Centaurea maculosa / spotted knapweed

Adjectives: exposed, invasive, non-native, noxious, striking, weed

Along about the beginning of August when all the “spring” wildflowers have all but disappeared, the silvery leaves and tight, thistle-like buds of spotted knapweed begin to show, soon followed by a frilly, bachelor-button-like, pink flower head. It is seemingly everywhere – at least along roadsides, in trashed fields, in pastures.

Like other weeds, spotted knapweed is a good colonizer of disturbed sites, i.e. it is an invasive weed. Once there, it spreads far and wide for several reasons.

• First, it is a prolific seed producer (up to 25,000 seeds per plant with soil residency times of about 8 years), and like many of the Asteraceae, the fruits are small achenes, each with a bristly pappus, like dandelion seeds.
• Second, once the plant is growing, it has a good taproot and is good at outcompeting its neighbors for water.
• Third, cows, deer, elk, moose and other grazers don’t like it – it has low palatability. Which also means that the areas between the knapweeds are likely to be overgrazed, making more room for yet more knapweed. On the other hand, sheep and goats seem to like it and are a good biocontrol… if you have sheep or goats.
• And finally, it seems that spotted knapweed releases chemicals from its roots that are toxic to other plants. That means that once established in an area, it is good at keeping other plants out. These chemicals, known as catechins, inhibit seed germination and growth, and tie up phosphorous in the soil. Interestingly, at least two Idaho natives, Gaillardia grandiflora and Lupinus sericeus, are resistant to these chemicals.

At this time, spotted knapweed is found on about 100,000 acres in Idaho alone. Because of all this, in addition to it being invasive, the State of Idaho (and many others) have officially declared it to be “noxious”, meaning that whosoever has it on their property is legally required to eliminate it. Good luck on that!

But on the plus side, spotted knapweed honey seems to be quite good. Or so they say.