Centaurea stoebe, the spotted knapweed or panicled knapweed, is a species of Centaurea native to eastern Europe. It is also an invasive species in southern Canada, and northwestern Mexico, and nearly every state in the United States; it has thrived in the western United States in particular, much of which has a dry climate similar to the Mediterranean. This species and Centaurea diffusa are tumbleweeds — plants that break free of their roots and tumble in the wind, facilitating the dispersal of their seeds.
Centaurea stoebe is a biennial or short-lived perennial plant, and it usually has a stout taproot and pubescent stems when young. It has pale and deeply-lobed leaves covered in fine short hairs. First-year plants produce a basal rosette, alternate, up to 6 inches (15 cm) 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 three feet tall. The flowers are a vibrant pink flowers with black-tipped sepals that look like spots, which is the origin of its common name. The fruit is an achene (about a quarter-inch long) with a short, bristly pappus. It is primarily dispersed by wind.
The common name spotted knapweed most often refers to Centaurea stoebe, formerly known as C. maculosa; however, there is some confusion surrounding the taxonomy of this genus. Two[specify] cytotypes of C. stoebe exist which have been considered as different species by some taxonomists. The diploid form of the plant is now called Centaurea stoebe L. spp. stoebe, while the tetraploid is known as C. stoebe L. spp. micranthos or by some taxonomists as C. biebersteinii DC.
Distribution and habitat
The plant grows on stream banks, pond shorelines, sand prairies, old fields and pastures, roadsides, along railroads, and in many open and disturbed areas.
Centaurea stoebe has been introduced to North America, where it is considered an invasive plant species in much of the western United States and Canada. In 2000, C. stoebe occupied more than 7 million acres (28,000 km2) in the US.
Spotted knapweed is a pioneer species found in recently disturbed sites or openings. 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 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 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 shipment. It was first recorded in Bingen, Klickitat County, Washington in the late 1800s. By 1980, it had spread to 26 counties in the Pacific Northwest. In the year 2000, it was reported in 45 of the 50 states in the United States. Spotted knapweed primarily affects rangelands of the northwest United States 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 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."
The roots of Centaurea stoebe exude (-)-catechin, which has been proposed to function as a natural herbicide that may inhibit competition by a wide range of other plant species. While this phytotoxic 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.
An 8-year study in Michigan 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 or glyphosate 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.
Thirteen biological pest control agents have been used against this plant and its congener, diffuse knapweed (Centaurea diffusa), including the moths, Agapeta zoegana and Metzneria paucipunctella; the weevils, Bangasternus fausti, Larinus obtusus, Larinus minutus and Cyphocleonus achates; and the fruit flies, Chaetorellia acrolophi, Urophora affinis and Urophora quadrifasciata. But in general, biocontrol has not been shown to be effective against C. stoebe. In some instances, root-herbivory on C. stoebe stimulates additional release of catechin, which may function as an allelopathic toxin. In addition, moderate levels of herbivory by biocontrol agents can cause compensatory growth.
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.
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