The flowerheads of knapweed look like the thistle, but they are to be distinguished by their lack of prickles and/or spines. Knapweed, it seems, grows everywhere; it is a very widespread species. The flower bracts supporting the flowerheads have fringed black triangular tips.
The uses of knapweed: "a medieval wound salve; used to soothe sore throats and bleeding gums. Also acts as a diuretic." (Clevely.)