Siobhan Dundee's fine woodland garden cannot be seen from her house, but its dramatic colours are worth seeking out

Hidden from the house, so you come upon it as a surprise, this relatively young garden is reached from a path that winds east through banks of rhododendrons flanking the castle. Moss-covered paths lead up and down the gully, allowing you to look directly into the blooms of the rare magnolias and rhododendrons for which this romantic garden is famed. Paths around the edge of the gully open up with tantalising views of the river, allowing you to gaze on a lazy tapestry of foliage.

The ancestral home of the earls of Dundee, Birkhill is now home to Alexander and Siobhan Dundee and their four children. The woodland garden was created as recently as 1930 by Alexander's grandmother, Panama, who was succeeded in the garden by her daughter-in-law, Patricia, followed by Siobhan. Previous generations of the family have tended the two-and-a-half-acre Victorian walled garden for far longer than that.

Leading the way through the mature trees above the woodland garden, Siobhan explains that it is well sheltered because of how the area was carved out in the Ice Age. It also benefits from a relatively warm microclimate and acid soil – both unusual in Fife. "When Panama thought this would be the most wonderful place to have a garden, it was dense woodland," she says, adding that the choice of location was also inspired by the primula-edged burn that runs down the middle of the gully.

Panama, a keen and knowledgeable gardener, was supplied with seed by gardening friends, including Lord Kinnaird of Rossie Priory. She began planting species of rhododendron and magnolia, including rare Magnolia campbellii. Nowadays, Peter and Ken Cox from Glendoick supply the rhododendrons that are slowly being added to the collection.

While Panama and later her daughter-in-law concentrated on rhododendrons and magnolias – with Patricia favouring dramatically coloured hybrids – Alexander's grandfather and father planted the hardwoods, including the Californian redwoods around the gully. "The redwoods are just babies – we've got a hundred years to go until they are wide enough to drive a car through them," Siobhan says.

When she first married and moved to Birkhill, Siobhan was too busy raising her family – the couple now have two grandchildren – to do much gardening, with maintenance to stop things getting out of control being the order of the day. But 15 years ago, when her youngest daughter, Lavinia, was five, Siobhan experienced an epiphany. She was at a christening when Hugh Lorimer, grandson of the architect Robert Lorimer, gave her a handpicked bunch of Helleborus nigra. "It was the very first time I realised it would be wonderful to be a gardener," she recalls. "And it was the first time I'd really focused on hellebores. So I went to a local nursery and bought some more and planted them in the walled garden. I was hooked from that moment on."

She began working in the walled garden, once a kitchen garden, on the far side of the gully. There, she found an overgrown garden laid out in traditional cruciform-style with central paths and narrow, south-facing, early 18th-century greenhouses, used for growing peaches and nectarines. Now maintained by gardener Graham Fulton, the vegetable garden is completely organic. The north-facing border is planted in the colours of the Scrymgeour tartan; rusty reds, oranges and yellows.

Gaining in confidence, she turned her attention to the woodland garden. "The first thing I noticed was that everything seemed to be so low, and you always had to look up," she recalls. The solution, which evolved gradually and often in response to events, lay in creating the paths. "Some were already there. Laid out with flagstones by Panama, they were just waiting to be uncovered," she says pointing out two flagstone-covered paths, one of which leads up to the house from the burn and the other running up the opposite side.

Later, the main path was extended from the top of the gully to the land east of the house, now consecrated for use as a family burial ground, with a yew hedge in the outline of a chapel. "We did this when Alexander's father died," Siobhan says, "and when we found it worked, we cleared the ponticum and laurel on both sides of the path." This path proved useful three years ago when there was a family wedding in the garden, prompting another development. "I suddenly realised that you couldn't see the Tay," Siobhan recalls. "So we quickly cleared some trees and shrubs down the hill to open up a vista." In case of rain on the big day, the family had organised 180 white umbrellas – but they didn't need them.

Another step forward came when a fence running across the lower part of the garden was removed and the banks on either side were cleared. "To me, the fence ruined the romance of the garden; it is a wonderful experience to wander through the woodland and come upon the garden. Now it is open to endless possibilities down to the Tay," she says. The main enemies of this garden are the deer who help themselves freely to young plants.

When it came to planting, Siobhan started slowly, moving seedlings already established in the garden and thereafter keeping new plants to a minimum. "I don't plant too many things," she says. "I like to think what it will be like in 25 years' time. And the lack of light does restrict how much we can plant." Concentrating on a soft pallet of colour, she favours rusty-leaved Japanese acers combined with magnolias, including M stellata, M "Butterflies" and fragrant white-flowering M sieboldii. Favourite rhododendrons include creamy Rh falconerii and lavender-flowered Rh augustinii.

Erythroniums, discovered in the undergrowth, were released into the light and are now spreading well in the mulch along with late-flowering Pheasant's eye narcissus, while recently planted clumps of trillium are starting to spread.

The gully is a surprisingly low-maintenance garden, with nature left alone to mulch the plants. It is maintained with help only one day a week from gardener Stewart Haart.

"He is very knowledgeable and works very hard," Siobhan says. "He also has a talent for finding and nurturing young seedlings. And we have the most wonderful team of helpers. Everything is so happy here – we are so very lucky." sm

- Birkhill Castle, Balmerino, Cupar, KY15 4QP; www.birkhillcastle.org.uk; Birkhill is open on 7 June for the Devoted to Life Walk, to raise money for the Marie Curie cancer charity, when it will be possible to walk all the way to the abbey at Balmerino, a 45-minute walk along the banks of the Tay – sturdy shoes are required. Weddings can be held in the walled garden.