It was our objective to determine whether plant–soil feedbacks confer weed suppression to a shrub–steppe ecosystem in Washington State. We found that soils developed by native bluebunch wheatgrass communities reduced knapweed growth from 18 to 5% ground cover. A pairwise feedback model developed from our data successfully predicted the native and weedy plant distribution seen on the landscape. We suggest that endemic soil-based mechanisms of weed control may be common and will produce novel techniques for the management of invasive plants.

Nomenclature: Bluebunch wheatgrass, Pseudoroegneria spicata (Pursh.); knapweed, Centaurea diffusa (Lam.).

Additional index words: Feedback, plant, shrub–steppe, soil, Washington State.

Abbreviation: SAS, Statistical Analysis Systems.