Knapweed has purple thistle-like flowers in summer, but though it grows profusely, the heads are much smaller than thistle heads. There are so many different varieties, I wouldn't like to hazard a guess which one this is.
The plants where once used as a poultice on wounds; in mythology the Centaur, which gave knapweed its classical name , is said to have used it to heal his hoof.
It's also used in traditional fabric dyeing, producing a yellow dye in spite of its purple flowers.
Knapweed provides late summer nectar for bees and butterflies and is pollinated in return.
This is a profuse clump outside the bird hide. I've become rather fond of it as its size increases year by year. Dried stems and heads will give way soon to new green growth from the base. Caught it against the lovely blue sky late afternoon. [husband says he's bored already, apologies to you if it affects you the same way.]