How many species can you really fit on one roof?
The birds and the bees have been the focus of much attention recently, and finding creative and exciting ways of sustaining them has become much more than a new trend. This is not a marketing report for Ann Summers, but a reference to the ways in which biodiversity targets have filtered into everyday consciousness, from ‘save the bees’ to green corridors.
Green roofs can play a big role in providing space to support wildlife, among their many possible environmental functions. Putting the right species on the roof according to its location and the function you want it to perform is always important, and common advice to increase the variety of species on the roof is generally designed to avoid the unhelpful symptoms of a monoculture sedum roof.
Putting more varied species on a green roof will help mitigate the loss of coverage should any species perform poorly in a particular year, and well as providing more visual interest and a better food supply for invertebrates at different times of the year. Common sedum roofs have a short flowering period, which is why meadowflowers are preferred for wildlife support.
Does it follow that the more species you put on a roof, the better? Some wildflower turf producers have taken this quite literally and seem to be aiming for a hundred species per roll. Certainly not all of these will survive on every roof, but the theory that starting off with the maximum possible and seeing how many last in the long term is interesting.
There are a couple of points worth considering before blasting your roof with as many species as you can. One is that if you are creating a habitat for particular species, then there will probably be supporting food plants specific to that kind of ecosystem. It may be more important to make sure these plants do well than to add lots of other species that might out-compete them. Another is that if the green roof is designed to gain BREEAM ecology points then there will be certain recommended species for the site, and finding a selection of these that are available at the time of planting may dictate how many are used.
Ideally with enough notice, your recommended plants can be grown to order. But this still leaves the question, how many species constitutes a good bio-diverse roof? The size of the planting area is important here. A modest roof area of 50 square metres with a good planting density of 30 plants per square metre will leave 1500 plants to order, trays of 100 plugs of a single species provide the opportunity for 15 different species. This allows quite a good range which, with consideration of flowering periods and local conditions, should provide well for many insects.
On smaller roofs this diversity can be achieved by planting plugs of key species which are long flowering and suitable for many insects, and adding sedum cuttings or seeded grass and wildflowers of other species to see what else can establish itself. With larger roofs one has the option to order just 100 of as many species as can possibly fit into the space. However it could be considered that once there are twenty different species present, all your main functions, flowering periods and support functions may already be covered.
Twenty is only a guide number and there are various exceptions, such as bio-diverse roofs with different substrate areas to support a mix of ericaceous and calcareous plants, and recreate different specific kinds of habitat. It is also advisable not to stick to the same twenty wildlife species on every roof, as this contradicts the essence of the word biodiversity and does not show consideration for climate and local ecology. I am merely asserting that twenty well-chosen species may be much more effective than one hundred indiscriminate additions that may have no relevance to the situation and ecosystem in question. I would also suggest that the distribution of the species mixed widely across the roof surface rather than in larger swathes of one or two species per section will be much more effective for the survival of a rooftop ecosystem with little maintenance, in contrast to normal ground planting schemes.
If in doubt, it is always best to consult an ecologist. However we aim to make some of the basic principles of roof greening accessible to a wider audience and with the help of ecologists at White Young Green and the online biological record database, we will be including in our catalogue pointers towards the best species for supporting biodiversity in a limited space.