Gardening: Haunts of ghostly gardeners: Who did what when? Anna Pavord sifts the layers of history in Walcot Hall's 300-year-old gardens, open to the public for a day

There is nothing to beat the excitement of turning in at a garden gate, not knowing what lies ahead. Like a sea anemone, all arms waving, you try to pick up clues from what you see about you. In a small garden, it will probably be the plants that seize you first. In an old garden you try to unpick the different layers of activity. Who did what when?

Walcot Hall, just outside Barnack in Northamptonshire, had me in a more than usually frenetic state of arm-waving. Until this year, it was not open to the public, but the view from the road offered enough glimpses of mature magnolias and wildly clambering roses to make you want to see more. On one day this summer, under the National Gardens Scheme, you can see it all.

The drive leads up to one of the prettiest houses - late 17th-century, neat, contained, five bays along the north and south fronts, nine along the east and west.

The house and garden were a celebration of the restoration of Charles II, after the upheaval of Cromwell's Puritan Commonwealth, when conspicuous consumption went out of fashion and landowners kept their heads down. But the garden either side of the drive - full conifers, huge wellingtonias, junipers and cypresses, overplanted and never thinned - is not redolent of the late 17th or even the 18th century; late 19th century, I would have thought. And a glimpse of a hexagonal boarded summerhouse at the end of an overgrown box-edged path also seemed to fit that period.

What you would expect around a late 17th-century house are stretches of water, intricate scrolls of box, and fancy, formal stuff borrowed from France and the Netherlands, whose styles had a big influence in England before Capability Brown swept them away.

The north side, where you arrive, provides none of this, but round to the west it all begins to feel quite different. Here you have a stupendous view over a huge area of grass, through ornate iron gates and along a double avenue of lime trees that climbs up a gentle slope and over the horizon, as if it might go on for ever.

The ground floor of the house looks down on the gardens of the south and west sides. Would an aerial photograph taken during one of our summer droughts show the ghostly swirls and curlicues of a forgotten parterre on this west side, now laid to lawn? This is just where you would expect one to be.

Though Darby Dennis, who lives at Walcot Hall with his young family, has spent most of his own life there, he has been able to find out little about the place; and as far as he knows, it has never been written about.

The house was built in 1670, he said, and spent a longish period in the hands of the Neville family. Then, in the late 19th century, it was bought by a Mr Dearden, who had made his pile in Yorkshire and came south to spend it.

That explains the conifers. And as you walk round the grounds - about 20 acres all round the house - you begin to realise the huge extent of Dearden's input. He laid out the ambitious Italianate water garden on the south side; and he built the Corinthian-pillared arcade in the secret garden - pure Hollywood, with columns curving behind a circular pool and roses tumbling off the top.

Mr Dennis is a farmer but not, he confesses, a gardener, and this is not a place to come for complex herbaceous borders or finished pieces de resistance. But it is an important garden, with magnificent bones, that several years' work would restore to its full glory.

The Dennis family has begun the long task of clearing the undergrowth. Some of the supporting cast of conifers have been felled, letting in the light and making room for Dearden's star trees to spread their wings - such as a superb handkerchief tree, an equally beautiful cut-leaf beech, swamp cypress, cedar . . .

Dearden's water garden - a complex late-Twenties arrangement of loggia, elegant curved bridge and water falling from a large lion's mask into two formal pools - ends in a semicircular balustrade. It lies at right angles to what I would guess is a much older stretch of water: a long rectangular canal, perhaps part of the original garden. Dredging and clearing would do it no harm, though it would be less romantic without its banks of bullrushes and flag iris.

The canal runs away from the house on the south side, a view terminated by another of Dearden's additions: a finely detailed rotunda with more Corinthian columns topped by a domed roof. Dearden signed and dated it (1925).

But his dog, Pompey, has the grandest monument: a vast structure, covered in a Latin inscription, at the end of another lime avenue that leads to a field still called the polo ground. The dog ('Canis amantissimus') died aged 16 in 1901, but what sort of dog it was we are not told. A mastiff perhaps? A bloodhound? I long to know. It would help in imagining what kind of man Dearden was.

Was he responsible for the extraordinary folly/grotto that stands in the forgotten far right-hand corner of the south garden? The Dennises have cleared a big area there - they are planning to make a rose garden - so the building stands rather more open and alone than it must have done.

It is a small, rectangular place with a hump-backed roof and walls of stone - quite a lot of it fake pudding stone, perhaps made by one of the many companies that specialised in reconstituted stone in the 19th century.

Or is it an 18th-century exercise in Gothic horror, with its upright tombstone (a full- length Norman knight and his lady) and fragments of old church windows? The mortar, cement- rather than lime-based, suggests a later date - but the cement may have been introduced during repair work.

For a more domestic view of Walcot, tackle the garden by way of the stable yard. The yard may seem familiar. It was used in the BBC's serial Middlemarch as the back of the coaching inn, the White Hart. It is still painted in the BBC's chosen livery of cream, and above one of the doors is a left- over prop notice: 'Coaches and Wagons to all Parts of the Kingdom'.

The yard, like the house, remains just as it was built: stables below, grooms' quarters above, the whole area neatly cobbled in quadrants leading to a central watering hole. Beyond lies a bed of stout hybrid tea roses and rows of neatly staked, potted chrysanthemums, signalling the bothy, headquarters of the Dennises' gardener.

After the wisteria-covered cottage on the left, you emerge into part of the old kitchen garden. It contains the best sort of slightly sagging lean-to greenhouse against the wall, packed with clivias and regal pelargoniums, all grown in comfortable clay pots. Beyond is the davidia, the handkerchief tree.

To those used to the brushed and combed gardens of seasoned garden-openers, Walcot may be a disappointment. But seldom will you find such a beguiling atmosphere, a palpable sense of place that 300 years' worth of owners have put their hearts into.

Walcot Hall (opposite the cricket club), Barnack, Northamptonshire, is open on Sunday 5 June (2-6pm). Teas in aid of Barnack Play Group. Admission pounds 2.

(Photograph omitted)