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 North American continent, but to the eastern Mediterranean.

Description

thumb|left|Centaurea diffusa basal rosette, first year plantDiffuse knapweed is an annual or biennial plant, generally growing to between 10 and 60 cm in height. It has a highly branched stem and a large taproot, as well as a basal rosette of leaves with smaller leaves alternating on the upright stems. Flowers are usually white or pink and grow out of urn-shaped heads carried at the tips of the many branches. Diffuse knapweed often assumes a short rosette form for one year, reaching maximum size, then rapidly growing and flowering during the second year. A single plant can produce approximately 18,000 seeds.

Synonyms

  • Centaurea microcalathina Tarassov
  • Centaurea cycladum Heldr.
  • Centaurea parviflora Sibth. & Sm., non Desf.
  • Centaurea comperiana Steven

Distribution

It is native to Eastern Europe and Western Asia, specifically the nations of Turkey, Syria, Bulgaria, Greece, Romania, Ukraine, and southern Russia.

thumb|Centaurea diffusa next to the Columbia River, Douglas County Washington

Invasive species

Diffuse knapweed is considered an invasive species in some parts of North America, having established itself in many areas of the continent. C. diffusa was first identified in North America in 1907 when it was found in an alfalfa field in Washington state. The seeds had presumably been transported in an impure alfalfa seed shipment coming from somewhere in the species native range. Now present in at least 19 states in the United States, it has naturalized in all contiguous states west of the Rockies and additionally in Connecticut, Massachusetts, and New Jersey. Portions of western Canada have also been colonized by this plant.

Areas in which diffuse knapweed has been established generally are plains rangelands or forest benchlands. Land that has recently been disturbed is commonly colonized. Research based on simulation models have shown that for biocontrol agents to be effective, they must kill their host, otherwise plants can compensate by having increased seedling survival.

Some of the more commonly utilized biocontrol agents are the Lesser knapweed flower weevil and the Knapweed root weevil. Individuals of these species lay their eggs on the seed heads of both diffuse and spotted knapweed. When the larvae emerge from the eggs, they feed upon the seeds of their host plant. As the females of this species can create from 28 to 130 eggs and each larva can consume an entire seed head, an adequate population of Larinus minutus can devastate entire stands of knapweed. The adult weevils feed upon the stems, branches, leaves and undeveloped flower buds. It is native to Greece and is now found in Montana, Washington, Idaho and Oregon. Insects are also used for biocontrol, such as the Yellow-winged knapweed root moth (Agapeta zoegana), and several species of Tephritid flies, mostly Urophora affinis and Urophora quadrifasciata.

References

Sources

  1. Washington State weed info: Diffuse knapweed
  2. Diffuse Knapweed (Centaurea diffusa)
  4. K. Bossick, Wood River Journal. A16 (2004).
  • This article is extracted p.p.max. from homonym one in Bugwood Wiki [http://wiki.bugwood.org/Centaurea_diffusa].
  • Species Profile - Diffuse Knapweed (Centaurea diffusa), National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Diffuse Knapweed.