Diffuse Knapweed - An Invasive Species

An 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 from 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 been recently disturbed—by human or natural processes—is favored for the establishment of diffuse knapweed. It grows in semi-arid and arid environments and seems to favor light, dry, porous soils. Areas with large amounts of shade or high levels of water discourage diffuse knapweed growth.

Dispersion occurs in the following ways:

• Agriculture – alfalfa contaminated with diffuse knapweed seed can promote the spread of diffuse knapweed;
• Wildlife – wild animals eating the seeds or transporting the seeds on fur;
• Wind – seeds blown out of their capsules held on the plant are distributed over a short range, but when dried out the plant may become a tumbleweed, rolling for great distances and releasing seeds along the way;
• Water – waterways carry seeds in their flow for long distances before depositing them onto a shore where they germinate.

Wind is the primary means by which diffuse knapweed seeds are spread.