TO CULTIVATE a plot of land is, they say, to grab a little slice of heaven on earth. God began by creating the Garden of Eden, after all; and even the grottiest allotment down by the disused railway sidings can, if you duck your head to block out the gas station, seem brushed by the fragrance of paradise. Maybe it's just that the grass is always greener, but city people are forever talking about rural idylls, pastoral bliss and country retreats - as if a bit of lawn with some roses and a drift of honeysuckle were in every way the cure for urban stress.

Martin Hoyles, in this lovingly tended history of horticulture in Britain, doesn't see it quite like that. Gardening, he argues, has complicated cultural roots - of the sort you can't pull up and burn. The blossoms that flutter in our bounteous summer zephyrs are planted in soil that has been fed by a swaggering imperial past. Green and leafy bowers sit, almost literally, on graves. In 1842, a million oxen were slaughtered in South America to provide manure for Britain; Europe's battlefields were ransacked for old soldiers whose bones could fertilise hollyhocks on the home front.

The book begins by noticing that because property is, er, theft, every time you mow your lawn you are handling stolen goods. The history of gardening is a history of enclosures, and this raises some important questions: 'Who is doing the enclosing, who owns the land, and who is being kept out?' The creation of Britain's finest gardens dates back to the energetic seizing of common land by aristocrats. Sir Philip Sidney's Arcadia was written in a park that was created by enclosing a village and chucking out the tenants. Between 1760 and 1867 seven million acres were hedged with thousands of miles of hawthorn, as well as a prickly web of defensive legal measures.

The strategy worked so well at home that Britons were swift to do the same thing overseas. International borders were not nearly so important as herbaceous ones. When the Virginia Company promised 50 acres of rich forest - magnolias the size of oaks] - to anyone willing to pay the transatlantic fare, Bertram Hobert stumped up for 13 trips and acquired 650 acres. All he had to do then was uproot the people who lived there.

Part of Hoyles's plan is to offer us a handy anthology of relevant cuttings. Each page blooms with sprigs of poetry and prose. Cobbett, Cowper, Marx, Marvell, Milton, Morris, Ruskin, Voltaire . . . the book is, apart from anything else, a nursery of pastoral quotations.

But this fidgety approach also means that the story promised in the title emerges only as a rough outline. First, there were the enclosed royal parks meant for hunting ('lawn' originally meant deer park). Then came country- house estates, dependent on the back- breaking hard work of low-paid labourers. Hardly any of the 'great' gardeners did much manual work: Capability Brown did not often take a scythe to the vast green expanses he rolled over the fake slopes of Blenheim and Stowe.

Finally, Hoyles narrates the growth of suburban gardens and the development of public parks. These were devised - with those hideous Victorian displays of bright bedding plants done up like heraldic devices - to give the proletariat a breath of morally improving fresh air, and to keep them out of the ale house.

The best section of the book describes the extraordinary role played by Kew Gardens, which became, more or less, the R&D lab of British Empire. Plants were pilfered from all over the world, nurtured in the hothouses beside the Thames, and used to reshape the world economy. In 1876 70,000 rubber seeds were smuggled out of Brazil, nurtured in Kew and forwarded to Ceylon and Singapore. Brazil, which at that time had 98 per cent of the world rubber trade, now has 5 per cent. Three quarters of the rubber plantations in South-east Asia belong to a family tree that goes back to 22 seedlings from Surrey.

Tea was plucked from China, and cinchona (which yielded quinine, the remedy for malaria) was filched from Peru and brought by ship to Liverpool. Again, the Kew greenhouses did the trick, and soon both plants had been successfully established in India. The abundant supply of quinine allowed Britain to reduce the death rate of its troops in Africa from 50 per cent at the beginning of the 19th century, to 7 per cent by the end.

This sort of global agribusiness is a long way from the traditional image of the British cottage garden, but without overwatering the links Hoyles makes both seem part of the same botanical story, which is an elaborate mixture of foreign influences. Horticulture was far more highly developed in the Arab and Aztec world than it was in Europe. Yet it is hard to recall anyone mentioning, during the blitz over Iraq last year, that the world's first institute of botanical research was formed in ninth-century Baghdad - at about the time Alfred was burning the cakes in Wessex.

Vast numbers of popular and seemingly domestic blossoms have come to us from the other side of the world: roses from the Middle East (like the Damask, from Damascus), rhododendrons from the Himalayas, camellias from China and Japan, marigolds from South America, and so on. Even today, plenty of our carnations and chrysanthemums have been flown in from Colombia, where 40,000 women suffer from asthma, bronchitis and cancer caused by pesticides.

In all of these areas, Hoyles has done some impressive cultural spadework. He has picked over the compost heap and given us a view of where the flowers come from. Churchill once said that the British had two abiding passions: war and gardening. Perhaps there's a connection. It's obvious that doomed men in the deadly mud of Flanders should dream of peonies and daffodils, but also true that there are more things buried in our gardens than meet the iris. I'll tell you now of some that I know . . .