Knapweed, A Growing Issue

Day 335: Invasives are a growing issue in our area, and even when they are recognized, no one seeems to be doing anything about them. Yes, I'm pointing a finger straight at you, Town of Eatonville. I've reported Knapweed along the Bud Blancher Trail multiple times, and it's spreading. In fact, there's some on the highway verge not far from my house now, and I have to consider the possibility that a seed was carried there on some vehicle's undercarriage or tires, maybe even my own. Knapweed is not the only unwelcome visitor. I've watched Lactuca (Bitter Lettuce) advance uphill from town to here in the space of only a few years. It and the Knapweed are two of the worst, although there are plenty of other weeds (I use the term in its ultimate form to indicate a non-native plant with the potential to set up a monoculture at the expense of natives) which have only recently begun showing up locally. The spread alarms me. It is occurring at a breakneck pace, boosted by negligence and the changing climate.

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.

A Not-So-Favourite Invasive

Day 314: There are half a dozen different Knapweeds (pure strains and crosses), all of which are considered Class B noxious weeds in Washington. They are prolific seed-producers and can crowd out native vegetation in a very short time, reducing the amount of forage for livestock and other animals where the plants occur. There are various controls which can be used for small infestations. Pulling or digging is effective, but mowing is not, due to the fact that the roots are long-lived and will re-sprout even in the same season. Biocontrols are an option when applying chemicals is undesirable. Methods include the introduction of Knapweed beetles or shading the plants out with more vigorous native species. That said, if Knapweed appears on your property, your first priority is to prevent it from going to seed.