The reason was explained by Keith Collins, Jarman's partner, who has looked after the garden at Prospect Cottage since the painter and film- maker died early last year. The purpose of some of the montages of driftwood, metal and old garden tools is to mark the site of a perennial plant that almost disappears in winter but will rise again in the spring. In the summer the living plants take over and smother their markers.
A second surprise is the plants themselves. Not all of them, by any means, are varieties of such as sea-kale, samphire, poppies, fennel and sea pea that have drifted in from Romney Marsh or the beach. These passing strays from the wild make up an important element of the garden but, to supplement them, Keith spends time rooting around in local specialist nurseries for pedigree plants that will take to the peculiar conditions of soil and climate. There are irises, roses, lavatera, curry plants and many others.
So peculiar are the conditions that not many residents of the other fishermen's cottages dotted along Dun-geness Road, in the shadow of the nuclear power station, have sought to make a garden. The cottages stand on shingle some 20 yards back from the road and about 300 yards from the sea. In many places the shingle is interspersed with thin grass, gorse, bramble and seakale. It does not look like a prime site for horticulture.
In his gardening diaries, published last week, Jarman explained that when he bought the cottage in 1986 he had no intention of creating a garden, either. It evolved first through making patterns in the shingle with flints gathered from the beach. Then he planted a dog rose and used a piece of driftwood as a stake.
That original rose is still there, just behind the back door, along with old-fashioned roses that Derek and Keith chose from Peter Beales's specialist catalogue. Soon, by digging holes in the shingle and filling them with composted manure, they had established a variety of plants.
One of the first gardening writers to discover Prospect Cottage was the veteran Christopher Lloyd, who wrote about it in Country Life in July 1990. Since then its fame has grown. Lloyd's own garden and nursery at Great Dixter are not far away and Keith now buys some plants there.
The qualities a plant needs to survive on the Kentish coast are a tolerance of salt and dry conditions - rainfall is low - and most of all indifference to the wind. "Everything gets blown flat here," says Keith. Thus many shrubs develop prostrate forms, spreading outwards along the ground instead of risking lifting their heads into the gales.
One is the blackthorn, or sloe, which seldom gets to more than 18 inches high here. It occurs in the wild, as does the prostrate form of woody nightshade, or bittersweet (Solanum dulcamara), and the sea pea (Lathy- rus japonicus). Keith has all of them in his garden, and his yellow "Canary Bird" rose also seems to be developing a prostrate habit.
"There are so many things I'd love to grow," he sighs. "I see something I really like in a nursery and then I read the label and see that it needs a sheltered spot, so I think, 'Forget it.' But a lot of it is trial and error. For instance that aquilegia there behind you has done really well this year." The most intriguing plants in the garden are the several varieties of seakale (Crambe maritima). A relative of the cabbage, this comes in numerous shapes of leaf in both green and purple, and proliferates on the beach.
"In the winter it's completely flat and subterranean, so you can't see it," says Keith. "In the front garden it's the main structual element, but it completely vanishes in the winter. Then you get lovely deep purple foliage in early spring and in May the incredible flowers that smell of honey. They last a month but they come up at different times.
"They're all different genetically because of cross-pollination: some have enormous leaves and some little tiny ones. They're all edible but you aren't allowed to eat the wild ones, only the ones you grow in your garden. There's a woman who used to live next door who ate them every day of her life."
Although the coast is exposed to biting easterly winds there is seldom a frost, which means that several half-hardy plants can survive the winter. A yucca clearly flourishes, as do fleshy cardoons, and wallflowers grow as perennials rather than biennials. There are few traditional garden pests, such as greenfly and black spot, but rabbits are a continual menace.
When a rare frost does come it has dramatic results. "The stones are completely frozen solid - it's an astonishing feeling because you're used to trudging through them rather than walking on top."
Keith is a Tynesider (in Derek's diaries he is referred to as HB: Hinny Beast) and therefore believes that no garden is complete without its vegetable patch. Although he has not gone in for the giant leeks and onions that are characteristic of the North East, he grows a healthy-looking selection of vegetables and herbs on a raised bed alongside the house. "I feel I've got to make a Geordie effort," he said, presenting me with two bunches of crisp and impeccable radishes, some parsley and a jar of clear honey from his beehives.
Since Derek died, Keith has expanded and developed the garden. "I keep changing it. I'm not going to keep it the way it was when he died because there was a lot of grass then. I spent all my time looking after Derek and so I didn't have time for the garden.
"Now I've put up more sculptures and cleared away an extra 10 feet of brambles. But I can't expand it any further because English Nature have written to me to say: 'Do not gardenise and more' - gardenise, that's the word they use.
"This area is designated as a site of special scientific interest and for English Nature this garden is a disaster. I've brought in foreign soil, foreign species and I've disturbed the shingle ridges - for English Nature it's a nightmare. They're trying to make it into a special area of conservation, where they pay you pounds 75 a year not to have a garden."
English Nature's letter followed a prickly visit from one of its inspectors. Charged with protecting the natural landscape, he clearly did not see eye to eye with Keith on several issues.
"He was especially scathing about the red valerian that I'd planted on the other side of the road, to stop people parking there. It didn't work, because people just drove over them, but the inspector objected because he said it's not an indigenous plant to the British Isles. I said it had been here since 1400, when it was brought from the Mediterranean. How much longer does it have to be here before it's indigenous?"
If the environment of Dungeness is being damaged, as English Nature fears, it is not so much by the garden as by the hundreds of sightseers who flock there to see the Jarman garden every summer weekend. It is not officially open to the public but, because the road is unfenced, there is nothing to stop people tramping round it, as they do in increasing numbers.
"On Sundays and bank holidays you may get 200 people here," says Keith. "A garden is meant to have a contemplative nature to it and be a place of peace and quiet. People used to knock on the door and ask if we minded them looking round but now they come and I catch them peeping in. I'll show you a noseprint on the bedroom window. I tell them I live here and they say they thought they read somewhere that it was open."
The publication of the book and the resultant publicity will do nothing to staunch the flow - nor, it must be said, will an article that Keith wrote for the current edition of the magazine Gardens Illustrated, in which he declares that visitors are "welcome and wander freely". So numerous have they become that life in the small cottage is untenable at times and he is occasionally driven to stay elsewhere. But it is still his home and he will carry on cultivating his shingle as long as the twin pressures from English Nature and insatiable sightseers do not force him to call a halt.
! 'Derek Jarman's Garden', with photographs by Howard Sooley, is published by Thames and Hudson, pounds 14.99