Extensive green roofs are traditionally dominated by sedums, for their extreme drought tolerance, or wildflowers chosen for biodiversity support. Plants need to cope with extreme weather conditions, poor soils, and have a good degree of drought tolerance so sedums and some wildflowers are natural candidates.
However there are many more species that can thrive in these conditions, particularly when you have 150mm substrate depth to allow for extra root growth and water storage.
Sedums are often chosen to provide fairly uniform winter cover (although there are deciduous varieties, and many that naturally turn red or orange in winter). Wildflowers are mostly deciduous and create big bursts of flowers and a great summer display, but can get large and unsightly when they have finished flowering. The subsequent removal of dead material can create a large maintenance burden.
To get the best aesthetic and biodiverse performance with the lowest maintenance requirement, a range of low-growing, evergreen plants with a wide variety of forms and flowering times would fit the bill perfectly. Some native wildflowers such as Bellis perennis, Helianthemum nummularium and Dianthus deltoides are strong contenders, and a range of different forms of evergreen Sedum can be used as well – Sedum oreganum , Sedum reflexum and some Sedum spurium varieties for example. Sedum spathifolium ‘Cape Blanco’ has a striking foliage colour for highly visible schemes.
There are many other alpines and cultivars that can serve, as long as they are hardy and drought tolerant. Petrorhagia saxifraga and Delosperma and Mesembryanthemum species are well tested, but underused. Some of the species I have been excited about more recently are the beautifully structured Saxifraga paniculata, Lewisia cotelydon, and Alyssum montanum.
More information about other types of planting such as herbs and bulbs can be found on our website design pages. There is a lot more to be done on other types of planting, such as shade tolerant plants to combine with solar panels. There is certainly no excuse for a boring green roof!