London Wetland Centre Rain Garden feature by Nigel Dunnett

Wetland garden

Wetland garden Raised timber boardwalks allow visitors to explore dryshod the plantings and features of Nigel Dunnett’s garden at the London Wetland Centre, Barnes. The areas beneath the paths are designed to flood during times of high rainfall.

Bring on the rain

With more frequent periods of high rainfall or drought, gardeners are having to manage their gardens differently. The RBC Rain Garden at the London Wetland Centre is a living example of how to manage water successfully Author: Richard Sneesby, garden designer, author and lecturer. Photography: Jane Sebire

The Garden | November 2012

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Towards better water stewardship One of the world’s largest man-made wetland reserves in a capital city, the London Wetland Centre was created in 2000 by the Wildfowl and Wetlands Trust (WWT) on 44ha (110 acres) of disused reservoirs by the River Thames downstream of Kew. Features included three demonstration Sustainable Gardens. To celebrate the centre’s 10th anniversary, the WWT decided to renovate one of these and commissioned Nigel Dunnett to design a visitor-friendly garden demonstrating good water husbandry by avoiding waste and absorbing runoff. Funded by the Royal Bank of Canada, its first RBC Blue Water Project in Europe, the RBC Rain Garden opened in September 2010.

November 2012 | The Garden

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Wetland garden

he Royal Bank of Canada Rain Garden is a public space like no other. Designed to illustrate how to deal with some of the issues resulting from climate change, it can inspire home owners to adapt their own gardens. The designer, Nigel Dunnett of the University of Sheffield, approached the challenge using his extensive knowledge, making this garden one of the most interesting to be built in the UK for some years. Much has been written about rain gardens over the past 15 years or so. The principles behind them are largely common sense: capture water on site; conserve it for use in dry periods; manage site drainage; and select plant species that will withstand wet and dry cycles. Capturing water requires an understanding of where rain is falling so that it can be collected – mainly roofs and hard surfaces. The roof of an average house in the UK might collect 45,000 litres (about 10,000 gallons) of water per year, nearly all of which ends up in urban drainage systems or soakaways. A large water butt might hold up to 1,000 litres, but the ability to store a small amount of the remaining, ‘lost’ 44,000 litres of water can make a big difference to a gardener’s watering regime. The main problem is not lack of water, but how and where to store it during times of drought. One option is to capture it onsite and store it at ground level in ponds and swales. The key is to contain enough for use in dry periods, and release the excess slowly into external drainage systems at other times of the year. This is the principle behind sustainable urban drainage systems (suds), which is implemented throughout the Rain Garden.

Ideas from the Rain Garden

A recycled shipping container has been reformed to be a garden pavilion: the roof collects rain water and channels it through a series of filtration systems.

Made from recycled materials, this sculpture is a habitat for wetland and garden insects.

Plants in this circular pool, such as Rudbekia fulgida var. deamii, astilbes, Rodgersia and Primula pulverulenta, can withstand large amounts of water.

In this pool the roots of Phragmites australis subsp. australis ‘Variegatus’ clean the water as it passes through, which itself has been fed from previous pools.

Paths and paved areas should be permeable and sit above the highest water level in times of flood.

Thoughtful planting

The Garden | November 2012

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Creating your own rain garden The techniques used in the RBC Rain Garden can adapted to domestic gardens:

The raised boardwalk sweeps through drifts of Gaura lindheimeri and Aster x frikartii with grass Calamagrostis x acutiflora at the back.

richard sneesby

What makes Nigel’s garden special? His circular design incorporates sweeping arcs of curved timber paths, carved through tall flowering grasses and herbaceous perennials. These paths are raised above ground level to allow the underlying ground (but not people’s feet) to get wet. A central axis is paved in permeable gravel with timber sleepers strategically placed across the main pond, allowing visitors to get up close. There is a chain of four circular pools that direct the water gathered from the green roof of a pavilion through a series of planted beds. The pools have different water regimes and planting, so that moisture level decreases from one pond to the next. This layout could be adapted for domestic gardens to clean grey water (from sinks, baths and washing machines) before discharging it into freshwater ponds or drainage systems. The water in the Rain Garden is not entirely left to its own devices, though. Should the systems dry out, there is a pump to send water back to the top. While Nigel has concentrated on ‘rain’ and ‘water’, he has not forgotten ‘garden’. Fortunately he knows his plants – he has chosen those with physiological adaptations that allow them to survive and thrive in this stressful environment. They look good, too (this is a garden after all), making this a place for enjoyment, visual stimulation and somewhere to put us in a good mood.

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✤ Water can be collected by diverting rainwater from roofs 1 . It is important to estimate (or accurately calculate) volumes of water to create appropriate storage areas. ✤ Ponds for wildlife 2 should be carefully designed with shallow slopes as well as deeper water areas. ✤ Use and choice of plants 3 needs careful selection. Know your soil type, aspect and soil depth, but also choose plants that will withstand the wet and dry cycles characteristic of this environment. ✤ Where soils and underlying geology are not suitable for storing water, waterproof membranes need to be installed. ✤ Paths and paved areas should be permeable, for example decks and gravel, and sit above any high water levels at times of flood (right). ✤ Maintaining the garden is important: replenish mulch to retain water in the soil, weed during the dry spells and keep ponds free of excess silt.

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Wetland garden His plant selection is thoughtful, creative and for all seasons. Many plants absorb excessive amounts of water and help control the moisture balance within the soil. The front of the garden is seeded with a Dunnett trademark, an annual pictorial meadow, similar to those at the London 2012 Olympic Park. There are aquatic and marginal selections such as Schoenoplectus lacustris subsp. tabernaemontani ‘Albescens’, Persicaria bistorta ‘Superba’ and Rodgersia pinnata ‘Superba’. These are supplemented with more ornamental subjects including Helianthus ‘Capenoch Star’, Astilbe chinensis var. taquetii ‘Purpurlanze’ and Rudbeckia fulgida var. deamii. The parts of the garden that swing between being excessively wet and dry contain plants that tolerate such fluctuations, including Caltha palustris, Ligularia ‘Little Rocket’, Iris pseudacorus and Lychnis flos-cuculi. Where there is no danger of permanently saturated soils you can find garden favourites such as Echinacea purpurea and Calamagrostis x acutiflora ‘Karl Foerster’ combined with Aster x frikartii ‘Mönch’, Kniphofia ‘Tawny King’, Gaura lindheimeri, Phlomis, Monarda and Helenium. The appearance of this garden might not be to everyone’s taste. It is more of a demonstration garden or scientific experiment, to be be used for inspiration rather than copied. There is no lawn, for example, or areas for growing food, so adaptation would be required. Despite our dreary wet summer, it looks highly likely that the principles illustrated here will become increasingly important in the future of UK gardens.