Kidney Vetch (Anthyllis vulneraria) is a UK native wildflower of chalk, grasslands, shingle banks, cliffs and Junes. It is very common on green roofs in Switzerland and can sometimes tend to dominate an extensive green roofs. However it’s profuse yellow flower heads are a real magnet for bees, butterflies and other pollinators.
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However it’s profuse yellow flower heads are a real magnet for bees, butterflies and other pollinators. In London, it is found on a lot of biodiverse and so-called brown roofs. On the Laban Dance Centre, it was established using local seeds collected from a brownfield site within the locality and is positively thriving. It flowers between May until the middle of August on green roofs.
It’s leaves are the larval food plant of the Small Blue Butterfly and is also an important nectar source for the Common Blue. Other Butterflies, Moths [including the Burnet Moth] and bees are utilise the flowers as a nectar source throughout the summer.
When the flower fades to seed heads, it can provide a visual treat. The flower heads firstly turn to orange and then to a lovely light brown. This happens at different times for individual plants and such that some plants are in the yellow stage, whilst others are in the orange and brown state.
The larvae of the small blue butterfly use the kidney vetch FLOWERHEAD (not the leaves) as their only food source, remaining initially fairly well hidden within the woolly heads after they hatch from the (normally single) eggs laid by the adult female butterfly. Later, as the caterpillars develop, they may become more visible on the surface of the flowerheads, burrowing into them to eat the developing seeds.
On any one site the breeding success of the butterflies is closely dependant upon the flowering success of the kidney vetch. This makes the conservation management for the butterfly even more tricky in the complex habitat of semi-natural chalk downlands, as in some years, according to the population of grazers and the weather that so closely controls the growth of the plants, the close grazing essential to keep the turf short and open may also remove the flowerheads if the balance and/or the timing is slightly out! This balance might be easier to manage on green roofs, where the open disturbed habitat that kidney vetch really likes might be easier to maintain.
The adult butterflies also use nearby longer vegetation such as grasses or even scrub as male territories for breeding purposes, and both genders use grass stems for communal evening roosting, so this might need to be taken into account when planning the vegetation of green roofs if the aim of using this plant is in part to encourage the small blue butterfly. To be honest this butterfly is reputed to be a very poor traveller from existing sites and is also perhaps currently near to the edge of its range in the UK, really liking warmer dryer conditions. It may be difficult to get small blue butterfly colonies to expand from their nearest existing habitats to establish themselves on otherwise suitable green roofs in urban areas. It would be interesting to try to encourage more green roofs in urban edge sites where development is taking place close to existing downland colonies.
Are there any instances of this butterfly, adults or larvae, being found on existing green roofs in Europe? I would be very interested in any records.
David Carey.
Hadlow in Kent, UK
David
I have only just realised that you add a comment to this back in June – apologies – we have seen small blue on some roofs though have no evidence of the presence of catarpillars. However they are relatively common on green roofs in Switzerland, where Kidney vetch can be the dominant plant on many green roofs. We hope that soon we have evidence that they do use green roofs in the UK. WE have been waiting for the first records of rare bees using green roofs. In september we found foraging brown carder bees on a new green roof field station in Barking – they were observed throughout the month – the only flowers on the site were on the green roof. So any thing is possible if we put the right ‘habitat’ up on green roofs!.