Creating a roof garden

In an exclusive interview with Reckless Gardener, Professor Nigel Dunnett, looks at the advantages of creating a rooftop garden, from the health benefits of exposure to wildlife and green spaces to the efficient drainage of rainwater. (Pictured left, the Royal Bank of Canada Blue Water Roof Garden for RHS Chelsea Flower Show 2013.)

Nigel Dunnett is Professor of Planting Design and Vegetation Technology and Director of The Green Roof Centre, University of Sheffield. He returns to RHS Chelsea Flower Show this year for the third year in a row with an urban rooftop garden designed for the Royal Bank of Canada in association with the Landscape Agency. He was one of the principal consultants, with Professor James Hitchmough, on the London Olympic Park and its transition into the Queen Elizabeth Park. He is the author of several books including “Small Green Roofs” (Timber Press 2011) and writes widely for the horticultural and gardening press.

Reckless Gardener: What are the steps that an average suburban gardener can follow to achieve a rooftop garden/area?

Nigel: “As a starting point, it is essential to make sure that the intended site can take the load of a garden or living roof: it is best to consult a professional if in any doubt. Make sure the space is thoroughly waterproofed – on smaller structures it is possible to use DIY materials such as a pond liner to protect the roof surface. Again this may need to be checked by a professional.

“It is not as challenging as you would think to create a fully sustainable and biodiverse roof garden. The first step is planting on any available surface – creative planting to reduce the amount of paved-over surfaces is the key. This type of planting will act to capture rainwater and minimise the amount of watering needed. Secondly, you could consider adding a bug hotel to attract different species of insects and encourage biodiversity. These are available in most garden centres, and can vary widely in structure and content to suit the needs and style of your garden.

“For the more adventurous, you could buy or build a habitat stack. These man-made structures use natural materials to encourage biodiversity by creating the ideal environment for insects and invertebrates to nest, feed and hibernate. They can be sculptural in appearance, adding an artistic element that can be designed to match the feel of your garden.

“If you start with these simple steps, you are on the road to creating your own water sustainable and biodiverse roof garden. Be bold, be creative and don’t be afraid to try things.”

Reckless Gardener: What sort of planting would you suggest they use?

Nigel: “Carefully selected flowers and plants can help attract and foster local wildlife without compromising on colour and creativity. It all depends on the depth of substrate or soil you can work with. Wildflowers from dry meadow habitats are perfect. Many alpines will love the free draining conditions. On thinner soils, succulents such as sedums are tried and tested. Hardy annuals will self-seed from year to year.”

Reckless Gardener: Why would someone want to create a roof garden?

Nigel: “With urban space at a premium, rooftops are becomingly an increasingly attractive option for city dwellers looking to create a garden of their own. There are many studies to show the benefits of having exposure to wildlife and green spaces that can have provide a break from the daily grind. Roof tops are often the only available space for introducing greenery and vegetation to a cityscape.

“Many of us assume that a terrace or a rooftop is only suitable for small-scale planting and container growth but it can do so much more than this – trees, lawns, and even small-scale woodlands and meadows are all viable possibilities and with the simple steps above, you will be surprised what you can achieve. Not only do these roof gardens provide an escape for stressed-out city workers in places where space is in short supply, but these gardens can help us address the environmental issues caused by increasing urbanisation. Also, small green roofs are perfect for garden buildings and sheds that would otherwise be not so attractive to look at.”

Reckless Gardener: Environmentally, what are the advantages of creating a rooftop garden?

Nigel: “One of the key issues in urban environments is the efficient drainage of rainwater due to the large expanses of concrete, which can lead to flooding. If every building had a rooftop garden or living roof – be it on an entire office block or simply a garden bin store – much of this flood water would be absorbed before it even hit the ground.

“The rooftop garden I have designed in partnership with RBC to be displayed at this year’s RHS Chelsea Flower Show is a ‘rain garden’, whereby every drop of water that falls on it is captured and recycled, rather than sitting on an impermeable surface and contributing to urban flooding pressures. They also provide a perfect place for wildlife and bring habitats and biodiversity into otherwise lifeless areas. Even if it is not possible to use the roof garden all the time, bringing plants, vegetation, green and colour into otherwise grey environments can have a real therapeutic benefit if they can be seen from inside the building, or from other buildings.”

Reckless Gardener: Do you have a particular favourite roof garden, perhaps one you have designed, and why is it your favourite?

Nigel: “The one that captured my imagination in the first place was the green roof on Chicago City Hall – a beautiful landscape with trees and meadows, and full of flowers. If they could do it there, I thought, it is possible anywhere.

“I am very proud of two very different roof gardens that I have designed: Moorgate Crofts in Sheffield has become a fantastic diverse garden, sparkling with wildflowers; and Sharrow School in Sheffield, has a ‘rooftop wilderness’ which is a colourful riot of plants, birds and insects.”

Thanks to Nigel for taking time out of his busy schedule preparing for Chelsea 2013 to highlight the advantages of rooftop gardening. An overview of his garden for the Royal Bank of Canada can be found on Reckless Gardener.