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Flowering meadow design: RHS steppe-prairie

by Timber Press on March 23, 2017

in Design, Gardening

In autumn 2013, the RHS prairie was a mass of lilac and purple from Aster turbinellus and Aster oblongifolius. Included photos by the author.

Using a distinct technique of sowing meadows from seed, James Hitchmough creates plant communities that mimic the dramatic beauty of natural meadows and offer a succession of blooms over many months. With case studies in some of the world’s most respected gardens, Hitchmough walks you though the gardening techniques and design strategies you need to capture this distinctive style in your own home garden.

RHS Garden Wisley
Steppe-Prairie
Wisley, Surrey, England

 

Plant communities
North American prairie with western North American and Eurasian steppe elements sown onto 100 mm of sand mulch, with open-weave jute erosion blanket; plug planting used at low levels, including 200 plugs of Pulsatilla vulgaris and Dodecatheon meadia ‘Goliath’ across the site.

Seed sources
Seed mainly from Jelitto Perennial Seeds

Client and conditions
This is the Royal Horticultural Society’s oldest and most heavily visited garden, very much a prestige horticultural location. The site has relatively warm summers for the United Kingdom and potentially quite cold winters (minima of –15°C, although only very occasionally so). Average rainfall is 650 mm. RHS Hardiness Zone H4, USDA 8b/9a. Soils are very sandy, moderately productive, and prone to episodes of severe drought.

Timescale
Site prepared in summer 2007, sown in January 2008

HISTORY AND DESIGN APPROACH
I was invited by the Royal Horticultural Society (via Tom Stuart-Smith Design, who were designing the bicentennial borders around the garden’s new glasshouse) to undertake this project. My 600 square m were the final and wildest portion of the planting sequence. It was an opportunity to respond to a different sort of site—one with relatively dry, drought-prone soils and a warm (for the United Kingdom) climate—with an expert horticultural workforce. I came up with a hybrid vegetation, based partly on North American prairie species often associated with mid-range to dry sites, combined with what were essentially steppe species from the central to southern Rocky Mountains and Eurasia.

Many of the steppe species were coming into flower by late May 2009, here dominated by Dianthus carthusianorum and blue Penstemon strictus, along with pink Phlox pilosa and scarlet Castilleja coccinea.

This project was also an opportunity to further develop the spatial form of my designed communities. To date, I had mainly worked with prairie species that formed a 1-m-tall layer out of which arose occasional large emergents. When sown at high densities, such as at the Sheffield Botanical Gardens, this creates a meadow-like vegetation with a topographically rather uniform, almost flat surface. At RHS Garden Wisley, I wanted to investigate whether with seeding alone it was possible to produce a much more topographically varied surface. To try to achieve this, I worked with a community composed of three layers of foliage: a ground layer of flat rosettes to low mounds up to 300 mm tall, primarily of steppe species; an intermediate layer of mounds or upright stems to 600 mm tall, mainly composed of prairie species; and then relatively few species with foliage taller than 600 mm, such as Aster turbinellus, and the basal foliage of giant forbs such as Silphium laciniatum.

To reduce shading of the generally shade intolerant lowest layer, the aim was to progressively decrease the density of species proportionate to their foliage height. This can be seen in the table, with some Penstemon (foliage mainly in ground-level rosettes) at up to 6 seedlings/square m and Silphium at 0.25 seedlings/square m. I used a spreadsheet to help conceptualize how many target seedlings would be allocated to each of the three canopy layers. Of a total target density of 77 seedlings/square m, 56 of these were allocated to the lower foliage species, 16 to the intermediate layer, and 5 to the tallest foliage layer. The actual values are not hugely important; it is the relativities that are key.

The drama increased further by mid June 2009.

Nearly all of the taller species chosen had leafless stems, that is, largely basal foliage with flowering stems with few leaves present. This both reduces shading and creates see-through visual effects. The other innovation was to reduce the target seedling rate, from 150-plus in my earlier prairie sowings to less than 80 in this scheme. This was a slightly scary decision to make in this high-profile site, with a preponderance of small and slowly growing steppe species, but a necessary one if a more complex topography more reminiscent of central European matrix planting was to be achieved. In essence, the entire site was sown with an edge mix. Although it seemed certain that the prairie species would eventually dominate the planting, it was unclear just how quickly this would happen. I anticipated that most of the steppe-like species would ultimately be eliminated except on the edges, where there was more light. Given the high-profile nature of the site and the horticultural expertise of the staff, I believed there was a good chance that these processes could be successfully managed.

