Kent is laid out like a patchwork quilt, full of little neatnesses

Kent is the most Cockney county. Londoners flowed out into Kent from the earliest days, to find a garden of their own. Nature, here, is as domesticated as a south London tabby. Even the castles of Kent (Leeds, for example, or Hever) are more like garden ornaments than fortresses. Or stage backdrops from Cockney vaudeville. Visions from the Old Kent Road.

Going through the Weald this afternoon, I never see a view without a building in it. Kent is a patchwork quilt of a county. Not broad acres, but little neatnesses. The reason is gavelkind. This sounds like some loathsome infection. But it is the opposite of the primogeniture rule which, elsewhere in England, gave everything to the eldest son. Under gavelkind, land was shared among the heirs.

So, Nigel Nicolson says, "There were Dukes but no Dukeries." He and his direct descendants were given the right to live at Sissinghurst Castle, after the National Trust took it over. It is called a castle, but everyone knows that the point is that it is a garden, created by his mother and father, Vita Sackville-West and Harold Nicolson - happily married homosexuals - around a tall, thin, redbrick Elizabethan tower.

Sissinghurst is stupendously popular. It is the sort of garden anyone thinks they might create if they had world enough and time.

Guidebooks call Kent "the Garden of England". If we followed the American model of a nickname for every state, this is what would go on car number plates. But these days, I decide, it is more like the Garden Centre of England.

January is a bad time. The weekend papers scratch around, trying to find jobs the gardener should do. But the garden centres of Kent don't despair. Forget about the place of the car in the British economy. Gardens are just as good a way to part people from their money. I pass four garden centres, but then my resistance goes. I edge in past the parked hatchbacks and stare at packets of dog and cat repellent, called Get Off My Garden. There are rose ties, plant twists, cane caps - and a widger. I read the label to find what this does. It is a sort of shoehorn for transplanting seedlings.

The seed packets are a bright dream of summer: sunflowers, sweet peas, stocks. Get your barbecue briquettes now, at winter prices. And what about special green waterproof bags, "the clean answer to that muddy boot problem"?

The rows of takeaway trees look very sad - no leaves, no fruit - except for the branchfuls of golden crab apples. Horticulture was one of the first global industries. Malus x zumi is a Japanese tree, brought to the United States in 1892. In 1949, an English firm bred the Golden Hornet hybrid. It was then planted at Sissinghurst, and in thousands of suburban gardens. The tree I am looking at is about as English as a Honda. Gardening ignores frontiers.

The most famous (or notorious) tree ever to come out of Wales is the leylandii cypress. It is an accidental cross between two north American Pacific cypresses which were never within 500 miles of each other till they were both planted out in a mid-Wales park. Garden centres promulgated this vigorous mongrel for instant hedge-making. More leylandii have been planted in Britain in the past 30 years than any other tree.

Here, between Sissinghurst and the fringes of real London, it is much hotter in the display sheds than in the freezing lanes outside. I feel I should write an elegy in a country garden centre. But one of the strongest garden passions is not here to be written about. All the tulip bulbs have been sold.

Tulips have driven men mad, as Anna Pavord reports in her new book, The Tulip (Bloomsbury, £30). No rational explanation has ever been put forward for the tulipomania in Holland, after the bulbs were brought to western Europe from Turkey. But it is the model for every speculative bubble since, including house-price cycles in 20th-century Britain. The scramble to buy outruns any possible resale value.

Some of the most spectacular tulips are due to a disease. A beneficial virus makes the bulb grow untrue. But neither the virus nor present-day genetic manipulation has yet produced a proper black tulip. It is a grail so elusive that Dumas pere wrote an entire adventure novel about it.

For years, tulips went out of fashion in English private gardens, because municipal park keepers loved them so much. The Royal National Tulip Society folded in 1936. The only tulip society to survive, Anna Pavord says, is in Wakefield. Perhaps early tulips and early rhubarb - Wakefield's other industry - go together, like a horse and carriage.

From the day Vita and Harold bought it, in 1930, Sissinghurst ignored fashion. Tulips were planted - but raggedly, the very opposite of municipal military precision.

And then came David Hockney. For about ten years from the late 1960s, vases of tulips were one trademark of a Hockney picture. They were reproduced on thousands of art postcards. The garden centre boom was beginning. The tulip was on its way again.

I must come back to Kent in tulip time.