When my horticulturist niece said she had been to a garden unlike any other, I had to find a way to get there, too.

There were a couple of snags: The place in question, Benmore Botanic Garden, was off the beaten track, at the edge of Loch Eck in Argyll in the west of Scotland. And the only time I could get there was in December, when the rainfall for the month averages 12 inches. On the days when the sun shines, it does so for about 5 1/2 hours.

This better be worth it, I thought, as I drove the twisting and misty road across the glens of Argyll's Cowal Peninsula.

I won't keep you in suspense: Benmore, essentially an arboretum, is one of the most powerful gardens I have seen, even in the dead of winter a place of vital mystery and enchantment.

The thing is, there is no dead of winter in coastal Argyll. Flowers are sparse, but the landscape is alive with the colors of a temperate rainforest: the pink-red of sphagnum moss, the maroons of fallen bracken, the golden yellow mountains capped with snow, and a million shades of green. Through 150 years of ambitious but careful garden-making, Benmore is the essential Argyll, gathered and intense.

The primal flora — mosses, lichens, liverworts and ferns — come alive in the winter and form great swaths of greenery on the woodland floors. The majestic conifers are awe-inspiring. Scots pines, western cedars and Douglas firs reach well over 100 feet, and the entrance to Benmore is marked by an avenue of 49 giant redwoods that are now so tall they appear to look down on the winter sun. It is a testament to the vitality of Benmore that only one redwood has died since the avenue was planted in 1863. Its creator was a brief owner of the estate, a wealthy American named Piers Patrick who clearly wanted the redwoods to remind him of home. They are now more than 165 feet tall.

The avenue is the first element you see after entering the garden and taking the iron bridge over the River Eachaig, carpeted with moss and bathed in the low, golden light of solstice time. By early afternoon, the trees are steaming where the sunbeams play on the flares of their trunks. The same North Atlantic Drift that warms the tree ferns and palms of Heligan, 350 miles to the south, is at work here, transforming a glacial northland into a place of temperate lushness. The air can be cold after an hour or two, and the frost is slow to leave the great lawns of the old walled kitchen garden. But the light, fleeting and precious, plays with the trees and makes their far-reaching shadows move in a sort of dance.

In the charming, converted stable block that is Benmore's offices, I met Peter Baxter, the curator, who had agreed to give me a tour. Technically, the garden is closed in the winter, reopening March 1, but all that means is that the ticket office and cafe are closed. If you want to risk the weather in the off season, you are free to visit, though there may be days when Baxter closes the garden or parts of it for safety or maintenance reasons. Best to phone ahead.

COLOSSAL CONIFERS

The wettest of the four major gardens controlled by the Royal Botanic Garden Edinburgh, 120-acre Benmore receives more than 100 inches of rain a year. Mix in the richly organic and acidic soil, and you find a perfect environment for huge conifers, tiny mosses and lots in between, not least the rare and old rhododendrons that have played such an important role in Benmore's history.

James Duncan, a sugar refiner, took over from Patrick and must have spent a fortune on Benmore, planting 6 million trees. One, a Douglas fir, is now, at 190 feet, the tallest tree here. He imported and installed a pair of elaborate gilded gates that had been displayed at the Paris Exhibition of 1878, and he built a fernery. This was a fancy greenhouse popular among the new-money industrialists of the Victorian period, a place to house exotic and tender ferns in an age when plant exploration gripped imperial Britain with a fever.

Benmore was purchased in 1889 by Scottish beer magnate Henry Younger, whose son, Harry George Younger, added many of the conifers that today are so colossal, including western cedars and larches. He also began to plant the exotic shrubbery that was coming to Britain from all its vast dominions and beyond, including the rhododendron species from the Himalayas and China.

When Harry Younger donated the garden to the public in 1925, it became the perfect setting, horticulturally and aesthetically, for the large and rare collections of rhododendrons sent to Edinburgh from, among others, intrepid plant explorer George Forrest. The garden boasts 350 species or subspecies of rhododendron, some as huge shrubs, others as small trees reaching 30 feet. The hillside paths lead through groves of rhododendrons in bloom from late winter to June.

Species such as Rhododendron barbatum and R. strigillosum bloom through March. The showy, large-leafed species peak in mid- to late April, Baxter said.

In May, fortunea rhododendrons open, along with deciduous species from Japan and North America. But to see Benmore only in its spring glory would be to miss the point. Early autumn, after the highland midges have waned and the landscape begins to color, would be a special time, I imagine.

OLD AND NEW

After about an hour of hiking up woodland trails, we found ourselves in the newer parts of the garden, the Bhutanese Glade, where endangered plants from Bhutan are now growing safely on a hillside cleared for the purpose. Carrying on through twisting paths, we reached the Chilean Rainforest Glade, planted from seeds Baxter helped collect in Chile.

On our descent down Benmore, we moved again into an old, established area, and Baxter pointed to a romantic ruin perched on the cliff above, like a small abbey. It is the stone shell of James Duncan's fernery. There is an active plan to restore it, he said.

Farther down, we reached another area under development, a native woodland that showcases the mosses that cushion the forest floor. Baxter said there are as many as 50 species of moss here. A cliff face, glistening from the constant moisture, holds tiny colonies of liverworts, clinging to the crevices. Mosses once were considered a bit of a weed, Baxter said. Now they are getting their due, though he is hard-pressed to distinguish the species. That takes a moss expert — a bryologist.

Benmore is near the mouth of the River Clyde and deceptively close to Glasgow. In less than two hours, you can be transported from modern city to another world. The effort to get there is well worth it. "Most people who do come can't believe it," Baxter said, without swagger. Or, I imagine, exaggeration.