Steppe by Steppe to plantings for the future

Interest in steppe plantings is increasing rapidly. That’s great, because steppe vegetation is adapted to harsh, exposed conditions, with regular summer water-deficit. These are difficult conditions anywhere, but typical of roof gardens and green roofs and other very urban situations where the urban heat island and dominance of hard surfaces creates very stressful conditions. I’ve been working for around 15 years with designed versions of steppe plant communities to create ‘Sky Meadows’ as a form of sustainable planting for roof gardens, green roofs and other exposed dry and stressed situations with thin soils.

Photos below: a) and b) Steppe Grassland with Stipa grasses and Campanula sibirica, on thin calcareous soils over limestone, Austria.  All photos by Nigel Dunnett unless credited otherwise  c) natural steppe grassland on the Ukraine/Russia border in spring with Tulipa sauveolens and Iris pupils (photo Ekaterina Bezuglova)

What is Steppe?

Steppe is a vegetation type of continental climates where there is insufficient water availability to enable trees to grow. Typically these are grasslands, and often consist mostly of grasses, and they occur across the Northern Hemisphere on low-fertility, free-draining calcareous soils and geologies. In fact the chalk and limestone grasslands of the UK on thin, infertile soils can be seen as a variant of steppe, on the very edge of its climatic range.   It’s what might loosely be termed as ‘steppe meadows’ that have interested me most, where there is a higher diversity of flowering plants in the vegetation, although the grasses remain very important.

Photos below: some European steppe plants in natural habitats: Dictamnus albus, Dianthus carthusianorum, Salvia pratensis, Euphorbia cyparissius, Centaurea montana and Geranium sanguineum

The beginnings of my research into steppe plantings

I initially started work with European dry meadow and steppe species in 1999 when I set up the first trials for green roof plants to be undertaken in the UK (the subsequent published research paper was also the first green roof paper for the UK). This trial looked at the influence of the amount of irrigation and the depth of the growing medium (substrate) on the growth and performance of those species.

Photos below: a) Green roof plant trials at the University of Sheffield, b) Trial of Alliums for green roofs

My first steppe planting: Moorgate Crofts green roof, 2004

In turn, this then allowed me to develop a suite of reliable and dependable plants for use in real green roof projects. The first of these was the green roof on Moorgate Crofts, Rotherham, which was planted in 2004. This was also the first time I used the ‘random planting’ approach to set up a naturalistic perennial planting, based upon several plant mixes and a detailed specification, but without a detailed planting plan. Moorgate Crofts has also, arguably, been the long-term green roof project that I have learnt the most from.

Photos below: steppe plantings at Moorgate Crofts, 2005.

Photos below: the spring aspect at Moorgate Crofts green roof.  Primula vulgaris (Cowslip), Pulsatilla vulgaris (Pasque Flower), Euphorbia cyparissias, Helicotrichon sempervirens (Blue Oat Grass).  

The autumn aspect at Moorgate Crofts.  a) Euphorbia cyparissias in autumn colour, b) Piet Oudolf and myself visiting Moorgate Crofts in Autumn 2015

Big scale application: The Barbican 2013 – present

The lessons from Moorgate Crofts, as well as on-going plant trials focusing on dry meadow plants and bulbs (e.g Tulipa and Allium) fed into the subsequent development of the plant mixes for the largest application of steppe planting in a roof garden context: Beech Gardens and the High Walk at The Barbican, London – an ongoing project that started in 2013. Here, combinations of designed steppe meadows and ‘shrub steppe’ formed the basis of the plantings in this very exposed, windy, dry public podium landscape.

Photos below: The Barbican Steppe Meadows in spring, with Primula veris, Pulsatilla vulgaris and Tulipa praestans, within a matrix of the grasses Sesleria nitida and Helicotrichon semperivirens.  Compare with the photo in the introduction of natural steppe with Tulipa and Iris amidst a matrix of grasses.  

Photos below: the grass component is essential to the steppe aesthetic.  These photos show Sesleria nitida in flower at The Barbican and a Sesleria spp in natural steppe

Photo below: Allium ‘Purple Emperor’ flowering in The Barbican steppe meadows in June

Photos below: late summer in the steppe meadows at The Barbican, with Echinops ‘Veitch’s Blue, Origanum laevigatum ‘Herrenhausen’ and late summer grasses

Photo below: Kniphofia ‘Green Jade’ in September in the steppe grasslands at The Barbican

Steppe planting on the ground

Of course, there are widespread applications at ground level for creating steppe meadows on dry, freely draining soils. The seeded landscapes I developed in the ‘Fantasticology’ zone of the London Queen Elizabeth Olympic Park indicate the potential of climate-change adapted, low-input vegetations for gardens and parks. And in cities, much of what we might regard as ground-based landscapes are actually ‘landscapes over structure’ and steppe vegetation offers great opportunities for low water-use plantings.  With ever-increasing probability of extended summer hot dry periods, steppe plantings are a perfect basis for plantings that are adapted to these conditions.

Photos below: a) and b)part of the sown steppe grasslands in the London Queen Elizabeth Olympic Park, with Echium vulgare,  Linum perenne, and Centaurea scabies; c) designed steppe plantings by Sabina Plenk in unirrigated roadsides in Austria, planted into a free-draining gravel mulch.

Key Challenges

In order to survive the harsh period of summer drought, many steppe plants flower in the spring and early summer, and then go into a sort of shut-down to withstand the driest and hottest part of the year. The challenge therefore is to find successful later-flowering species to extend the season beyond early-mid-summer. Partly this can be achieved through a switch from a focus on flowering interest to foliage, textures and seed-heads later in the year, but partly it is also about introducing very tough drought-adapted species from natural steppe planting. This is a focus of some of my current research, in collaboration with researchers from Russia, investigating the late summer aspect of the so-called ‘Mammoth Steppe’.

In the steppe meadows at The Barbican, Aster amellus is the last species to flower in August to October

In working with steppe vegetation, I don’t take a purist approach of trying to recreate in detail specific plant communities from different parts of the world. Instead, it’s very much a mix-and-match approach of ‘designed ecology’ – combining species from different areas to create new cosmopolitan designed steppe plantings that might not bear any resemblance to anything you would find in the wild.

Photo below:  The Barbican steppe meadows in late summer, with additions such as Kniphofia and Verbena bonariensis to extend the flowering season

A new approach to roof gardens

There’s long been an artificial distinction between roof gardens and green roofs. Roof Gardens (intensive green roofs) typically are formal, with deeper soil depths – and usually look like a garden on the ground transferred to a roof, requiring a lot of maintenance. Green roofs (extensive green roofs) are seen as more ‘ecological’ with a greater diversity of plants, increased value to wildlife and requiring little in the way of maintenance. There’s no reason why roof gardens cannot also be ecological, biodiverse, require low inputs and maintenance and also look fantastic. The use of steppe planting and sky meadows is one exciting way to achieve this.