The long view: One family has been tending to Bickham House's ancient hedges and trees for over 300 years

As yellow as the daffodils that are waving in our garden, the new directory listing gardens of England and Wales open for charity is now stashed away in what is still quaintly called the glove compartment of my car. Sticky-sweet compartment. Ancient-OS-map compartment. Guilty-parking-ticket compartment. But gloves? Never.

I like to know it is there. Dreams are packed away in this little paperback (NGS £9.99); 3,800 interpretations of an ideal, private world. Just reading it, stuck in a traffic jam, keeps the blood pressure down. "Victorian double-flowered hyacinths" in Timothy Clark's superb garden at Netherhall Manor, Tanners Lane, Soham, Cambs CB7 5AB (open tomorrow from 2-5pm, admission £2), "expedition plants from worldwide mountainous regions" in Professor Haszeldine's garden at Copt Howe, Chapel Stile, Great Langdale, Cumbria LA22 9JR (open Easter Monday from 12.30-4.30pm, admission £4), "arboretum with spring bulbs and blossom" in the gorgeous Arts and Crafts garden at Misarden Park, Miserden, Stroud, Glos GL6 7JA (open tomorrow, 2-6pm, admission £5).

This dreamy process, trying gardens on for size, as it were, is quite different to an actual visit. Then, my antennae are wobbling like some crazed ant's. What does one learn from the approach to the garden? What wild trees and shrubs are growing around? This will give some indication of the kind of soil to be found. Is the site sheltered or exposed? If exposed, how have the garden owners coped with the problem? Is this a garden where plants rule? Plantsmanship is a wonderful madness, but the best gardens always offer more than a collection of plants.

Increasingly rare is a sense of timelessness in a garden. We live in a restless, fidgety age. To spend eight years in the same house is thought to be extraordinary. To have spent a lifetime there, unimagineable. But gardens benefit from owners who stay rooted in them. They take the long view. They plant trees. They understand that gardening is a process, not a product.

So it was a treat to amble through the garden at Bickham House in Devon, where Julia Tremlett's family have been entrenched since 1685. My antennae had, anyway, been wobbling like mad, because the approach is by way of a long, narrow ribbon of lane with rich Devon pasture on either side. You don't expect the lane to end, quite suddenly, in a grand semi-circular entrance with pillars, finials and superb gates. Camellias in red, pink and white push through the railings.

The drive curves round to give you, at last, a view of the house, sitting with its back into the hill. The old bit, which you don't at first see, is round the back. The entrance front was added around 1830 with a loggia running round the front and the south side of the house. A myrtle, set in the shelter of a big projecting bay, reaches almost to the eaves.

The handsome park railings which curve down alongside the drive, divide the garden from the sloping land below, with a lake, made in 1990, sitting at the bottom. An osprey once came to inspect the lake, said John Tremlett, but sadly decided not to stay. Moving round the house, under the loggia, where vast old wisterias wrap themselves round the columns, you look out over a huge lawn, carved out of the slope, said Julia Tremlett, for her grandmother's coming-of-age party in 1912. She was very keen on tennis and there's room here for two grass courts, side by side.

The land rises quite steeply on the right, with huge old rhododendrons and camellias bending over banks of old-fashioned daffodils. An ancient, mossy-branched mulberry that stands there looked just as venerable in 1903, said Mrs Tremlett, when her grandmother was photographed under it, aged three. By the time the Tremletts began to take the garden in hand, little remained in the flowerbeds but a red hot poker, which is still there. But they did have the vast shrubs and trees, including the biggest tulip tree (Liriodendron tulipifera) I've ever seen.

Magnolias do well there, too, and the higher path through the shrubbery allows you to look straight into the canopy of the trees growing lower down the slope – Magnolia sprengeri, Magnolia x soulangeana – underplanted with shrubby peonies and lacecap hydrangeas. Hidden under the domed heads of Japanese maples are the remains of an Edwardian Japanese garden once threaded with intricate paths and pools. Newer plantings include the sulphur-yellow-flowered Rhododendron macabeanum and some fine camellias such as 'Margaret Davis'.

The path through the shrubbery brings you down to the back of the house and a completely different scene: a formal, paved courtyard garden with a rectangular pool surrounded by clipped box beds. Here, another vast old Japanese maple weeps branches down the bank to a retaining wall of mossy stone. The wall looks as though it has been there forever, but the Tremletts made this courtyard garden quite recently and used the stone from a wall that was doing nothing in the yard beyond. Mop-headed hollies stand either side of a tall arched window and tubs of tulips are lined out in pretty ironwork stands.

The courtyard came about when the Tremletts divided the house, giving one half to their son. They then had to make themselves a new kitchen – and a new view.

At this stage you've already seen enough to wonder how these two keep the place looking so beautifully gardened – the beds all weeded and tickled up, paths raked, edges cut. But there's more. Wandering down past the potting shed you come to a cobbled passage, hung either side with forks and spades. This leads through into a walled garden, itself covering at least an acre, with walls of cob and stone topped with an intricate arrangement of tiles and coping stones.

The first thing you notice are the greenhouses – six of them. One, "a new toy" says John Tremlett, is set up as an alpine house, with clay pots of miniature treasures sunk into beds of sand. Others shelter the plants that he propagates and brings on for sale on the garden open days. Knowing, now, what meticulous gardeners the Tremletts are, it does not surprise that all the labels in the pots of seedlings and young plants in the greenhouses line up in parallel rows. Order combined with profusion. The perfect mix.

The bottom half of the kitchen garden is still used for fruit and veg. The top half is now laid to grass, with a row of trachycarpus palms (grown from seed by John Tremlett) leading to a summerhouse that their son, a joiner, made for the millennium.

From here you can look over to Bickham Cottage, home to Steve Eyre and his collection of South African plants; nerines alone he has in 450 varieties. Mr Eyre gardens for the Tremletts (lucky them) and his own patch is as immaculately maintained as his employers'. Both gardens are open tomorrow and also Tuesday and Wednesday (2-5pm); the combined admission is £4.50 and it's free for children. If you miss the Easter opening, try 13, 15 and 16 May. Find them a mile off the A38 at Kenn, near Exeter, Devon EX6 7XL. For more information, call 01392 832671 or e-mail jandjtremlett@hotmail.com.