plant diseases, using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also inv ...
agents have been used against this plant and its congener,
diffuse knapweed ''Centaurea diffusa'', also known as diffuse knapweed, white knapweed or tumble knapweed, is a member of the genus ''Centaurea'' in the family Asteraceae. This species is common throughout western North America but is not actually native to the ...
(''C. diffusa''), including the
moth Moths are a paraphyletic group of insects that includes all members of the order Lepidoptera that are not butterflies, with moths making up the vast majority of the order. There are thought to be approximately 160,000 species of moth, many of w ...
s, '' Agapeta zoegana'' and '' Metzneria paucipunctella''; the weevils, '' Bangasternus fausti'', '' Larinus obtusus'', '' Larinus minutus'' and ''
Cyphocleonus achates ''Cyphocleonus achates'' is a species of true weevil known as the knapweed root weevil. It is native to southern Europe and the Mediterranean and is used as an agent of biological pest control against noxious knapweeds, especially spotted kna ...
''; and the
fruit flies Fruit fly may refer to: Organisms * Drosophilidae, a family of small flies, including: ** ''Drosophila'', the genus of small fruit flies and vinegar flies ** ''Drosophila melanogaster'' or common fruit fly ** '' Drosophila suzukii'' or Asian fruit ...
, ''
Chaetorellia acrolophi ''Chaetorellia acrolophi'' is a species of tephritid fruit fly known as the knapweed peacock fly. It is used as an agent of biological pest control against noxious knapweeds, especially spotted knapweed (''Centaurea maculosa''). The adult fl ...
'', '' Urophora affinis'' and ''
Urophora quadrifasciata ''Urophora quadrifasciata'' is a species of tephritid or fruit flies in the genus ''Urophora'' of the family Tephritidae. The host plant for the larvae is usually a knapweed (''Centaurea'' sp), and because of this, it is used to control ''Centaur ...
''. Although the number of seeds is significantly reduced, this is not enough to make biocontrol truly effective against ''C. stoebe''. In some instances, root-herbivory on ''C. stoebe'' stimulates additional release of
catechin Catechin is a flavan-3-ol, a type of secondary metabolite providing antioxidant roles in plants. It belongs to the subgroup of polyphenols called flavonoids. The name of the catechin chemical family derives from '' catechu'', which is the tanni ...
, which may function as an allelopathic toxin. In addition, moderate levels of herbivory by biocontrol agents can cause compensatory growth.


Prescribed grazing

Prescribed grazing may be an effective means of controlling infestations, as all growth forms of ''C. stoebe'' are nutritious to sheep. High-density infestations can be controlled by fencing in the affected area with sheep until the desired level of removal is achieved.


Detection

Detection dogs A detection dog or sniffer dog is a dog that is trained to use its senses to detect substances such as explosives, illegal drugs, wildlife scat, currency, blood, and contraband electronics such as illicit mobile phones. The sense most used by ...
can be trained to locate small infestations of invasive, non-native weeds. Previous methods of detection involved lining up dozens of volunteers to locate the small rosettes buried somewhere on a large parcel of land covered with other vegetation. Montana State University successfully completed field testing in 2004. The dog, Knapweed Nightmare was finding low densities of the non-native invasive knapweed plant in the field with an overall success rate of 93%. She followed it up with 98% in the final trials in open fields, demonstrating that dogs can effectively detect low densities of invasive plants.


Toxicity

Handling the plant barehanded is rumored to possibly cause tumors, particularly if one has an open wound, but there is little evidence to support this theory.


References


External links


Species Profile - Spotted Knapweed (''Centaurea stoebe'')
National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for spotted knapweed.
Spotted Knapweed, Aliens Among Us
Virtual Exhibit of the
Virtual Museum of Canada The Digital Museums Canada (DMC; , ''MNC'') is a funding program in Canada "dedicated to online projects by the museum and heritage community," helping organizations to build digital capacity. Administered by the Canadian Museum of History (CMH) ...
. {{Taxonbar, from1=Q624368, from2=Q4117851
Centaurea stoebe ''Centaurea stoebe'', the spotted knapweed or panicled knapweed, is a species of ''Centaurea'' native to eastern Europe, although it has spread to North America, where it is considered an invasive species. It forms a tumbleweed, helping to increa ...
stoebe