England in the fall

Peter Gregory surveys the overflowing supplementary car parks at Westonbirt Arboretum and declares: "I wouldn't have done it. I wouldn't have encouraged all these visitors." The place is teeming with people, dogs, children on Heelys, Portaloos and burger vans as Westonbirt struggles to cope with the 200,000 October nature lovers who arrive to see what's considered to be the best display of autumn colour this side of the Atlantic. "But I was wrong, very wrong," he smiles. "Because all these people who come, they fall in love with the trees; that's why they come back again and again."

Gregory used to be one of the co-managers of Westonbirt, our national tree collection, before taking well-deserved retirement. However, he's still a visitor - these days he even turns out for the Enchanted Christmas, when trees are lit with coloured light and visitors can enjoy a rare night-time walk around. "It's really, really magical," he enthuses. In fact, Gregory's here often enough for both he and his dog Gillie to be cheerfully greeted by current members of staff. Which is hardly surprising, because he's a walking reference library when it comes to one of the most important elements of Westonbirt's October colour scheme: the Japanese maples.

"They come in every size, they come in every colour," he enthuses. Many of the trees have personal memories for him because he was involved in actually selecting and planting them, but he loves them all. I have done some swotting up before arriving by reading Gregory's Pocket Guide to Japanese Maples (published by the appropriately named Timber Press, £14.99). I mention how much I love the fact that the text gives all the poetic Japanese names for the different cultivars. "I know!" says Gregory, delightedly, seizing the nearest branch: "This one, Beni shigitatsu sawa, means 'Red snipe flying up from a winter swamp'." Tony, the photographer, and I gaze at the leaves, which look marbled with pink and green turning to red. Both of our faces look appropriately amazed.

Most visitors to Westonbirt tend to follow a marked trail to the autumn-colour area in the Old Arboretum, where there are magnificent Victorian specimen trees planted by the estate's original owner, Robert Holford, and then his son, Sir George Holford. The Holfords laid out an arboretum for themselves with views protected by their very own acts of parliament. While the trees in the Old Arboretum are sumptuous, Gregory is keen to get Tony and I up into the so-called New Arboretum, where there is a National Collection of Japanese Maples.

"Well," we say to Gregory after a 10-minute walk, "at least we now understand why you were so insistent." After a stroll through wilder woodland, a soft glade of mature larch opens up before us, utterly foreign and glamorous. The vivid green of the drooping larch needles seems a sudden touch of the exotic, but the firepower of the New Arboretum is multiplied tenfold by the presence of the Japanese maples. Spots of bright orange, red and yellow, studded into the green, the trees line elegant serpentine pathways moving through the forest glade.

"But who designed all this?" I ask, innocently. Gregory smiles as he explains that, in fact, this area began as a government genetics experiment (the larches) and a consumer experiment for Which? magazine (in the case of the maples). These prosaic origins could hardly have produced a more stunning result - I just stand and gawp. These days, further planting is being supported by a large grant from the local Rotary Club in celebration of its centenary; looking at the delight of visitors tumbling through the woodland, I think it's safe to say that the Rotary Club made a great decision.

But if you don't make it to the maples this autumn, Gregory has a surprising treat for you. "This isn't even my favourite time to see the maples," he says with a grin. "I think they are at their most beautiful in mid-May. Maples go bright colours in autumn because they withdraw the green chlorophyll from the leaf, leaving the reds and oranges behind. And during spring they go through exactly the same process in reverse. The early leaves have no chlorophyll yet, so they aren't green, they are all these fantastic colours. But not so bright, rather more subtle, and personally that's my favourite time to see them." Looking through his book, I am gobsmacked by photos of the sticky emerging leaves of Higasa yama, the Popcorn maple. The tight curls of pink and green look like Dale Chihuly's glass art. So there's one to look forward to, I think, as I put it in my diary for a spring visit next year.

Westonbirt's Enchanted Christmas, weekend evenings from 30 November to 23 December, 5pm to 8.30pm, last admission 7.30pm. Tel: 01666 880 220