Back in fashion

They adore hot summers and never droop, because rather like camels they carry their own water supply with them. They have fleshy tubers and these nobbly growths looked so delicious to the Spanish conquistadors that they actually imported them as a food for King Philip IIs court in the mid-17th century. Nobody found them palatable and even the king's cattle refused to eat them.

Some of the Mexican species (like D.imperialis)are so treelike in proportions that their hollow stems were used as irrigation pipes and one of their colloquial names is cocoxchitl', an Aztec word meaning hollow pipe'.

The European name of this member of the daisy family honours Andreas Dahl a Swedish botanist who died in 1789 aged 38. When Dahl was alive, five species of roughly 30 or so known were in cultivation. The dahlias we tend to grow now are usually deliberately bred cultivars produced from hybridising species and varieties. This has given us a range of colours and flower forms.

However in recent years the fashion pendulum has swung away from the enormous blooms bred for the show bench, to the smaller, willowy forms with daintier flowers.

Some of the show bench varieties can measure a foot in diameter (30cm) and these dinner-plate blooms look hugely out of place whether grown in the garden or on the allotment.

Perhaps it was their sheer sock-it-to-you presence that caused them to plummet in popularity in the years following the Second World War. They were even branded as vulgar, perish the thought.

And they were hit by an unknown virus as well and many nineteenth century varieties were lost at that time. John Brown, who lives or lived in the Oxford area, collected as many varieties as possible during the 1960s and there is now a National Collection which conserves and sells two thousand varieties based at Varfell Farm near Penzance. ( see below) Dahlias are tender plants and frost zaps their foliage instantly, but doesn't kill the tuber. Once frosted the foliage blackens and gardeners used to dig up each tuber, turn them on end and dry them before placing them in a cool frost-free place. My uncle stored his under the bed before the days of central heating. Increasingly gardeners are leaving the tubers in the ground and even last year's winter saw most come through unscathed on freely drained soil. If you garden on heavy soil it may be trickier to get them through the winter.

I have always loved loud dahlias and I grew them in their wilderness years for their vibrant presence in the border as well as the vase. The secret of growing them is to start off with tubers, rather than fully grown plants. These can cost as little as £1 each. The dusty brown roots usually arrive in mid-spring and can be potted up in April, preferably in an unheated greenhouse. This keeps them small and hardy and when the four-inch high plants are put into the garden in early June ( after the frosts) they soon romp away without any intervention from the gardener. Experience has taught me that large dahlias planted out when almost in flower need substantial amounts of water to establish and they flag as soon as they're stressed. Always use three canes and green garden twine to support newly-planted dahlias, or grow them with a taller support plant like Verbena bonariensis.

The main enemies of the young dahlia are the slug and snail and dark-leaved varieties are more prone to damage. However who can resist the dusky charms of the Bishop of Llandaff', a dark-red single dahlia with a clear-yellow centre. Moonfire', a soft-orange single with a bronze centre and Roxy' , a shorter bright-pink single, are also excellent doers with dark foliage. All single dahlias are more attractive to bees, butterflies and hoverflies than double forms, but a will need their pointed seed-heads removed after flowering. Don't confuse them with the similar bun-shaped buds.

I also grow fuller-petalled doubles and semi-doubles to mix among the singles; their more substantial flowers last longer than the singles and there's less deadheading. I rate Art Deco' (a burnished orange-pink double, David Howard' ( a soft peach-orange double) and Arabian Night' (a sultry dark-red). They're all as garish as a Mexican's sombrero I admit, but dahlias come in pastel pinks, pure-whites and pale-yellows too. The soft-pink, water lily flowers of Gerrie Hoek' or the spiky-petalled White Moonlight' will mingle in among traditional herbaceous borders should you lack the courage to go completely dahlia loco'.

The National Collection of Dahlias (2000 vars) Winchester Growers Varfell Farm Longrock Penzance Cornwall TR20 8AQ tel 01736 335851 (catalogue available)