Mission Impossible

Day 295: Invasive Knapweeds (Centaurea species) are spreading rapidly across western Washington, but I was still surprised to find them in such abundance along Hwy 12 above Packwood, and horrified to see a monoculture covering approximately an acre at the junction with Hwy 123. These are lands administered by WSDoT (Washington State Department of Transportation), as I noted in my report to the Invasive Plant Council. Come on, Washington! Do your part to control these plants while there's still some hope of eliminating them!

Much of the time, it's difficult to identify a Knapweed as a specific species because they hybridize readily and any particular plant may be a genetic stew. That said, the key to identification is in the scales of the involucre (the "cup" of bracts from which the petals arise). The manner in which each bract terminates is crucial; some may be fringed or have papery margins which others may narrow down to a single point.  When fringe is present, it may be sparse or abundant, or it may be brown, black or white. It may be modified into spikes, as is the case with Yellow Starthistle. Between scales and flower colour, a reasonable guess at the genetics of a hybrid is possible if a specific species can't be singled out. In the case of the Hwy 12/123 population, they seem to be largely Centaurea diffusa (left) and C. stoebe x diffusa (right). As I got closer to Packwood, C. stoebe became more prevalent, leading me to conclude that C. diffusa is in migration across White Pass from eastern WA where it has been problematic for decades, now hitchhiking to new territory in tire treads and mud caked on the undercarriages of vehicles. Y'know, now that I think about it, maybe I ought to wash my car more often to prevent unwitting transport of invasive species.