Paths grow over with lightning speed, but sometimes you can pick out the line they took by a straggle of box trees, which originally formed a low- clipped hedge, now just a gappy line of full-grown evergreens.
Garden archaeologists have far more precise ways of mapping forgotten layouts. By taking core samples at regular intervals, they can work out which areas were cultivated, which were top- dressed as paths, and which probably contain remnants of built structures.
Last week, I swished rather wetly with members of the Welsh Historic Gardens Trust through the undergrowth of Talacre, an early 19th-century house near Prestatyn, in north Wales. Arriving at a totally unknown site, such as this, your nose starts twitching like a bloodhound's. What was in the minds of the people who made this place? What did they want their garden to be? What clues will the garden reveal? What red herrings will detain you along the way?
The house - there still is one, which helps to give a clearer picture of the garden - was built in the 1820s for the Mostyns, the local big wigs, who made a pile from lead mining. The architect was a Chester man, Thomas Jones, and the style is heavy-handed Gothic; a solid house rather than a beautiful one.
It was placed to take advantage of a long view out over the Irish sea (now mostly caravan sites), with a cloistered walk curving round the front of the house to provide cover when it rained. Irish yews, still intact, march in a line along the front of the terrace and remnants of roses and lilac still struggle through the weeds.
To the side of the house is a large eight-sided stone basin with a broken- down fountain. Behind it stands a bank of rockwork, not real rocks but brick and clinker, covered with a porridge-like render to give the effect of natural stone. Planting holes, worked into the layout, are all empty now, but you can imagine this fake rock face hanging with ferns and periwinkles and looking very effective, in a damp kind of way.
There are two seats on the terrace, made in the same way from broken bricks, covered with cement, and sculpted into fanciful, almost rococo forms. You can scarcely imagine anything more uncomfortable to sit on, but, like the rockwork, the seats have a bizarre appeal. They look like wedding cakes that have been left out in the rain, the cement running like icing down the sides.
To the left of the house, a rough track leads past a deep pond to the vast kitchen gardens. The walls are brick and long ranges of derelict lean-to greenhouses slump against the south face. Behind that are apple stores, potato clamps, a mushroom house, a boiler room and a fine multi-stemmed hornbeam.
Derelict kitchen gardens are particularly poignant. The slide from order to chaos is always most marked here, the remnants of planting most heroic. Ancient espaliers still flower against the old walls. Eighteenth century, said Dr John Savidge, of the university of Aberystwyth, a botanist and enthusiast for old kitchen gardens. This one was left over from the previous house on the site.
Dr Savidge also explained the large pond lying close to the kitchen garden. 'That's where they dug the clay out to bake the bricks to build the wall,' he said. Self-sufficiency ruled. The biggest surprise in the kitchen garden is the small banqueting house, in a precarious state of decay, perched on the wall that divides one enclosure from the next.
The vines have long since disappeared from the vine houses, but you can still see the carefully carved stone gullies where the vine stems were led from the outside to the inside of the building. The only living thing in the glassless greenhouses was a ribbon fern, Pteris cretica, seeded into a sheltered corner by a door.
The estate was already beginning to run down in the Twenties, when the Mostyns sold the house and its 35-acre garden to a Benedictine order of nuns. They surrounded the place with a high stone wall and Talacre quietly withdrew from the world - and from people's memories.
Five years ago, when the nuns' community had dwindled to an impossibly small number, the place was once again put up for sale. It was still boom time then, and George Begg, who bought the house, intended to turn it into a hotel. The grounds were acquired by a local dentist, Brian Nuttall, who hoped to develop it with holiday chalets. Neither project has yet been realised.
The Welsh Historic Gardens Trust was set up shortly after these events, not because of Talacre, as it happens, but because of unease over the uncertain future of the late 18th-century park at Middleton Hall, Llanarthne, and the much earlier gardens at Aberglasney, Llangathen, both in Dyfed.
The trust is no chain-yourself-to- the-bulldozer outfit. It sees itself, says its dynamic projects manager, Michael Norman, as a catalyst, a bringer-together of people, an enabler. The need for cash is always firmly in his sights. 'No good bringing a place back to life unless there are funds to run it,' he says briskly.
Members of the trust are helping to compile a register of all the important parks and gardens in Wales, amassing dossiers about gardens in their area from the archives of county record offices. Sara Furse, who herself has a garden with a history in Clwyd, has painstakingly done the detective work at Talacre, an irritatingly ill-documented property.
A garden in divided ownership, such as Talacre, throws up particularly delicate problems of diplomacy for the trust, which is negotiating with Mr Begg and Mr Nuttall to provide a site survey and a management plan for the place that will give it a future without destroying its past at the same time .
Central to the discussion is the survival of an extraordinary look-out tower, combined with a shell house and underground grotto, built on the hillside behind the house. To set it in context, you have to think of Mary Shelley, who published Frankenstein only a decade or so before Talacre was built, of the kind of self-induced, romantic terror that was satirised by Jane Austen in her novel Northanger Abbey.
This was the Mostyns' own Frankenstein castle, with weird underground chambers, all built of the same mortar-covered brick and stonework as the seats and the rockery. A headless hermit sits in one chamber, a ghost stretches out a skeletal hand towards you in another. A Cyclops leers out from a rocky promontory with his beady, central eye. The chamber is guarded by a fierce sort of Cerberus, arranged so that you can light a fire in his snarling jaws. All this, set in a wet afternoon in north Wales, provided a scene of wonderfully self-indulgent gloom. I enjoyed it hugely.
For details of the Welsh Historic Gardens Trust (membership pounds 10 a year) contact Michael Norman, Freepost, Plas Tyllwyd, Tan-y-groes, Cardigan SA43 2ZZ (0239 810432).