If you want bees to home in your green roof, there are many flowers that will attract them. However the star plant and therefore the star attraction has to be Viper’s bugloss (Echium vulgaris).
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This tall biannual is an elegant addition to the green roof flora. It can grow on brown roofs – if the rubble is reasonably deep and will self seed and spread on most low nutrient green roofs. And, whilst most of the really good green roof plants for bugs and bees are yellow, the Echium is bluey purple.
One flower head in May and June can have a crowd of bees and other invertebrates hanging out on its drooping flower heads. I have seen Common blue and Burnet moth on green roofs in London during the summer.
Even in very poor substrates, with little organic material, it will flourish, though the plants maybe slightly stunted. Another important attribute is that some plants will flower right into late August, when nectar source on a green roof can be quite limited.
I always recommend planting a few plug plants of this species, as well as seeding. The seeds can take some time to germinate and to offer up their flower heads to bees. Plugs depending on the individual plant can provide a valuable nectar source int he first season of a green roof.
The name derives from it’s purported medicinal ability to help cure the effects of a Viper bite, though one would be lucky to get bitten by an adder on a green roof.
[...] were over twenty different bees of at least 5 species on the roof. Although there were only two Vipers bugloss in flower they were certainly the most popular of the plants on the roof, although the cornfield [...]