All is not lovely in the garden: they've taken the forks and spades, the cupid and now the fish pond.
Most visitors to Powis Castle in Wales are content to take away a postcard or some other National Trust memento of the site. Three years ago, an unknown group of visitors took things several steps further: three lead statues - a shepherd boy, a piping faun and an antique Venus - and two baroque urns were stolen from one of the terraces. They were the work of the Flemish sculptor John van Nost, dating from the early 18th century, and the National Trust put their value at around pounds 100,000.

Garden theft started to hit the headlines about ten years ago. This was the time that firms such as Sotheby's in Billingshurst, West Sussex, began to develop a specialist market in antique garden statuary, seats and urns. Victorian cast-iron twig and vine benches which had mouldered for years under dripping summer trees suddenly acquired astonishing value. As gardening and gardens reached into the smartest magazines, the market for 18th-century cupids, fountains and slumbering lions increased exponentially.

The National Trust, whose gardens are home to a remarkable collection of antique urns, statues and furniture, has been a particular target for theft. Thieves at Wallington, a Trust property in Northumberland, were disturbed as they were trying to remove the fine lead statues which decorate the walled garden. The garden lies down a long track leading from the road, and the tenant of the portico house in the garden discovered the intruders when he returned home late at night. The 18th-century statues, attributed to the English sculptor John Cheere, were said by the Trust to be "irreplaceable".

The police put thefts from gardens in the same category as burglaries from houses, so no one can put an exact figure on how much is stolen, but it is thought to be at least pounds 70m worth of goods a year. In a recent survey of 22,000 subscribers, the magazine Gardening Which? discovered that 14 per cent of its readers had suffered from some sort of garden theft. Most commonly stolen were garden plants (nearly a quarter of those who had suffered from garden theft had lost trees and shrubs, sometimes whole hedges), followed by tools (including lawnmowers) and containers such as tubs and hanging baskets.

In an extraordinary raid earlier this month, police unearthed an enormous haul of stolen garden machinery, statues and tools at Radstock in Avon, said to be worth at least pounds 200,000. The spoils included 50 lawnmowers, 30 strimmers, generators, garden tractors and power tools, all of which were stacked in outbuildings and trailers at a deserted country house near Radstock. "It was just like a scrap dealer's backyard," says Detective Sergeant Mike Veale. He thinks that the Garden Shed Gang, as they have inevitably been dubbed, would probably have sold the equipment on through car boot sales and second-hand shops in the area.

"We believe there is a big network of small thieves feeding one or more main handlers," says Det Sgt Veale. "We have got 185 items here to identify, most of them stolen in the Thames Valley area and Hampshire, then shipped into Avon for redistribution. This is big business. So far, we have identified the owners of about 30 items, by serial numbers or other distinguishing features. Some pieces were marked with ultraviolet pens."

The raid has been a great success for Operation Bumblebee, an initiative launched by the Metropolitan Police and recently taken up by the Avon police force. The object is to target particularly prolific burglars and handlers of stolen goods.

Stealing lawnmowers and strimmers is the outdoor equivalent of lifting televisions and videos from houses. Reprehensible, but comprehensible. What is more surreal is the way that an entire pond, together with fountain and fish, can disappear in a night, as happened to a gardener near Crewe in Cheshire. Bill and June Williams of Cray's Pond in Oxfordshire had an entire hedge stolen: 20 expensive two-year-old Leyland cypress, each of them 5ft high. The fact that in this overcrowded island, thieves can dig up, with roots intact, 25ft trees from a front garden without being spotted is rather remarkable.

All of those who love gardening wince more painfully at news of plants being stolen than they do when told of purloined lawnmowers or strimmers. We invest some of ourselves in our plants, as we do in our children. As well as being animate, plants are personal, in a way that a ride-on mower, however gleaming and expensive, can never be.

Those who have watched the slow birth of a collection of families of plants were shocked when the entire national collection of pseudopanax, strange spiky plants with hooked and toothed leaves, originating in New Zealand, was stolen from the botanic garden at Ventnor on the Isle of Wight. The curator, Simon Goodenough, who had built up the collection over 17 years, thinks that the theft could only have been the work of a highly specialised gang of plant thieves, stealing virtually to order. "There is no doubt they knew what they were looking for," he says. "Pseudopanax are not everyone's cup of tea, but they took the entire collection, 14 species and eight cultivars."

There was another highly specialised plant robbery earlier this month when more than 400 bulbs disappeared from the national collection of hyacinths held by farmer Alan Shipp at Waterbeach in Cambridgeshire. "The sad thing is that the thieves may not know what precious bulbs they have got," he says. "The collection is in unmarked plots and there are five varieties that only exist in this collection. These thieves may have wiped them off the face of the earth. You couldn't get these five varieties anywhere else in the world."

Mr Shipp is hoping that somebody may notice his bulbs at a car boot sale. "If anyone sees hyacinths being sold before the 10 August, then there is a strong possibility they may be mine. Dutch bulbs don't come in until then. I think the thieves lifted them now so they could dry the bulbs off and sell them later in the summer."

The stolen bulbs were lifted with a wide tined garden fork and Mr Shipp hopes that when he has finished working carefully through all the disturbed ground, he might find some small bulbs, offsets, which may have slipped through. Sadness rather than anger is what he feels about the affair.

Others beside thieves are poised to benefit from such losses. Insurance companies, always quick to spot a new opening, are now offering specialised garden insurance to home-owners. Policies vary in their comprehensiveness. Some cover plants in conservatories and greenhouses, but not those growing outside. Some, such as the Home Plus policy from Norwich Union, cover hedges but not individual shrubs and trees. General Accident has gone into partnership with the Homesitters agency to offer a policy which substantially reduces insurance premiums if a house and garden are not left unoccupied for long periods.

In 1993, more than pounds 2.6bn was spent on plants and garden equipment in this country. "With such high investment, people must ensure that their gardens are more secure and that they are properly insured," says Ray Morley of Commercial Union. The insurance firm CGA Direct offers a policy called GardenGuard which provides cover for fencing, hedges, lawns, patios, tubs, urns and garden gnomes, but does not cover plants. It costs pounds 15 a year. Richard Playle of CGA Direct's marketing department says that any decent home contents policy should also cover garden machinery such as a lawnmower, if it is stored under lock and key. If it were stolen while standing in the open, the maximum payout would be pounds 250. CGA also has a specialised home insurance policy which offers pounds 1,000 of cover against the theft of plants, but garden owners have to pay the first pounds 50 of any claim.

"We are talking premium plants here," says Mr Playle, who adds that garden theft is the "designer crime" of the Nineties. "We have just done a survey of claims. It showed that house burglaries seem to be dropping, but that theft from gardens is rising. It's a soft option isn't it? And the car boot sale gives thieves a wider potential market than they used to get in the pubs."

"Thieves", though, come in many guises. Stephen Anderton, horticultural chief of English Heritage's northern gardens, recalls a trio of Mother's Union members who were creating havoc in the gardens of his region, regularly hiding plants and cuttings in a selection of large handbags. "The oldest was 80," he says. "She was the mother of one of the women in her sixties and they said it was all her idea." The cartoonist Giles's Grandma is evidently alive and well and living in Northumberland.