Green roofs are a great way of supporting pollinators, especially struggling bees, by providing an extra source of nectar. But with so many green roofs composed of sedums only, how well does this single genus contribute to biodiversity support?
A recent article in the telegraph http://bit.ly/1m04934 explains that bees experience a summer gap in food supply, and have to travel further to find food than they would in spring or late summer/autumn. I have noticed on my wildflower roofs that a lot of species like Leucanthemum vulgare and Lotus corniculatus, favourites on biodiverse roofs, finish flowering in early summer and come back again for a second flush later in August.
Sedums have a relatively short flowering season, but that season does bridge this summer gap, providing nectar throughout june, july and early august. Also these plants are extremely drought tolerant, so when other summer-flowering species are struggling with lack of water and hosepipe bans, the flowering succulents suddenly become more important.
I don’t think a roof entirely composed of one genus can really claim to be biodiverse, but I would recommend sedums alongside other plants as a good element of a wildlife-friendly scheme.
Some of the other plants I would recommend that help extend the flowering season for bees and other pollinators:
• Centranthus ruber and hypericum perforatum are good summer flowerers if you have a decent depth for a semi-intensive roof (120mm+).
• Centaureas (knapweed/cornflower species) and various scabious are great summer and early autumn performers
• Prunella and Dianthus species cover the summer and are some of the latest flowering autumn species we usually stock.
• Thymus and Origanum are great summer flowering herbs
• Linarea purpurea, although not technically a native, has been great so far this summer and popular with the insects.
• Primulas are my favourite for early spring flowers before everything else gets going, and seem to flower through tougher conditions.
Now go forth and feed those bees!