That'll be the dahlia

Mary Keen's Late Summer Flowers 1: Dahlias & Salvias; When summer peaks, many gardens lose their looks. In the first of a three- part series, Mary Keen finds flowers to keep the borders fresh

Dahlias Were Regency favourites. Their exotic brightness appealed to fashionable gardeners of the 1820s. The Victorians loved them too, especially the enormous flowers with lolloping heads developed by breeders of the time. Along with everything else from the age of the antimacassar, dahlias were discarded early this century. For 75 years, nobody wanted to grow such stuffy and outmoded flowers. Antimacassars are still out, but dahlias are back in time for the turn of the century. They lack the traditional associations of the rose and they have no smell, but new gardeners heading for the millennium may soon lean more heavily on dahlias than roses for late summer colour out of doors.

About 15 years ago, a fiery red dahlia with dark purple leaves called the "Bishop of Llandaff" began to appear in more interesting gardens. The grower would point it out, usually adding defensively: "I hate all dahlias but the Bishop of Llandaff is all right." Apart from that, only the sublimely confident could get away without commenting on the fact that they were actually growing a dahlia. Once the first taboo had been broken, though, gardeners started looking for other varieties. Gradually prejudices were shed and spotting a dahlia became less of an event.

Four years ago, Christopher Lloyd dug up the rose garden at Great Dixter and replanted it with tropical plants, including orange, magenta and scarlet dahlias (which are native to central and southern America) in colours that clashed with such exuberance that it made you laugh out loud. Now it is hard to imagine a time when, along with gladioli, they were at the top of the list of the least desirable flowers.

There are still people, especially those over 50, who have an aversion to these herbaceous perennials. The star-shaped cactus dahlias seem strange to the unconverted and their names win them few friends. "By the Cringe", a purple cactus form, is not for the wary. Other names to scare off the conventional are "Kung Fu", "King Soccer", "Disneyland" and "Chorus Girl". Size is another barrier. Some dahlias are too large for the average garden. "Playboy" and "Go American" have flowers that are more than one foot in diameter.

The types to concentrate on are the decoratives, the singles and perhaps the smaller pom-poms. "Arabian Night" with its darkest velvet crimson rosettes, is a winner with everyone who sees it. "Porcelain" is as pretty a pink as it sounds. "Nina Chester" is snowy white, and "Fascination" rich pink. All these have green leaves and flowers between 4in to 6in across. Dahlias with bronzy purple leaves I find irresistible. "David Howard" is a decorative with soft orange flowers, and "Yellow Hammer" has single yellow flowers against dark purple leaves. All the dark-leaved forms are dramatic, but they are not for the pastel shades brigade. For the timid, the pom-poms may be the ones to choose. "Small World" is white, "Rhonda" palest pink and "Butterball" is bright yellow.

Dahlias are not difficult to grow. In the southern counties and in the micro-climate of London they are hardy. Anywhere else they will not always survive the winter. When the stems are blackened by frost, dig up the whole plant, cut off all but a couple of inches of stem and put it upside down, balancing on the stems to dry out on some newspaper. When the tuber (like a potato), is dry, wrap it in several layers of paper and keep it somewhere cool but dry - a shed or garage will do - all winter. Have a look around Christmas to make sure the tubers are healthy. The following spring, unwrap them, water them so that the tubers plump up and lay them in a box of peat in a sunny place until they sprout. When they have proved they are alive, put them into 6in pots of compost and keep them frost free. They can be planted out at the end of May. If all this sounds too exhausting, just plant the tubers outside in May, they will flower about three weeks later. Food, water and sun is all a dahlia requires of life. The larger ones will need staking and all of them will need protection from slugs and earwigs.

Dahlias grown from seed are less glamorous looking, but "Rigoletto" and "Burnished Bronze" have both been given awards of Garden Merit by the RHS, so they are not to be despised.

Like dahlias, salvias have been in the wilderness for years. The scarlet "Blaze of Fire", so popular for parks bedding, is still not much sought after by private gardeners, but since the early Eighties, when tender perennials started to be popular, salvias have become a favourite plant for discerning gardeners. Because they are less greedy than dahlias, they do well in poor conditions and hot summers. Not all salvias are tender. S sclarea var. turkestanica is a biennial and usually seeds itself liberally. The furry grey leaves are handsome in early summer, and through July and August they throw up 3ft spikes of pinky blue flowers. If you brush against it, though, the smell is fetid. S pratensis (Meadow-clary) comes in pink or blue and usually survives the winter. It is an airy, papery looking plant which flowers for about six weeks if you dead-head it.

Much smaller and slightly tender are the shrubby, small-leafed forms from Mexico. They flower all summer in all shades of red, pink and yellow. The best are the pale yellow "La Luna" and the rich red "James Compton". These salvias like poor, hot places: a nitrogen-rich diet will make them grow too many leaves at the expense of flowers. In warm gardens they will survive for years. Cuttings are easy to strike now and will grow to perform well next year. Pinch them out to make them bushy and keep in a frost- free place.

Two showy salvias are "Indigo Spires", with deep blue spikes, and the curious S involucrata "Bethellii" with fat, shocking pink blobs of flowers. Both of these will flower from July until the frosts, but they will not survive more than a few degrees of cold. If you get hooked on salvias there are several amazing varieties from Brazil to grow. S uliginosa is intense forget-me-not blue all late summer. This one is at home in a swamp, so it needs moisture, but not in winter if it is to survive outdoors. S confertiflora is exotic with browny orange spikes and its large leaves look terrific in a pot.

!Next week Mary Keen on fuchsias and lobelias