Proponents of the power theory would say that it is the sheer bulk that attracts: massive meaty tubers, hugh fleshy leaves, stalks like drainpipes, flowers as big as punchballs. But this is too simple an explanation. There are plenty of growers, particularly in the South-West, whose passion is the pom-poms - tight-lipped little flowers, round and hard as a piece of porcelain, no more than two inches across, balanced perfectly on long, thin stems. I have seen veterans handling blooms of "Hallmark", a deep lavender-pink pom-pom of symmetrical perfection, as though they were touching the cheek of an angel.
There are plenty, of course, who loathe dahlias; they usually know very little about them, but are conditioned to give a little shudder, to wince sensitively, if the word comes up in polite conversation. In these circumstances, the best thing is to say, "Oh, but Gertrude Jekyll loved them", which that renowned landscape architect did.
Because they have such a high profile as exhibition flowers, dahlias are surrounded with a great deal of mumbo-jumbo. They are actually extremely willing plants, not difficult to grow adequately; adequately enough, that is, to please you in the garden even if not to meet the stricter requirements of judges at the show bench.
Raising from seed is not the usual way to get hold of dahlias; it is more likely that you will buy tubers, or cuttings; or well-grown plants. Tubers are sent out by specialist growers between November and March. Plants raised from cuttings are dispatched between late April and June. Ken Aplin, a Dorset postman, who grows superb dahlias - more than 300 of them on his allotment by the River Frome - says that good stock is vital. Nothing worthy will ever come from a wizened, pre- packaged tuber that has been hanging round longer than it ought in a garden centre, he says firmly. If you go for rooted plants, these can go straight in the ground at the end of May.
They need good ground, a position where they will get sun for at least half the day. Mr Aplin rotovates plenty of muck into his ground before planting, and he top-dresses with Vitax Q4. He does no extra feeding, but plenty of watering. At the rate they grow, dahlias lap up water faster than dehydrated dingoes.
Staking is vital. There is a lot of wind resistance in a dahlia dressed overall. Three canes and plenty of soft twine is the usual answer. Through June and most of July, all you will have to do is watch them grow. Then comes the fiddly bit: disbudding. For show dahlias this is essential, but a little population control is good for garden plants as well. Without your intervention, too many flowers will be produced and the bush will become a muddle.
On most growths, there is a boss bud and two lesser outriders. Although it runs counter to all one's natural sympathy for the underdog, the subsidiary buds, rather like the sideshoots of tomatoes, must be nipped out before they get too big. The remaining bud will then have unlimited resources to develop into a fine long-stemmed bloom.
The giant decoratives are the dahlias that hypnotise me, like a rabbit in front of a blazing headlamp. They are not the easiest varieties to keep on their feet. Some grow more than five feet high and need not so much staking as buttressing. But who, having seen "Trelawny" in fighting form - whirling wheels of bronze, almost a foot across - could fail to be transfixed? Who could pass by the sumptuous magenta purple globes of "Night Editor" without a second glance?
Mr Aplin could. He is not a giant-decorative man. He is, as he put it, "more of a pom-pom and ball man". His champion this year is "Catherine's Cupid", a miniature ball of slightly salmony pink. "It'll be a pleasure to cut those," he said.
From the Gardening page of `The Independent', Saturday 2 September 1989