Bee Study (Swiss and UK)

Bees are known to use green roofs and images of them are often used in green roof company literature. In the UK, there has been no long-term study of green roofs. However, bees were collected as a by-product during the London PhD study, though were not the focus of the study.

A study of bees on green roofs in Switzerland showed that roofs planted with a mix of both wildflowers and sedums were used throughout the foraging season [April to September], whilst roofs merely planted with sedums were only visited by bumblebees during June/July, when sedum species were in flower.  The full research paper is available. 924.64 Kb

The Swiss study recorded 77 different bee species, of which 54 occur in the UK. Of the 54 species of which 21 were recorded during the long-term study of green roofs and invertebrate biodiversity in London.

Green Roofs for Foraging Bees

Bees collect pollen for larval food and the adults feed on nectar. Certain species of bees gather food from single species of flower (monolectic), others feed on closely related flowers (oligolectic). Long-tongued bees, such as leaf-cutter and some species of bumblebees collect nectar from labiates and leguminous flowers, whilst short-tongued species, such as Colletes, visit flowers from the daisy family. Other short tongues species tend to visit flowers with short corolla such as Rosaceae, Asteraceae and Apiaceae.

Biodiverse green roofs and brown roofs with a wide range of wildflowers attract many foraging bees. It is crucial for bees to find suitable foraging sites from early spring to late summer. Therefore green roofs, brown roofs and living roofs that are planted with a wide selection of wildflowers and sedums can provide an important forage habitat for bees in the urban environment.

Bees also frequently use moss as drinking source even where water is readily available. Mosses are relatively common on most green roofs, and can provide an important water source for bees on roofs.

Below is a series of tables of several green roofs in London where certain bee species were observed during 2009 and also a list of plants the bees were observed to being foraging on.

Bees on Green Roofs

From these studies it can be seen that green roofs provide a good forage source for bees in the urban environment and that it is imperative that mix of both sedums and wildflowers need to be used to ensure that the roofs provide benefit throughout the foraging season.

Written by Dr. G. Kadas and Dusty Gedge