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

Dusty Gedge

Liquid Roof

Waterproofing:
The waterproofing system beneath any green roof MUST be root resistant in the long term preferably to the German FLL standards. If the waterproofing is not root resistant then the green roof system must include a root barrier. The installation of the waterproofing and its detailing to perimeters, outlets, protrusions through the roof etc., MUST take into account the depth of the green roof build up. The waterproofing should always be 150mm above soil level at internal upstands and protrusions, and at least 50mm at external perimeters. The waterproofing system must be leak tested and certified as such IMMEDIATELY before the installation of the green roof.

Green Roof System:
No matter what species of plants are specified, for them to flourish the green roof system MUST provide them with:
◦Moisture
◦Drainage
◦Aeration to the plants root systems
◦Nutrients
To achieve this, a successful system must basically replicate nature but within a very compressed build up. The build up should ideally consist of:
◦Moisture retention/protection layer
◦Drainage layer
◦Filter layer
◦Manufactured/Recycled Growing medium
◦Appropriate components i.e. outlet inspection chambers etc. The type, size, and design of each layer will depend on the proposed vegetation; as will the need or otherwise for irrigation
◦In the absence of a British Standard, green roof systems should conform to the German FLL standards. This standard is generally recognised as the benchmark for green roof quality, certainly within Europe