With so many ifs, buts and variables in green roof design, I thought I’d have a go at putting forward my idea of the perfect green roof build up. This is what I would choose if I had a nice garage-size roof space to fill, assuming both the view and the maintenance responsibility were mine.
Firstly I know I want a mix of sedums and meadow flowers which will support each other and provide variety through the seasons. I want to provide food for pollinators without having an entirely bug-focused roof with habitat features.
This is leading me towards a mix of low-growing alpines and wildflowers with a little height variation, but I will exclude the taller, wafting wildflowers that will need cutting back. I will limit myself to 12 species otherwise I will get over-excited and choose too many different species, allowing for about two plants apiece.
Four succulents sound like a fair balance: a Sempervivum, Delosperma cooperii, and two of my favourite sedums. Petrorhagia saxifraga and Armeria maritima, will provide good long flowering season between them, and I’ll add a Thymus and an Allium as I want some herbs in there.
For a little interest, a small tufty grass like Koeleria macrantha would nestle well between these and give a bit of movement without getting too big, and I would add a Lewisia as my favourite ‘new’ experimental plant – such beautiful flowers and foliage. Most of these species would get along quite well in a low nutrition substrate, so if I round off with Lotus corniculatus and Trifolium pratense which will happily fix their own nitrogen and provide some coverage straight away, that should work well. These two are also fantastic for wildlife.
On 120mm of ultra- low nutrition substrate this will settle in nicely in the first 2 years, if I accept that the Lotus and Trifolium will steal the show in the first year. It may be a little biased towards the pinky/purple end of the spectrum but some white- and yellow –flowering sedums will help.
There were many plants I could happily have added in here – maybe a Prunella and Bellis perennis would have been safer than the Koeleria and Lewisia? – but actually this meets my desired balance between a beautiful roofscape and a haven for biodiversity, so I will leave it there.