First published twenty years ago, the book of this name by Nicola Ferguson went on to be a seminal piece of gardening literature. 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. That is because the idea is simple, and fundamental to successful horticulture. The idea is this: put plants in conditions which suit them, and they are much more likely to do well!
Gardeners have a range of soil types to deal with, whereas on a typical green roof it can be assumed that the growing medium will be an engineered, free-draining mineral substrate – hopefully designed according to the needs of the situation. However there is still variation between substrates for extensive, low maintenance roofs and more profusely flowering semi-intensive roofs.
The key point to recognise on green roofs is that on a roof plants are exposed to greater extremes of temperature and harsher winds than they would expect on the ground. Even this inhospitable microclimate can provide different conditions: sheltered or exposed, shady or full sun. Given that we are asking these plants to work extra hard with very little care, getting them in the right place to start with is more important than ever.
SkyPlugs are already grown to suit roof conditions anyway, but we encourage green roofers to think about which plants will suit which areas to maximise their performance. We still maintain that mixing plants up rather than planting large swathes of one species will improve the roof’s performance.
So why not plant all the shade tolerant species in the shaded or north facing areas of the roof, and the sun-loving plants in full sun? To this end we have produced a list of more shade tolerant species to cope with tricky areas that are short of sunlight.
As well as looking at shade and exposure to wind, on roofs with more of a gradient it may often be helpful to plant more drought tolerant species at the higher points of the gradient, and less drought tolerant plants at lowers points nearer the drainage outlets.
The right plant, right place theory is just another example of how we should be using good horticultural practices, which are commonly accepted at ground level, to improve the performance of green roofing.