Roof gardens are nearly always small, and have to contend with exposure to wind as well as to sun. Wind is not only physically destructive but it also dries out soil extremely fast. Since the beds on roof gardens are usually raised, the soil is already less deep than in ground-level beds and a system of drip irrigation may be essential. Some form of boundary will be necessary for protection from wind as well as for safety. Solid boundaries like walls may simply create eddies but a semi-permeable form of screening, such as pierced walls or panels of trellis, slatted timber, bamboo or wattle fencing would be more effective, filtering the wind and reducing its force. Heat too is more concentrated higher up, so a light canopy of slatted timber may be needed to give shade and protect plants. Select plants, such as sea buckthorn and pyracantha, to cope with the extremes of rooftops. Despite all their drawbacks, there are some wonderful small gardens in the sky.
S. IS FOR SEASONS
The whole of the tiny space is usually on view from the house so making it attractive all year round has to be a priority. Several small shrubs blossom in spring and many carry either fruits or foliage colour in autumn - include these in the foundation planting of the garden. Herbaceous perennials and bulbs reflect seasonal change, so choose those plants which pop up and fade with no fuss and require little tidying up afterwards.
Many spring bulbs can be fitted in at the base of shrubs. Include the early winter aconites (Eranthis hyemalis) and wood anemones (A. blanda) in shaded corners and follow them with early small daffodils like Narcissus 'February Gold' or the really miniature ones like N 'Minnow'. Small spaces are the place to appreciate fully the delicate beauty of Cyclamen coum, with its pink and white flowers above silver-patterned dark green leaves. The small Iris reticulate hybrids also become important in intimate spaces. Crocus species like C tommasinianus are more delicately built than their progeny; when open to the sun, they are a welcome sight after winter; Scilla sibirica can be massed without taking over. For formal scenes, tulips provide repeating vertical forms with a vast range of flower colour and shape, and some smaller species, like Tulipa tarda, open like crocus in sunlight. By summer, alliums will merge with most schemes: there are tall, slim drumsticks like A sphaerocephalon and A hollandicum, plus many smaller ones like the yellow A moly.
If you want to indulge in masses of herbaceous flowers, start early with doronicums and feast on old-fashioned pinks (Dianthus), chrysanthemum hybrids, Leucanthemum maximum, Campanula lactiflora 'Pouffe' and, later on, rudbeckias, phlox, Japanese anemones (A x hybrida) and asters, followed by nerines and kaffir lilies (Schizostylis coccinea). Infilling with annuals like cornflowers, poppies, linaria, limnanthes, nigella, nicotiana, flax and cosmos will produce flashes of summer colour where you need it. Bedding out with tender perennials such as verbena, impatiens, tagetes and the tender sub-shrub Senecio cineraria will fill any remaining gaps and bulk up a young garden before it matures.
By the end of the year the cotoneasters, roses, pyracanthas and skimmias will be carrying red, orange, yellow or white berries. Seedheads like the fluffy ones of clematis, the papery ones of honesty (Lunaria annua), dead agapanthus flowers and superb allium globes carried on tall stems enhance the garden for months from autumn until they are battered by snow. Autumn is also the time of brilliance in colour, when the foliage of acers and many azaleas takes on fiery hues and vires flare into scarlet crimson reds.
T. IS FOR TREES
There is rarely enough space in a truly small garden for real trees unless you choose very slim ones like Prunus 'Amanogawa'. For a softer effect, the weeping pear (Pyrus salicifolia 'Pendula') is quite small, with trailing branches and fluttering silvery foliage. In a small site the chosen tree must offer more than a mere two or three weeks of pleasure, discover those that offer good value and beware of instant appeal at the garden centre.
For sheer elegance, Japanese acers are the front runners for small spaces. Acers are very slow-growing and seen from beneath, with sunlight filtering through, they look magnificent. Some, like A palmatum 'Osakazuki', have outstanding autumn colour and others, such as A palmatum 'Dissectum Atropurpureum', are primarily grown for their finely dissected foliage.
In sun-filled sites, treasure small trees which give flickering shade. Silver birches like Betula pendula 'Fastigiata' filter sunlight effectively, as do some of the rowans. Some of the thorn family (Crataegus) are relatively small but densely solid, making deep shade beneath. C tanacetifolia is slow growing but ornamental because the greyed leaves are deeply incised and the fragrant white flowers are succeeded by yellow fruits.
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A-Z of Really Small Gardens is taken from Jill Billington's RHS Really Small Gardens, published by Quadrille, available from bookshops. To order your copy for the special price of pounds 20 (a saving of pounds 5), including p&p in the UK, call the credit card hotline on 01256 302 699 quoting ref GLR 991