Large flower, boxes or butts can be used to grow a very wide variety of plants on flat roofs, including a number of shrubs. While careful choice of plants is obviously essential, a sound soil mixture, good , and proper are equally important. John Innes Compost (preferably No. 2) is an excellent growing medium for most plants. It is advisable to replace the top few inches of the or soil (2 parts fibrous loam, plus 1 part each of peat and sand is an alternative to the John Innes mixture) every spring. Replace with the same amount of or soil mixture.
To allow surplus water to drain away, raise tubs etc. off the ground. Drainage holes at the bottom of tubs, boxes and pots are, of course, vital, and should be covered with crocks (pieces of broken flower pots). A layer of pebbles can go above the crocks. Water in when planting and subsequently whenever the soil is at all dry.
Many amateurs like to grow roses in tubs. Note that the tubs must be at least 2 ½ ft. deep, 2 ft. long and 18 in. wide. Poles can be inserted in the tubs and climbing varieties chosen. Selection of the right kinds is vital — very vigorous varieties like Golden Glow, Dr W. van Fleet and Alberic Barbier are hardly suitable and the shorter growers which flower more or less continuously should be grown. Examples are Aloha, Coral Dawn, Danse du Feu, Golden Crest, New Dawn and Parade. Ramblers are usually too rampant but the relatively new Crimson Shower is an exception. It is in bloom from the end of July well into September (and sometimes as late as October).
Floribunda roses are also very effective, especially the extra tough varieties like Frensham, (probably the best for this purpose), Elsinore, Silberlachs, Tivoli, Orange Triumph, Masquerade and Gold Cup. Among the shrubs and climbers suited to tubs are hydrangeas,ternata (Mexican orange blossom), Berberis Darwinii, Jasminum nudi-Florum, Rosmarinus officinalis (the common rosemary which is hardier than the white and silver-striped varieties), Lemoinei, clematis, viridissima.
Bedding plants, e.g. petunias, salvias, antirrhinums, are useful for roof gardens, hardy border perennials like geums, heucheras,maximum Esther Read, Cobham Gold and Wirral Supreme; gladioli, provided they are given a mulch of peat, hop manure etc. during dry spells; hardy like clarkia and godetia.
The Allwoodii pinks, being more continuous than the older varieties like Inchmery, Mrs Sinkins and White Ladies (though the latter should not be despised), are particularly suitable. Good varieties are the orange-scarlet Robin which stands up to stormy weather, the bright crimson Winston, and Susan,with black centre (the last two are very fragrant). Most spring-flowering bulbs do well. Lilium regale which blooms in July is one of the best lilies to plant. Dwarf azaleas and can only be grown if planted in lime-free soil with peat and mould and kept moist in dry weather during spring and summer. Slight shade is advisable.