Plant of the Month: Spotted Knapweed

Spotted Knapweed (Centaurea biebersteinii, synonym Centaurea maculosa). Photo by Mara Trushell.

By Mara Trushell

Spotted Knapweed (Centaurea biebersteinii, synonym Centaurea maculosa) commonly grows as a biennial forb, beginning its cycle merely as a basal rosette. During the second year, the plant develops into a widely branching forb topped with vibrant purple flowers. Spotted Knapweed is a composite, meaning each seemingly single flower is actually a cluster of multiple flowers. The cluster is held by unique bracts that are each lined with narrow, teeth-like projections and come to darkened points. The points arrange into a visually stimulating, checkered pattern that circulate the visual pathway around the flower cluster.

These intricate knapweed flowers attract a variety of pollinators from June through October and occur within the majority of Arizona’s plant communities. Spotted Knapweed is native to Eastern Europe and, along with several additional species, have become well established in a wide range of plant and biotic communities, not only in Arizona but across the United States. In several biotic communities, including grassland and prairie, knapweed species have taken over large swaths of land, diminishing species diversity and even generating monocultures. Centaurea species are on many state’s Noxious Weed Lists and Invasive Species Management Lists. In Arizona, seven species are listed, including Centaurea biebersteinii.

Research shows that a key component to the success of the species are the chemicals they release from their roots. This chemical not only assists the plants in absorbing nutrients, it’s also shown that these chemicals change the soil composition. As the knapweed roots grow and spread, the soil becomes toxic and uninhabitable for most of the native species. Thusly, this provides a delightful monotypic habitat, at least from the perspective of the knapweed.


Mara Trushell is a local natural science enthusiast.


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