Centaurea nigra | knapweed

A future plant for an unpredictable world

Six spot burnet moth, Zygaena filipendulae, on Knapweed, Centaurea nigra - two of my favourite things. Knapweed is hands down one of my top plants of all time. Easy to grow, flowers for ages and supports a huge number of beautiful flying insects. What’s not to love?

Need to know information about Centaurea nigra

Originating from Europe, including the UK, and the very north of Africa, Centaurea nigra is a widespread plant. It sits within the Asteraceae family which means it’s closely related to all daisies. Though it may not look like a daisy, the similarity is evident when we appreciate those tufts of purple-pink are actually composite flowerheads containing hundreds of tiny floret flowers compacted together.

It grows to about thigh to waist height with a beautiful open structure. Preferring full sun and well drained soils, though it is very tolerant of richer and wetter soils, as proven by the plants on our smallholding. Typically you find it in low grassland habitat, which in the UK means anything from meadows to roadside verges.

How to propagate Centaurea nigra

Grow more knapweed by pinching off some seed when the seedhead is totally brown and the seed comes away freely in late-summer to autumn - but be quick because small birds, especially goldfinches, will try to beat you to it!

Sprinkle the seeds where you want the plants to grow and they’ll appear next spring. You can help them by making sure they make contact with soil, though they’re tough and should make it themselves.

If you have even one knapweed plant in your garden or neighbouring gardens, it will almost certainly self sow naturally anyway. Especially if the birds give you a helping hand. Helping beak?

Is knapweed edible?

Not really for humans, except those pink florets, but it is very edible for insects.

Is knapweed good for wildlife?

As you’ll already have realised, the answer is a resounding yes! One of the main reasons I love knapweed is because not only is it a fantastic looking plant, it attracts so many fantastic looking animals.

On our smallholding I often see meadowbrown, small copper, large white, peacock and red admiral butterflies fluttering around the plants. As well as day flying and night flying moths.

Butterflies adore it, various bee species jostle the butterflies to get on it, hoverflies, wasps, moths - the lot! Like so many plants labelled ‘weeds’ in the past in the UK, they can actually be among the most useful plants for wildlife, just like ragwort.

In this study, knapweed came second among meadow perennials for producing nectar sugar mass per flower. No wonder it’s the most popular joint for wildlife.

Joking aside about birds, there’s a reason they eat the seeds, because they are nutritious too.

Using knapweed in design

I cover this more in my book Wild about Weeds, Knapweed is fantastic for contemporary design. It is one of those ideal plants for naturalistic planting, whether that means a meadow or other planned plant community.

Because it’s a grassland plant, it can naturally grow among competition from other plants. Making it easy to grow in a border.

Its structure is open and airy, with thin, strong stems and small leaves. Not dissimilar to Verbena bonariensis, a kinda stockier version. Producing bright pink-purple flowers over many months from mid to late summer.

Come autumn and winter, it holds its structure and the seedheads become ornamental, especially when covered by frost, snow or flocks of goldfinches!

Sustainability wise, because knapweed has a wide tolerance of conditions from baking hot drought to soggy soil, it’s a top plant for future gardens. No gravel garden required (see my 10 concerns about gravel gardens). Combine that with all the other benefits, it’s hard to see why any gardener would ever call it a weed at all.

Are there any downsides to knapweed?

The only downside to knapweed is that like so many successful wildflowers it self sows readily. You could remove seedlings but this is impractical so only grow it in dense plant communities where you want it, such as a meadow or a modern naturalistic plant border that replicates those conditions, so no bare soil between other plants. Otherwise, this is a fantastic garden plant.

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For all of these reasons, Knapweed, receives a yes from me. One of my very favourite plants.