Knapweed
Consistently among the highest-scoring wildflowers in nectar surveys of British grassland, and one of the few that produces at that level well into September. Knapweed's thistle-like purple heads are worked by bumblebees, honey bees, butterflies and solitary bees together, and greater knapweed produces more nectar sugar per flower than almost anything else measured in the temperate flora.
Where not to plant it
Several Centaurea species, including spotted and diffuse knapweed, are serious invasive rangeland weeds in North America. The European common knapweed is well behaved in its native range and should not be planted outside it.
Value to bees
- Nectar
- Very high
- Pollen
- High
- As a honey crop
- Minor honey crop — a surplus only locally or occasionally
- Which bees, and why
- Worked by an unusually wide range — honey bees, several bumblebee species, solitary bees, butterflies including marbled whites and meadow browns, and day-flying moths. The composite head is a landing platform of many small florets, so a visitor can work it without flying.
Individual florets are tubular but short enough for honey bees to reach, which is why this plant serves both the short-tongued and long-tongued ends of the range.
- Worth knowing
- One of the very best wild nectar plants in northern Europe, and unusually it delivers over months rather than weeks.
Flowering
- In flower
- June to September (northern hemisphere)
A long season and a late one, carrying grassland forage through the gap after the clovers finish.
- When nectar flows
- Exceptional nectar sugar production per flower — British nectar surveys repeatedly rank greater knapweed at or near the top of the wild flora. It secretes well in dry weather.
- Year-to-year variation
- Highly dependable. Not a commercial honey crop, though it contributes substantially to late meadow honey.
What it does to the honey
Contributes a warm, faintly spicy, slightly astringent character to late-summer meadow honeys, and is a significant component of many British and Irish wildflower honeys.
Growing it
- Conditions
- Full sun, any soil including chalk and clay. Extremely tough and long-lived.
- Size
- 30-90 cm.
- Native range
- Europe; naturalised in North America and Australasia, where it is often a weed.
- Suits
- Meadow or wildflower area, Large gardens, Small gardens
- One of the easiest and most rewarding wildflowers to establish in a meadow or a rough corner, and it persists for decades.
- Leave the seed heads standing — goldfinches strip them through autumn and winter.
- Greater knapweed is the better bee plant of the two and prefers chalk or limestone.
What it looks like
A tough wiry perennial with narrow leaves and hard dark bracted buds opening to purple thistle-like heads without any of the spines. Greater knapweed (Centaurea scabiosa) is larger, with dissected leaves and showier ray florets.
Sources
- Field Guide to the Bees of Great Britain and Ireland — Bloomsbury (2015) · Field guide
- Bumblebees: Behaviour, Ecology, and Conservation — Oxford University Press (2010) · Monograph
How sources are selected and weighted is set out in the sources and evidence policy.
Guides covering this plant
- The threats facing beesBee decline is not one problem with one cause. It is a set of interacting pressures whose relative weight differs between managed honey bees and wild species — and confusing the two produces bad solutions.
- Plants that attract honey bees specificallyHoney bees have short tongues and work in large numbers, which means they want different plants from bumblebees. Half the flowers sold as bee-friendly are out of their reach entirely.
- The best plants for beesA shortlist chosen on three criteria: how much nectar and pollen the plant produces, how accessible it is to different bees, and when it flowers. Season coverage matters more than any individual plant.
- Summer plants for beesSummer looks abundant and contains a real gap. The weeks between the spring blossom finishing and high summer starting are thinner than most gardens realise, and colonies are at peak population when it happens.
- Making a wildflower area for beesA meadow is made by reducing fertility and weakening grass, not by scattering seed on a lawn. The single most useful plant in the process is a parasite.
- A bee-friendly garden in Britain and IrelandAround 270 bee species, one honey bee among them, and a climate that gives you flowering months at both ends of the year that most of the world does not get. The British gaps are February and late June, and both are fixable.
- Plants for bumblebeesLong tongues, big bodies, cold tolerance and an annual colony that has to build from a single queen. Bumblebees can use flowers honey bees cannot reach, and they need forage at both ends of the year when honey bees are indoors.
Tools for planning planting
- Flowering calendar
What should I plant so that something is always in flower for bees?
- Bee-friendly garden planner
What should I plant in the space I actually have?
- Plant recommendation tool
Which plant fills this specific gap?