Knapweed is a member of the daisy family and is sometimes known as Hardhead. It is a distinctive hairy perennial which is common in the UK often reducing crop yields if left unchecked and invading grassy lawns rapidly due to its fast seed production. Its hard dark buds which are thistle like or even described by some as tiny bristly pineapples (from which it gains the name Hardhead) open up between June and September. The flowers are a pretty purple colour and a favourite among Bees and butterflies making it an important species for these insects. The flowers appear atop the bristly buds further adding to its similarities to the thistle. It is a native species to Europe but has spread to North America.
What Culpepper says
KNAPWEED
Description. The common sort hereof has many long and somewhat dark green leaves, rising from the root, dented about the edges, and sometimes a little rent or torn on both sides in two or three places, and somewhat hairy withal; amongst which arises a long round stalk, four or five feet high, divided into many branches, at the tops whereof stand great scaly green heads, and from the middle of them thrust forth a number of dark purplish red thrumbs or threads, which after they are withered and past, there are found divers black seeds, lying in a great deal of down, somewhat like unto Thistle seed, but smaller; the root is white, hard and woody, and divers fibres annexed thereunto, which perishes not, but abides with leaves thereon all the Winter, shooting out fresh every spring.
Place. It grows in most fields and meadows, and about their borders and hedges, and in many waste grounds also every where.
Time. It usually flowers in June and July, and the seed is ripe shortly after.
Government and virtues. Saturn challenges the herb for his own. This Knapweed helps to stay fluxes, both of blood at the mouth or nose, or other outward parts, and those veins that are inwardly broken, or inward wounds, as also the fluxes of the belly; it stays distillation of thin and sharp humours from the head upon the stomach and lungs; it is good for those that are bruised by any fall, blows or otherwise, and is profitable for those that are bursten, and have ruptures, by drinking the decoction of the herb and roots in wine, and applying the same outwardly to the place. It is singularly good in all running sores, cancerous and fistulous, drying up of the moisture, and healing them up so gently, without sharpness; it doth the like to running sores or scabs of the head or other parts. It is of special use for the soreness of the throat, swelling of the uvula and jaws, and excellently good to stay bleeding, and heal up all green wounds.