Planting a podium deck
Jodie Jones investigates the growing trend of roof and podium landscapes
A podium landscape is a growing environment built above a solid base, but the magic of a successful project is that the uninitiated will have no idea that the grass on which they eat their lunchtime sandwich may be supported by as little as 150mm of growing medium on top of a concrete slab.
From Chavasse Park in Liverpool to Jubilee Park in Canary Wharf, podium landscapes are increasingly common, particularly in cities where land is at a premium and subterranean structures are prevalent. Not only do they enhance the aesthetic and amenity value of a space, they also boost BREEAM value, help ameliorate airborne pollution, manage rainwater run-off, moderate temperatures and increase biodiversity. No wonder developers are increasingly keen to include podium landscapes in their planning proposals.
Sort out the soil
Soil specialist Tim O’Hare, who spoke at a recent Palmstead Nurseries Workshop on this topic, has noticed a massive increase in such projects, but also a corresponding increase in problems of maintenance. “We consult on a lot of failing podiums, and this is usually caused by issues with the soil,” he says. “It is crucial that the project architects and structural engineers are involved from the outset. The classic constraints are limitations on loads, access restrictions and lack of natural drainage (so every raised bed must be constructed with weep holes). And starting from scratch means there is no inherited soil structure, so you have to engineer that in from the outset.”
BSI soil standards are not relevant to podiums, but there are a number of suitable alternatives available. In general terms, you want a sandy soil (not claggy clay or heavy silt). O’Hare recommends buying from a specialist soil company, which will do a lot of work on product development and ongoing assessment, and being sure you know the permeability, porosity, water-holding capacity and bulk density of your soil.
Appropriate drainage is vital. Typically, this takes the form of either a plastic drainage mat of 40-100mm thickness, or a traditional layer of gravel drainage. Either way, it is equally important to stop the soil falling into this drainage layer. Engineers are keen on membrane, but O’Hare has found this generally unsatisfactory. “Often the membrane itself becomes clogged. A better solution is to include a blinding layer (what the Americans call a ‘choker layer’) of medium to coarse sand to a depth of 100mm.”
Even with the groundwork in place, problems can still be introduced by over-zealous management in the longer term. “I see more over-irrigation than anything else,” says O’Hare. “You can decide the level of fertility and how moisture retentive you want your growing medium to be, so you shouldn’t need masses of irrigation in the long term. I’m all in favour of putting in well-watered pots, then watering them a few times as they settle in, but the aim should be for the whole thing to be self-sustaining.”
Pick the right plants
The celebrated podium landscape at the Barbican in London, designed by Nigel Dunnett, proves that this ideal is achievable. “We didn’t use any form of irrigation system,” says Dunnett. “It’s fair to say there was serious initial scepticism from the maintenance team, but they are learning to cope with this new planting style. They can spot water by hand in extreme situations, but it’s not really necessary because, quite simply, we chose plants that were suited to the conditions.”
Inspired by steppe grassland, Dunnett developed a matrix planting of up to 20 species to give an extended season of interest, using plants naturally adapted to exposed conditions and low rainfall, avoiding potentially troublesome spreading cultivars in favour of clump-formers.
“In general, I plan on having interest from two or three plant species at any one time, to create a big impression with modulating seasons of interest. At the Barbican, I favoured later-flowering species and put in relatively few things chosen for early-season interest. I also included a number of short-lived quick flowering species for rapid impact combined with slower developing species, embracing the fact that the look will change over the next six or seven years.”
Early in the season Euphorbia characias subsp. wulfenii combines with Tulipa praestans ‘Fusilier’, which produces four to five flower heads over a period of five or six weeks and has proved reliably perennial. The spring flowers of Sesleria nitida add to the display, and continue to look good for many months. Salvia nemorosa ‘Caradonna’ and Libertia formosa then take up the baton, enhanced by masses of Allium ‘Globemaster’ and ‘Purple Emperor’. Next into the spotlight are Kniphofia ‘Tawny King’ and Crocosmia ‘Emberglow’, followed in later summer by Kniphofia ‘Green Jade’, Echinops ritro ‘Veitch’s Blue’ and Origanum laevigatum ‘Herrenhausen’, all softened with Sesleria autumnalis and Miscanthus.
The key in podium planting, as in any traditional border design, is to choose the right plant for the site. As a general rule, as little as 60mm will support a sedum mat. With 150mm, it is possible to grow amenity turf, given appropriate irrigation and regular feeding. With 300mm of growing medium, a good range of shrubs and herbaceous perennials will be perfectly happy, and there is always the option of localised mounding over areas with extra structural support.
Dan Pearson MSGD has produced many such landscapes, from his own first roof garden in Vauxhall, London, to the extraordinarily accomplished landscapes at the Amanyangyun hotel in Shanghai, China. “Podium planting has its challenges, but it is all about being adaptable, looking at the habits of a plant and understanding the conditions of certain environments, like arid steppes and the seaside. The most difficult dynamic I have found is the combination of shade and lack of soil depth. Your plant palette shrinks drastically, since shade plants tend to be more moisture and shelter dependent, but there are always options.”
The one category in which even the plant literate can become daunted is in the selection of statement trees or large shrubs. Loads are often limited, meaning that large root-balled trees are not an option. Pearson favours air pots, which tend to have a wider and shallower root run but, as ever, he cautions that it is not enough to just find a tree that will physically fit into the depth of growing medium provided – it must also be happy to survive there. “We have had great success with willows – in particular Salix purpurea ‘Nancy Saunders’, Salix gracilistyla and Salix lanata. Figs and bay also grow well with a limited root run.”
Podium landscapes may be the futuristic answer to 21st-century problems, but it turns out that the key to their success is as old as gardening itself - get your soil right and choose the right plant for the place.