As the urban population of cities continues to grow we are faced with larger and denser urban areas, and our urban green spaces are struggling to cope with the pressures of increased active use. As landscape architects, we are often seeing more requests for ecologically friendly proposals to combat the loss of amenity, habitat, and disturbances caused by development. These ecologically minded proposals often happen in places that are not visible from the street, with many of our projects featuring a biodiverse roof or amenity areas situated on a roof terrace. As the practice continues to explore the urban ecological networks that exist in our cities, the ExA London office visited several examples of projects which highlight different methods for dealing with limited urban green spaces, ecological function, and approach to ‘re-wilding’ our cities.
The first stop on the rooftop walk was to 201 Bishopsgate which was completed in 2008 by SOM Ltd. The building is twelve storeys and features a biodiverse roof with stunning views over London. We were fortunate enough to have the sun on our side as we reviewed what was working well and examine a real life example of an established biodiverse roof. The roof featured a mosaic of habitats which included planted areas, and some areas devoid of planting with rocks and rubble instead to encourage burrowing insects. There was an array of ground nesting bird boxes, most likely targeted towards encouraging the very rare Black Redstart (Phoenicurus ochruros) which is on the Red List of Birds of Conservation Concern. These small creatures are unique as they have adapted to live in industrial and urban centres, but they are at threat of extinction as there are fewer than 100 breeding pairs in the UK. Many of their main food sources such as insects, spiders and seeds can be found on a well designed biodiverse roof. 201 Bishopsgate features rocks, rubble and log piles to encourage insects and spiders.
The range of planting on 201 Bishopsgate appeared to be a typical representation of the kind of vegetation that grew in that location over the past century. Seeds can stay viable in the soil for up to 100 years and it is more than likely that subsoil was used from on site excavation to create this roof. While this method of letting nature populate and take over the space as it would in any natural environment may be beneficial, it should be noted that biodiverse rooftops do often require a stringent regime of eco-management to ensure it is functioning as intended. This is evident in some images of the site where grass has been left to seed and is slowly taking over from the south corner, likely due to prevailing south-west winds. Overall, it was a great start to our rooftop walk and was a particularly useful precedent study.
The next stop was the SkyGarden at 20 Fenchurch Street which is a well known example of an emerging trend of ‘gardens in the sky’. We observed how the planting was faring seven years’ on from completion and how important it is to choose the right plants. There were some plants like the ferns which were thriving towards the upper terrace where the humidity is higher with water misters to create a suitable microclimate for them. The misters may also be causing problems with some of the other plants in terms of fungi and disease. In humid conditions where water remains on the leaves due to insufficient air movement, the plant may be unable absorb the moisture and as a result it becomes susceptible to disease. The struggle with an indoor elevated garden such as SkyGarden is the constant battle between simulating a real outdoor environment with supplying enough light, moisture and other elements plants need to survive, and reconciling that with human occupation needs within a building.
There were several other brief visits to a range of rooftops around London with our final stop for the day ending at the Barbican to see Nigel Dunnett’s re-planting project. Nigel’s approach to planting explores the concept of natural planting hierarchy and our director Sam Martin talked about Nigel’s methodology for choosing, designing and setting out his plants. The plants at the Barbican were set out species by species across the entire site starting with ‘anchor’ plants, then ‘satellite’ plants, and finally ‘filler’ plants. The proportion of each species was chosen for the desired end effect as well as natural dominance. Care was taken to ensure that the planting would function all year round with no gaps, and we could see on site that the planting still looked great as we are coming into autumn.
We finished our rooftop walk with a trip to a local pub and reflected on the variety of rooftops we visited that day. As designers, we are conscious that our role as landscape architects is shifting to include not just the ground plane but is moving towards more ambitious elevated parks and pushing nature to perform in non-traditional environments as we grow our cities upwards. The release of the new City of London Corporation’s City Plan 2036 recognises the importance of urban greenery. It stipulates that all new and refurbished developments in London’s Square Mile must include ‘urban greening’. As the city considers building up to form a cluster around the iconic 20 Fenchurch Street building, the impact of this new policy will be a game changer. At ExA, we design to consider the evolution of flora and fauna in an increasingly urban world and their importance in ecological function, amenity value, and the health and well-being of those living in dense urban populations. As ecological pioneers, the future is presenting opportunities to push the boundaries and explore what we can do to design for the evolution of nature in urban cities.