Maggie Fennell, Technical Green Roof Expert at Boningale GreenSky, explains why adopting the “right plant, right place” theory to green roofs will help to maximise their success.
It’s a simple idea but one that is fundamental to successful gardening: to put plants in conditions that suit them so they are more likely to thrive.
The book Right Plant, Right Place was first published 20 years ago by Nicola Ferguson and went on to become hugely influential among gardening aficionados. Its influence can be seen across seed and plant sellers’ catalogues; even the Royal Horticultural Society’s online plant finder tool is inspired by the same idea. The same premise can be applied to green roofs.
Whereas gardeners have a range of soil types to deal with, it is assumed that the growing medium for a typical green roof will be an engineered, free-draining mineral substrate that should be suitable for where it is used.
Even so, there is some variation between substrates. Some biodiverse roofs will even use different substrate types on the same roof.
Whilst waterproofing and drainage are crucial to any green roof’s success, it is the plants and their supporting layer that are providing the essential ecosystem services and benefits beyond those of a grey roof. By keeping a focus on the ‘green’ and creating the best plant solutions for UK climates and requirements, the performance that is so vital for creating a sustainable urban environment can be improved, and therefore the real value will be maximised.
Plants that are growing on roofs are exposed to greater extremes of temperature and harsher winds than they would expect on the ground. They are also subject to varying conditions; sheltered or exposed, shady or full sun.
These plants are being asked to work extra hard with very little care and for this reason, making sure they are the right plant for their location is vital.
Boningale GreenSky’s SkyPlugs, which are specifically engineered for green roofs, are grown to suit roof conditions. But, green roofers are encouraged to think about which will suit which areas to maximise their performance.
We believe that using a mix of different juxtaposed plants, rather than planting large swathes of one species, will improve the roof’s performance. This is backed by academic research and development by green roof technology experts at the University of Sheffield’s Green Roof Centre.
It’s also useful to remember that roofs that have a steeper gradient may benefit from more drought-tolerant species at the higher points of the gradient but less drought-tolerant plants at lower points near the drainage outlets.
And if your roof has more shade, ask for our list of shade-tolerant species that can cope with tricky areas that are short of sunlight.
By transferring the right plant, right place theory to green roofs, in addition to using the right kind of substrate for the conditions, you have a much better chance of the plants thriving and thus creating a sustainable green environment.