What is a prairie?
Essentially it is the North American equivalent of a wildflower meadow – about three-quarters grasses and one quarter wildflowers (usually called ‘forbs’). Unlike European wildflower meadows which have a flowering peak around Midsummer, they tend to peak in late summer or even early autumn – this is one reason why there is so much interest in them amongst European gardeners. There are lots of different types of prairie, but the one which we tend to focus on is the ‘tallgrass prairie’ of the upper Midwestern states, eg Illinois, Iowa, Wisconsin. It is very species-rich, with the late-flowering component (many of them members of the daisy family) being much appreciated by butterflies and other pollinators.
Why would we want a prairie in Britain or elsewhere in northern Europe?
Gardeners and those interested in managing public space started to get interested in prairie in the 1980s. European native wildflower meadows do not really work on fertile soils as you tend to end up with grass, grass, more grass and the occasional knapweed – our most visually and biologically diverse wildflower communities tend to flourish only on thin, calcareous soils. A dense ‘natural’ plant combination such as prairie can be very low-maintenance and notably weed-resistant, so it looks like a viable proposition for public parks, corporate landscapes and larger gardens. I would add that in small gardens, strips of prairie planting, only a metre wide, can be very attractive. They are very good for pollinators, and the seedheads are much appreciated by seed-eating birds in the winter.
How viable are the plants so far from home?
We have a longer history of growing North American perennials than you might think. From the late 18th century a very wide range of species have been grown in northern and central Europe, in both botanical and private gardens. The magnificent herbaceous borders of early 20th century gardens contained many prairie species, particularly of Aster, Helianthus, Rudbeckia and Solidago. Since the 1980s there have been more introductions from the US, particularly since American growers themselves have now turned to using natives in a big way. Amongst the ‘new’ prairie plants are Solidago rugosa ‘Fireworks’, which doesn’t run like the old goldenrods, Vernonia species – dramatic tall violet-purple flowers for the very end of the gardening year and Veronicastrum virginicum, a statuesque perennial with a long season of structural interest.
There is a ‘but’ – the grass component does not do so well with us. The reason for this is that American prairie grasses have a C4 route for photosynthesis, which needs more heat, hence their being called ‘warm season grasses’. All the forbs have the normal C3 route and our lack of summer heat is compensated for by the exceptional amount of light they receive from May to July; this is why many are surprisingly successful as far north as central Sweden, and certainly in Scotland. The grasses however need a lot of heat to get them growing, which means that in spring the forbs will swamp them as they are slow to get going. I have seen American grasses flourish as far north as St Petersburg, but only in a conventional border, where they have more space than in a dense prairie planting. Many gardeners, myself included, use Calamagrostis x acutiflora ‘Karl Foerster’ as a ‘cool-season’ substitute.
How can we make a garden prairie?
European prairie plantings are not real prairie, as we focus our interest on the flowering plants, with only a small grass component. In making a prairie we are aiming at creating a dense intermingled mass of perennials – I stress ‘intermingled’, as simply creating big blocks of perennials is not a prairie, just a giant herbaceous border. Dense intermingled planting helps create a mass of interlocking roots and stems, which is strongly weed-resistant.
The site needs to have full sun, a fertile soil and be as weed-free, and as free of weed-seed as possible. There are three methods of planting a prairie: standard nursery-size plants, plugs and seeding.
Using nursery-grown plants is expensive, unless you can get them from a wholesaler or grow them yourself. It gives the quickest and most predictable results. Plants should go in at 9 per square metre, more or less randomly. Weed control should be rigorous for the first year or two – after that the clumps will have tended to mesh together. If you are including prairie grasses, such as varieties of Panicum virgatum, they do however need more space around them as they tend to develop more slowly than the forbs, both early in the season and from year to year.
It is possible to grow plants from seed in plug trays and then plant them out. However seedlings tend to germinate at different rates, which is where I got a bit unstuck in making a prairie planting at home – if you plant these quick developers out, you end up with a colourful but rather rank planting and it is then impossible to add the later-developing plants. It is better to keep the quick-developing seedlings waiting a year and plant them out with the slower ones – it won’t do them any harm to keep them pot-bound for a time. Plugs can go in at 16 or 25 per square metre.
This is the best method as the seedlings will form a dense carpet of growth, which will rapidly develop a plant community that will completely dominate the soil surface. The perennial seed specialist Jelitto sell a prairie mix, based on James Hitchmough’s work at Sheffield University. Seed can also be obtained from American suppliers but since some do not sell to Europe, they may need to go via an American friend, and may get heavily taxed if sent on by post; importation is however perfectly legal.
