spotted knapweed

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 15 centimetres (6 inches) 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 0.91 metres (3 feet) tall. Protruding from black-tipped sepals, the flower blooms from July to September. The flower head is 10 millimetres (3⁄8 in) wide, with vibrant pink to lavender (or more rarely white) disc flowers around 2 cm (3⁄4 in) long; the corolla of each has five narrow lobes. The fruit is an achene about 6.4 mm (1⁄4 in) long with a short, bristly pappus. These are primarily dispersed by wind, sometimes assisted by the plant's tumbleweed mode.

The species is 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 US in particular, much of which has a dry climate similar to the Mediterranean. 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.

Centaurea stoebe has been introduced to North America, where it is considered an invasive species in much of the western US and Canada. In 2000, C. stoebe occupied more than 7 million acres (28,000 square kilometres) in the US. Spotted knapweed is a pioneer species 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 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. A study conducted in 2003 has shown that with future global atmospheric carbon levels, C. stoebe shows increased growth with increased atmospheric carbon which could potentially expand its range and outcompete native species.

History in North America. …