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Water chestnut (Trapa natans)
Photo credit:  © Frank Philip Gröhl/Flickr

Diffuse Knapweed

(Centaurea diffusa)

Report this Species!

If you believe you have found this species anywhere in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

Species at a Glance

Diffuse knapweed occurs in every state west of the Rocky Mountains and is particularly widespread in the Pacific Northwest; it also occurs in Nebraska and is scattered in several eastern states. The worst infestations are reported to be in Idaho, Montana, Oregon, and Washington. It is very aggressive and can quickly dominate disturbed habitats. Diffuse knapweed produces an allelopathic chemical that inhibits the root growth of other species and prevents them from competing for soil moisture and nutrients. It is most abundant in disturbed and overgrazed lands but it can also invade undisturbed grasslands, shrublands, and riparian communities. Management is difficult, and usually requires a combination of methods over more than five years.

Similar Species

Diffuse knapweed can be confused with spotted knapweed (Centaurea stoebe) and others species of Centaurea.


Diffuse knapweed is a terrestrial species, preferring to inhabit open areas such as plains, rangelands, and forested benchlands with light, dry, and porous soils.


Diffuse knapweed is native to the eastern Mediterranean region. It was first discovered in North America in an alfalfa field in Washington state and has presumably been distributed throughout the United States via contaminated seed. It is now found in states directly east and west of Pennsylvania, as well as in several Canadian provinces.

Environmental Impacts

Once established, diffuse knapweed can create monotypic stands which deplete soil and water resources, displace native species, and reduce biodiveristy. It produces an allelopathic chemical that inhibits root growth of desirable species. A single knapweed plant can produce up to 18,000 seeds, some of which can remain dormant in the soil for several years.

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