Research suggests that sowing into a 5cm deep layer of sand is a good way of minimising weed seed competition, but the site will need irrigation until the plants have rooted through into the soil beneath.
Very few prairie species’ seed need winter chilling, so spring sowing is best.
One point is worth mentioning – many daisy family species are prolific self-seeders (aster and solidago in particular) and can become a weed problem in the rest of the garden, but only if there are two different clones growing together. Using a cultivar means that you are using a clone, so it can’t produce seed; at home I have masses of Solidago rugosa all derived from one plant (a clone) and it never seeds, but when I grew S. flexicualis from seed, and kept around a dozen plants, it soon became something of a nuisance.
There are two main tasks: dealing with end of year growth and weed control.
The mass of dead growth at the end of the year is most easily dealt with by chopping down with a brushcutter or hedgetrimmer sometime after Christmas and just left to rot down, which creates a natural mulch layer that is particularly beneficial for invertebrate biodiversity. You may choose to burn it however – see below.
Anyone who is growing plants through a gravel mulch will need to keep this clear of organic debris, which means growth needs to be removed for composting.
The main weed problems are native pasture grasses, which have an advantage over prairie species in that they grow over winter, along with nettles and goosegrass. Low level non-grass species such as creeping buttercup are not a problem as the taller prairie plants overshadow them. Taller weeds such as nettles are best hand-pulled but patches of grass are best dealt with by spot-spraying with a glyphosate-based herbicide, to avoid disrupting the soil and bringing up yet more weed seed.
Any discussion about prairie soon comes around to the question of burning. It’s a colourful topic, much inflamed by a slight pyromaniac tendency amongst prairie gardeners (there’s a You Tube video of me setting fire to grasses somewhere). In North America, prairies were largely created by fire, either natural or lit by Native Americans to create good grazing for buffalo and game. An end of winter (January or Februrary) fire burns off invading weeds and gets rid of the debris, without harming the prairie species, which do not emerge until later. I have found it a fantastically successful way of getting rid of goosegrass seedlings; this species is a particular problem as it climbs up and over plants during the summer. However our west of Britain climate is rarely dry enough for this to be relied on as a method of routine maintenance, and in urban areas it is also distinctly anti-social.
Using a gravel mulch
The highly successful prairie at Cambo House in Fife, Scotland uses a gravel mulch as a weed suppressant. Here are head gardener Elliott Forsyth’s notes on this:
- Gravel prairies due to their stability can be very long lived.
- At Cambo we used a warm-toned gravel called ‘Harvest Gold’ at 12-20mm grade and aimed for a depth between 5-7cm. Pegs were put in to ensure depth was right. Smaller amounts are available by the tonne bulk bag, which can be delivered to your door.
- Gravel quanities: 1m x 1m x 5cm = 90kg or 1m x1m x 6cm = 108kg.
- We have found that we need to replenish it every 5-7 years.
- We cut back in January-February with a long-handled hedge trimmer (medium length).
- We lift and shred into other plantings with no mulch layer or burn. Other options could include composting. We then give it a good going over with the leaf blower to insure there is no organic build up on the surface which doesn’t’ look that good and can be a place where weeds can germinate. Of the gravel we also weed and spot treat any visible perennial weeds with glyphosate at this stage. We use weed wands, which make the job much more pleasant.
Late-summer flowering prairie plants are very long-lived and constantly regenerating. Their dense root and above-ground growth can create an extremely tough and resilient long-term vegetation, which needs little care and which, like a wildflower meadow, will change gradually from year to year, so you have the satisfaction and interest of having created something which has a life and a dynamic of its own. Bee, butterfly and bird life will be an added bonus.
Research at Sheffield and in Germany and the Netherlands is increasingly focusing on adding spring and early summer components to prairie mixes: camassia bulbs, species of summer-dormant perennials like species of Dodecatheon and Mertensia, or even oxlip, Primula elatior, or winter annuals such as Viola tricolor. Smaller daffodils also combine very well.
Words: Noel Kingsbury is internationally known as a writer about plants, gardens and the environment. He also works as a garden/planting designer and horticultural consultant and is best known for his promotion of what is broadly called an ecological or naturalistic approach to planting design.
Images by Robert Mabic
All the images here were taken at Dutch garden Lianne’s Siergrassen. The prairie-style display garden is attached to a nursery specialising in ornamental grasses.