WHAT WORKED AND WHAT DIDN’T
I first visited the site in May 2008 for a meeting with the Wisley staff, who I think were convinced the sowing was a complete failure. At the one-leaf stage, 70 to 80 seedlings/square m looks like no seedlings at all to the uninitiated. It was obvious to me that, despite their concerns, emergence was mostly on target. The site had been irrigated from March to my visit, and this continued to the end of June.

By September 2008, the site was approximately 50–75 per cent covered in the foliage of the sown species. There was much less shading by the sown species than in the more conventional prairie sowings, so weeding continued as required across the summer. The vegetation was cut down and the limited amount of dead material removed in spring 2009. Flowering started in earnest in May 2009 with Penstemon and Dianthus, and it was quite an extraordinary sight. Most of the Rocky Mountain species worked very well, including Oenothera macrocarpa subsp. incana, which often fails to perform in mixed communities in Sheffield, where summers are just too cool.

By early August 2011, the structural composition had changed from steppe to prairie.

In 2010 the community was flash burnt in spring for the first time to check the build-up of annual winter-growing weeds, such as winter grass (Poa annua). This burning is tolerated by Dianthus and some Penstemon, but it probably accelerated the switch to prairie dominance. In late July 2011 Echinacea pallida was visually dominant, signalling the anticipated switch over from steppe to prairie dominance.

The site has also provided an interesting study of how maintenance staff interact with naturalistic vegetation. From 2009 on, the Royal Horticultural Society was in a state of flux, with the departure of many senior gardening staff. The average gardener is now much younger, and the rotation of garden team supervisors on a regular basis has led to issues of continuity in understanding this planting. The latter is a general issue in the management of this type of vegetation.

Pulsatilla vulgaris also produces exceptionally attractive seed heads in May.

Because of my London Olympics commitments in 2012, I did not visit in that year. By 2013 it was apparent that there was a pressing need to remove excessive Aster oblongifolius and Aster turbinellus, which were beginning to dominate and shade out the other species, including Echinacea. In Tom Stuart-Smith’s home garden the same species also began to dominate, but Tom as the guiding force saw the need to respond to this early on. At Wisley this did not happen until I hit the panic button in 2013, and when it did, the approach by the new supervisor was to clear large areas of asters, thus creating large openings, rather than remove plants selectively bit by bit. Predictably, these big gaps just became weedy patches, making matters worse not better.

Staff rotation has made it difficult to develop an understanding of how to work more ecologically. Since 2013 management has been more proactive, but there are still issues with prioritizing maintenance resources. Because these areas are viewed as “natural” or “wild”, the management of weeds appears to be seen as discretionary, to the detriment of visual quality.

The rare European native Pulsatilla vulgaris, with attractive flowers in March and April, has proved to be robust and has established throughout the prairie from the original seedlings.

Despite these problems, the vegetation in the main continues to look reasonably good. The major events are in April, when the long-persistent and effective self-seeder Pulsatilla vulgaris comes into flower, then mid summer with Echinacea pallida and Asclepias tuberosa, and then the asters from September on. The major problem is the August gap, as not much flowers at this peak time. Although we have identified species to address this gap, these have rarely materialized.

If I was to design another version of this vegetation for a similarly dry site, I would increase the density of the steppe layer, decrease the density of the intermediate and tall foliage layers, and probably add Parthenium integrifolium, and Liatris scariosa or Liatris pycnostachya for August interest. I would also increase the grassy component, but prairie grasses are problematic in these not quite warm enough summer climates; the large ones potentially shade too much and the small ones tend to get eliminated by shading. Sporobolus heterolepis persists but is both very expensive and quite difficult to establish via sowing.

 

James Hitchmough is an expert in the design, ecology, and management of herbaceous vegetation. His techniques have been used to make meadows and meadow-like communities at prestigious sites worldwide. Hitchmough is head of the Department of Landscape Architecture at Sheffield University in the UK.

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