diffuse knapweed (Invasive Plants of the Kaibab National Forest)

diffuse knapweed

Centaurea diffusa

Summary ⁷

Diffuse Knapweed (Centaurea diffusa), also known as White Knapweed or Tumble Knapweed, is a member of the genus Centaurea in the family Asteraceae.

Distribution ⁸

More info for the terms: cover, shrub

Diffuse knapweed is native to grasslands and shrub steppes of the eastern Mediterranean and western Asia [66] and was introduced into Central Europe and North America [71]. It is thought to have been introduced to North America as a contaminant in alfalfa (Medicago sativa) seed from Asia Minor-Turkmenistan or in hybrid alfalfa seed from Germany [58]. The earliest record of diffuse knapweed in North America is from an alfalfa field in Washington state in 1907 [86]. It is currently found from Yukon in the north, throughout most of western Canada, east to Ontario. In the United States, the primary range of diffuse knapweed is the western states, from Washington, Idaho, and Montana south to New Mexico and Arizona [42]. Maddox [59] notes that diffuse knapweed is more common on the western side of the Great Basin, and spotted knapweed is more common on the eastern side. Diffuse knapweed has also spread east into several midwestern states and is found in Massachusetts, Connecticut, and New Jersey on the east coast [42]. The Plants database provides a state distribution map of diffuse knapweed in the United States.

The following table reflects estimates of diffuse knapweed acreage as reported by surveyed states or provinces in 1988 and again in 2000 (from [20]): State/Province 1988 Acreage 2000 Acreage Arizona not reported 1,800 California not reported 5 Colorado 30,000 83,000 Idaho 1,450,000 1,800,000 Montana 10,349 27,523 Nevada not reported 500 New Mexico not reported 200 North Dakota 0 30 Oregon 1,200,000 989,000 South Dakota 1,000 200 Utah 25 1,300 Washington 427,800 500,000 Wyoming 5,000 4,000 Alberta 0 scattered British Columbia not reported 75,000 Total 3,482,558

Although inventories are more common and more accurate in the year 2000 than in 1988, 50% of these states reported only 50% accuracy, while 31% reported 51 to 75% accuracy, and 2 states reported 75 to 100% accuracy [20].

The following lists reflect ecosystems and cover types in which diffuse knapweed is known or thought to be invasive. Diffuse knapweed occurs in some midwestern and eastern states and provinces, primarily along roadsides and in "waste places", but it is unclear, from the available literature on these areas, which ecosystems and cover types it occurs in. These lists are not, therefore, exhaustive, as the plant may be invasive in other types not listed.