Early may is when bees are flying strongly and butterflies are out of hibernation and looking for food. It’s also when UK green roofs start to bloom in earnest.
One of the first plants to bloom on extensive green roofs is the meadow saxifrage, saxifraga granulata. This native wildflower likes to live on well drained grassland but also thrives in shallow layers of green roof substrate.
Its name, saxifrage, comes from the Latin word for ‘stone breaking ‘ probably because the close relatives of meadow saxifrage tend to live in stoney ground. The species seems to be drought tolerant and because it is low growing with fleshy leaves it is quite happy on a green roof. It certainly looks lovely blooming its socks off on top of my brother’s shed.
The Other white flowering native plant normally found on an Enviromat green roof is sedum album – the white stonecrop. Coming into bloom just after meadow saxifrage this flower is like a magnet to honeybees and bumblebees as well as attracting butterflies who enjoy this rich source of nectar.
Sedums are well adapted for living on green roofs where there is a shallow layer of growing medium. Typically these living roofs are lighter than deep substrate roofs and are ideal for retrofitting on to existing buildings or installing on to garden sheds. Where the building can support more weight it is possible to have a deeper layer of growing medium and subsequently a more diverse range of plants.
Other white plants for green roofs
If there is a minimum depth of 15cm of well drained green roof growing medium, other white flowers such as daisy, wild strawberry or even mountain avens should do well but beware of plants that have invasive roots – for they may ultimately damage your waterproofing – and be aware that taller plants may be damaged by strong winds.
There is work being done to assess the value of Stachys (lambs ears) on green roofs. It is thought that the plant’s silvery white leaves will do an excellent job of reflecting heat away from the roof. Having grown stachys in my garden, I personally would be concerned about it crowding out other plants. But that’s just me.
Remember the old adage “right plant, right place “.