Growing Wild Knapweeds

Everyone has their favourite wildflowers.  Mine include showy species such as bluebells, meadowsweet, cowslips and scabious as well as more subtle beauties especially  moschatel, water crowsfoot and eyebright.  If however, I were choosing my favourites from the point of view of bringing wildlife to my garden, then my first choice would always be the knapweeds.   Robust plants with pink, blue or purple flowers, the knapweeds and their close annual relative the cornflower, must be amongst the very best of plants for encouraging garden wildlife.  The ease with which they will adapt to almost any garden situation makes them really special.

The knapweeds (species of Centaurea) belong to the daisy or Compositae family so a large part of their wildlife attracting ability lies in the fact that each flower head is made up of hundreds of small florets – tiny individual tubular flowers brimming with nectar.  If you observe a butterfly or bumblebee on a knapweed flower you will see that it spends a great deal of time probing its long tongue again and again into the tiny separate flowers, moving methodically over the flower head.  It’s a little bit like visiting a supermarket for all your food, rather than many individual small shops!  This means that the insect saves a great deal of energy by staying in one place for some length of time, although this method of feeding has the potential to increase the risk of predation.  That risk must be worth taking however, as anyone who has watched a clump of knapweed flowers will have noticed.

We have one annual wildflower member of this group – a flower with a hue so intense that it has a colour named after it.  Mention ‘cornflower blue’ to anyone and the phrase will immediately conjure up the true pure blue that is characteristic of this wonderful flower.  Sadly lacking from our countryside now, the cornflower was once a common sight in cornfields together with corn poppy, pheasants eye, corn marigold and corncockle, all of which have suffered at the hands of weed free agriculture.  These annual flowers are easy to grow in our gardens in a sunny spot, either together in swathes of gorgeous mixed colours, or in single species groups.  Sow this month by scattering the seed onto the surface of weed-free, turned and raked soil, and press them gently into the soil surface with hands or feet.

The cornflower has great wildlife value and is especially good for attracting the smaller butterflies to the garden.  Common blue, small copper, the skippers and the brown argus, a species found increasingly in gardens in the south, all seek out this flower for its nectar.  Honeybees, bumblebees and hoverflies also visit it for pollen and nectar.  If it is one of the few butterfly nectar sources in your garden (or on your allotment – it makes a wonderful cut flower as well as encouraging beneficial insects) it may also attract the small tortoiseshell and other larger butterflies.  Easy to grow, a stunning colour and a self-seeder too.  There are many cultivated varieties in a range of colours if you prefer, some shorter and more manageable than our native flower and all seem to be good at bringing insects to the garden.

Throughout Europe the knapweed group is represented by a range of perennial species, some more akin to the closely related thistles than our familiar British plants.  A few of these are now naturalised in our countryside and may be worth trying in your garden if seeds or plants can be located.  The yellow star-thistle is one, its pale lemon knapweed head providing a pleasing contrast to our own purple flowered species.  This plant is pollinated by bees as is another non-native interloper, the perennial cornflower, a common cottage garden plant sometimes occurring as a garden escape.  I always find a place for this plant in a border somewhere, as much for its heady scent of coconut (reminiscent of tropical holidays!) as its attraction to bumblebees.

Common and greater knapweed are plants that are well known to most of us who appreciate the countryside and wildflowers.  Often seen on roadside verges, in waste places, on railway embankments and cliff tops, these species are an important component of the nectar supply for bees and butterflies in natural habitats.  Both were once common in hay meadows, a habitat in short supply now.  If you have a small garden meadow or intend to create one, make sure that you include one or both of these plants to ensure a good nectar supply for summer butterflies and bees, as well as long lasting colour into September.

Common, lesser or black knapweed is the coarser of the two, having tighter, thistle–like purple flower heads resulting in its other common name of hardheads.  In the garden it will grow to 60 cms or so, its tough stems resisting almost any gale, making it useful for a windy spot.  The dark green leaves are rough to the touch and are pale and felted with hairs beneath.  Many insects will visit the purple flowers which bloom from June to September, making it a great plant to include in a late summer border.  Once the flowers are over, this plant, like all its close relatives, produces a seedpod full of large nutritious seeds, which, much like teasel seeds, will bring goldfinches from miles around.  It will self seed gently (few seeds ever escape the finches) so it never becomes a problem in the garden.  In a meadow it is a long lasting perennial, surviving for many years.

Greater knapweed is similar to common to the untutored eye, and the two can easily be confused.  Generally though (but not always) common lacks the distinctive ray florets – the pretty feathery bits around the edge of the flower head which give greater knapweed a more shaggy appearance.  It is also a more pinky purple to my eye and the leaves are more dissected.  In truth it doesn’t matter which species you grow as long as you manage to find true native stock.  They are equally brilliant at attracting bees, butterflies, hoverflies and goldfinches.  To some, greater knapweed is the prettier plant and there are few more wonderful sights in mid summer than a clump of this wildflower covered with small tortoiseshell, peacock or marbled white butterflies.  In my garden last summer, the visiting clouded yellows found a rich nectar supply in both greater and common knapweed in my wildflower meadow.

The perennial knapweeds will germinate without any kind of pre-treatment, needing no period of cold weather to break their dormancy.  The seedlings do however appear rather erratically, so sow in spring in small seed trays, cover lightly with grit or a fine layer of compost and be prepared to tease the little plants gently from the tray as they germinate, moving them into small pots or plugs.  They are robust seedlings, quickly producing a good root system and they transplant easily.

If you are relatively new to growing wildflowers in your garden you could do no better than starting with the knapweeds.  Easy to germinate and grow, robust, suitable for border, meadow or wild corner, plus brilliant for wildlife  – there is no better place to start your wildflower collection. 


© Text and photographs Jenny Steel 2017