Black Knapweed is a tonic, and an old wound herb

Black knapweed, Centaurea nigra, is also called common knapweed, lesser knapweed and hardheads. The knapweed genus Centaurea contains many different plants, only a few of which are active medicinally, including greater knapweed, common cornflower and mountain cornflower, all of which are related to this plant.

The word “black” refers, not to the flowers themselves but to the lower part of the flower head, which is formed of bristly layered bracts (modified leaves), giving the flowers a similar appearance to thistles, but they are not closely related.

Black knapweed is classified as a noxious weed in the US state of Washington, and may not be grown there. It is possible that it is banned in other parts of the world as well – check local laws before growing it. In its native Europe, it is a food plant for several species of insect and moth.

Black knapweed is a survivor. A hardy perennial which will grow in any well drained soil, reaching a height of around 2 feet (60cm), it can tolerate poor soil, very alkaline soil and drought, and will survive and self-seed even in very weedy soil, though it will not grow in shade. It’s also one of the few plants that is unlikely to be eaten by deer. It’s a pity it’s not more attractive, as there are many gardeners who would choose it on this ground alone if it was.

For continuing good supplies of black knapweed, plants should be divided every 3 years in Spring. Take the opportunity of harvesting some of the roots at this time, which should then be dried by laying out in a single layer in an airy place out of the sun. Check and turn over every couple of days until completely dried, then store in an airtight jar, label with the name and date, and put the jar somewhere cool and dark (or use a dark colored container).

Flower petals are edible, and sometimes added to salads. The parts used in medicine are the roots and the seeds. You can make a decoction of either or both of these, using 30g (1 ounce) of dried root/seeds which should be put in a pan containing 570ml (2.5 US cups, 1 UK pint) cold water. Bring to a boil, then simmer until the liquid is reduced by half and strain. The dosage is 75ml up to 3 times a day. If you’re not using it all in a day, transfer it to a labeled container and store in the fridge for up to 1 week.

Black knapweed can be used as a tonic, a diuretic and to induce sweating. It was once highly regarded as a wound herb, but I can find no instructions for its use. If you have information about this, please pass it on via the comments.

Plants intended for medicinal use are best grown organically to avoid their intrinsic properties being altered or eliminated by the presence of foreign substances, and the herbal remedy black knapweed is no exception to this rule. To find out more about growing organic herbs visit the Gardenzone.

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1. Greater Knapweed
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