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A wide variety of plants growing in the Medieval Herb Garden on a sunny day

Medieval herb garden blog 79: Common knapweed

Tuesday, July 14, 2026

Week 79 volunteer update

This week involved more dead-heading, cutting back and a little weeding. We finally had to resort to watering the garden, which we don’t normally have to do. It was becoming like a desert out there! Some of the plants were really suffering.

However, the 40 or more sun-loving lavenders are serving.

A row of large lavender bushes stretching into the distance with lots of tiny purple flowers

Lavender in bloom in the Medieval Herb Garden

Plant of the week: Common knapweed (centaurea nigra)

'Centaurea' comes from the Greek 'Chiron'. It refers to the mythological centaur, a half-man, half-horse creature. The centaur was known for using these flowers as a poultice (herbal treatment) for wounds. 'Nigra' is Latin for 'black', referring to its brown / black flower buds.

The name 'knapweed' is thought to come from the old English word 'knap' / 'knop'. This means a 'knob' or 'button', describing the rounded flower head. Its other names are bell weed, hardhead and matfellon.

Common knapweed is native to central Europe. It is an upright, hardy, evergreen, herbaceous perennial. It will grow in any moist or well-drained soil type, in full sun or partial shade. When in flower it can grow to a height of 3 ft (90 cm) and a width of 18 inches (45 cm).

Common knapweed produces a basal rosette of leaves. These are usually simple or pinnately lobed, hairy, grey-green and up to 1 ft (30 cm) long. It has tall, stiff, ribbed stems that branch at the top.

The plant has tap roots which form associations with mycorrhizal fungi. This improves nutrient uptake. It is part of the reason knapweed does well in nutrient-poor grasslands.

Common knapweed flowers between June and September. It has tightly packed, thistle-like, purple flower heads of small tubular florets. These are surrounded by dark brown or black bracts.

The plant has hard, brown, flattened, oval-shaped seed heads. These have short bristles (clocks) and an elaiosome, which helps them to be spread by ants.

A pale purple puff-ball flower with lots of thin florets making a thistle-like effect

Common knapweed in the Medieval Herb Garden

Culinary uses of common knapweed

Common knapweed petals are very bitter. They are best used in small quantities. They are sometimes scattered over salads and desserts, or placed in drinks for colour.

Use of common knapweed as a dye plant

The flowers produce yellow, yellow-green or olive shades depending on the mordant used.

Folklore and other facts about common knapweed

In the Victorian language of flowers, knapweed was sometimes associated with delicacy, industry or perseverance, humility and resilience.

In traditional European and British folklore, the plant was believed to have the power to ward off witchcraft, sorcery and evil spirits.

Its flowers produce nectar and pollen. These attract butterflies, such as common blues, marbled whites and meadow browns. They also attract bees and other pollinators. The seeds provide food for birds.

Medieval medicinal uses of common knapweed*

Common knapweed was used as a wash or poultice for cuts, wounds, bruises, sores and scabs, especially on the fingers. It was also a treatment for nosebleeds, mouth ulcers and sore throats.

“These plants are of the nature of Scabious, whereof they be kinds, therefore their faculties are like, although not so proper to physic's use”.

*As always, this isn't to be considered medical advice today. Please don't use any plants mentioned in these blogs as medicine without advice from a doctor.

Medieval herb garden blogs

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