Toby Buckland advises on what’s best to plant on top of the extension, shed or even bird box
Worthy things often lack panache, but a green roof can turn the dreary into something instantly appealing. At a stroke, drab tiles or an expanse of asphalt are transformed into a verdant meadow, as if a garden had slipped anchor and floated up into the sunshine.
Green roofs have been shown to reduce the heat-island effect of stuffy cities by as much as 7C. They also trap tiny particles of diesel pollution and reduce floods by absorbing heavy rainfall, which is becoming more important as our winters get wetter.
Once installed, they need little care, but offer the joy of looking for new specimens that will thrive. I’ve always got an eye out for plants in unlikely places such as walls and gutters — if they thrive in wisps of soil there, they will also be happy up on a roof.
Whatever the type of roof you are greening, you need to create an edge that holds the plants and compost in place and stops them sliding off. On small structures, the simplest option is to use timber planks or battens (depending on the depth of the soil), fixed around the sides of the roof to create a sort of tray.
Line the tray by pinning plastic sheeting to the inside, to protect the underlying roof, then fill with lightweight compost made from 50/50 multipurpose compost and water-retaining perlite. Pierce the plastic on the bottom edge, so that excess water can escape, and install
a gutter on sheds — green roofs drip for days after it rains.
Never fix, nail or screw the tray into the roof below, unless you want it to leak like a sieve, and don’t forget to water until established.
What to grow
Although many plants will grow on roofs, the most ubiquitous is sedum, aka stonecrop. You can buy the plants in kits, where they are grown on mats: these come in boxes, like pizza, and include everything needed for installation and to keep them happy. They are heavy and hard to handle, so always opt for small sections if you’re laying the roof yourself.
Sedum is by no means the only plant to grow. If the roof can take a few inches of extra soil, the range of possibilities expands to alpines, herbs and wild flowers. On really stout roofs, it’s possible (with the advice of an engineer) that you will have enough soil for trees.
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The real-deal roof garden
If your roof can take the weight, you can really go to town, with containers the size, depth and even shape of garden borders. Requiring serious engineering, and usually the preserve of concrete tower blocks, these gardens have the advantage of living above the frost, but are exposed to the full force of the wind. For this reason, the best trees include ”pioneers” of the type you’d see on a mountain – silver birch, mountain ash — as well as those that are able to cope with sea winds. In their shelter, plant silver-leaved shrubs to sparkle against the skyline.
PlantsSea buckthorn, Elaeagnus ebbingei, olives, lavender, artemisia, hebe, Scots pine, birches, grasses.
Care This kind of garden requires watering, feeding and regular weeding. A rooftop compost heap is a must, too.
A green roof doesn’t have to be big to be beautiful, and your building skills needn’t be on a par with those of Thomas Chippendale — a leak and the odd untidy edge won’t be disastrous here. Choose a table topped with a flat or shallow roof, then add a layer of capillary matting over the plastic tray to help keep roots hydrated. Fill with compost and mix plants together to create a colourful tapestry. An old trick that fans of alpines use in rocky displays is to make a slurry from blended manure and soil to “stick” specimens in place, but if they have plenty of roots before you plant, and are watered carefully until established, this smelly glue won’t be necessary.
Plants to use Sedum acre (sold as matting or in pots), sempervivum, Sedum spathulifolium ‘Purpureum’.
Care Water infrequently but regularly during dry spells.
The tar-topped sheds that hide our ever-growing collection of recycling bins make ideal candidates for a DIY green roof, turning ugly utility into useful beauty. Low and easy to reach, they’re ideal for tactile plants such as ornamental grasses or culinary herbs, which you can pick as a crop. These will thrive if given 4in-5in of compost.
PlantsProstrate rosemary, silver, golden and creeping thyme, chives, oregano. Alpine pinks (dianthus) and sea thrift (Armeria maritima) will add colour.
Care Use gravel as a mulch to hold in the compost and stop local cats from using the roof as a latrine. Water once a week in summer, and feed with a sprinkle of slow-release fertiliser granules in spring.
On a garden office, garage or a shed that can bear more weight (not all are suitable), you can get away with an extra layer of growing medium to provide root room for a wider range of plants, including grasses, herbaceous perennials and wild flowers, alongside the more regularly used sedum. Greater diversity brings more wildlife: in my garden, birds peck over the plants for aphids and the roof buzzes with bees and butterflies. Soft and naturalistic when added to a modern building, the look is stunning, especially if it is going to be viewed from above.
PlantsPurple moor grass (Molinia caerulea), ponytail grass (Stipa tenuissima), rudbeckia, achillea, chives, red valerian (Centranthus ruber).
Care The roof will need occasional weeding, so provide a safe place to prop a ladder. Sow the seed of ornamental grasses in spring, rather than adding them as ready-grown plants, as they will establish more reliably. Water in dry spells.
Elevated well above ground, a shed roof lifts up into the sunshine plants that would struggle in the shade at ground level. Sedum roofs are the simplest, but with extra growing medium, a wider range of herbs, alpines and flowers is possible. To keep weight off the centre of the roof, box around the edge and fix treated joists horizontally, creating a series of terraces one foot apart. Line these with plastic, fill with compost and plant in groups and drifts to break up the straight lines of the timber.
Plants Cerinthe major, Stipa tenuissima, Verbena ‘Lollipop’, mesembryanthemum, Lychnis coronaria.
Care The more ambitious the scheme, the greater the need for irrigation. When creating the garden, fix a leaky hose that can be plugged into a tap along the ridge. Feed the soil with slow-release fertiliser twice a year
On the house
The big advantages of sedum roofs are that they are largely maintenance-free and can be laid at any angle, although anything above 45 degrees will need the technology used for living walls to hold it in place. They are especially effective for hiding ugly flat roofs and protecting the bitumen from the elements, which means the roof will last longer.
Plants Sedums, Polypodium vulgare ferns, navelwort (Umbilicus rupestris).
Care Water when new and weed occasionally