If I said “Westonbirt” you would rightly think of autumn colour in the arboretum, or the modern garden festival.

But say Westonbirt to Sir Roy Strong and he thinks about “one of the most amazing and exotic built garden structures to survive from the High Victorian era”.

Say it to Peter Dennis, professional gardener extraordinaire, and he thinks of orchids: “Just google it — there are orchids from Westonbirt everywhere behind modern breeding.”

The link in all this? Robert Stayner Holford, once one of the richest men in England, who built the neo-Elizabethan Westonbirt House in 1868. He began the world-famous arboretum, he made the formal Italian garden that Sir Roy so admires, he collected and bred orchids.

It is his wife who is celebrated in the ironclad rhododendron ‘Mrs R.S. Holford’. And as if all this were not enough, he built a collection of Old Masters — Velázquez, Rembrandt — that was said to eclipse the Royal Collection. Art and trees were his life.

But bad times are always round the corner. Holford’s son, George, who did so much to develop the arboretum, died childless, and his indirect heir soon sold up. The arboretum eventually went to the Forestry Commission in the 1950s. Westonbirt House (and garden) became a private school and has now spent more years in that role than under the Holfords.

But, at long last, there is a chance to visit garden and arboretum this week, to see what Holford was really about. The school is opening its garden, with — from Monday — a string of daily events for young children; and this week should also be peak time for autumn colour in the arboretum’s famous Acer Glade, literally just across the road. Good trade for both, I should say.

At the school there will be face- painting, storytelling, nature trails, scavenger hunts and fairy workshops. Where better to do it than in a garden with such a wealth of vegetable and architectural hideaways? For the last weekend there will be plant sales, too (Derry Watkins from the famous Special Plants Nursery has a stand).

Frankly, I’d visit it for the garden alone. It’s fascinating, a true piece of Victoriana, like Brodsworth Hall in Yorkshire but on three times the budget. It’s a monster of a house à la Waddesdon, with great terraces and urns and an orangery so big that it was turned into the school’s theatre; but, unlike Waddesdon, its grand parterre is not set in front of the house, it’s tucked away in an enclosed Italian garden.

There are elegant walks, dingley grottoes, formal pools, all those evergreens so fashionable then, and a cosy, early 19th-century park made by W. S. Gilpin.

Be clear, none of this is in great order, but this is the time to see it, as the tide turns. For the world is on Westonbirt’s case now. The school and arboretum have got their heads together to form the Westonbirt Heritage Partnership, to keep the Holfords’ project intact and in good health on both sides of the road — garden, park and arboretum.

Major lottery funding is offered, but the usual 60 per cent match-funding must be found by the partnership. Meanwhile, some work is already afoot. From a private donation, the school has restored the pretty little Camellia House, where, today and tomorrow, Victorian teas will be served.

The Finnis Scott Foundation has provided funds to revitalise the Italian garden, and this is where the real excitement lies; this is what Dennis has been brought in to do. Its layout is simple enough. Across the top is a high wall with a fabulous domed pavilion at either end.

The broad terrace in front of them is filled with a parterre of stone-edged beds and stone vases set in lawn. The land then drops slightly to a lower terrace, presently containing only two rectangles of grass. Down either side of the whole space are broad walks flanked by rose borders.

At present the upper terrace’s beds are filled with crisp blocks of box, lavender and santolina, which look satisfyingly bold. The border against the top wall contains climbers and wall shrubs — Fremontodendron, Eccremocarpus and wintersweet. But over the next year things will change.

That line of climbers will turn into a subtropical border; expect to see ginger lilies, the 3m Dahlia imperialis, maybe bananas. Out will come the blocks of lavender and santolina to be replaced with full-blown Victorian bedding-out, in the most prime of primary colours.

But here’s the interesting bit. Those two great grass plats on the lower terrace — each 9m x 30m — will be dug up to put back two, well-researched, apsidal-ended flower beds. Two herbaceous borders 9m deep. Two enormous, Victorian island beds. Who says that the Victorians only had bedding-out!

What a sight they will be, just the kind of scale that the naturalistic, Piet Oudolf, grasses-and- perennials school of design would love to get their hands on. But here the pattern of planting has still to be confirmed. It will be remarkable and a complete change to see perennials manipulated so differently and on such a scale.

There is a two-deck pool, too, whose upper stone filigree walls are lined with thick glass. Was it to reflect light from the pool below or to allow the Holfords to watch fish swimming in the pool above, like an aquarium? Either way it’s a great idea. Those Victorians were full of them.

NEED TO KNOW

Getting there Westonbirt House, Tetbury, Glos GL8 8QG.

Garden open now until Sunday, November 1, 10.30am-4pm.

Children’s activity week starts Monday, October 26.

Plant sale Oct 31-Nov 1.

Further details holfordtrust.com

Note that there is a separate entry charge for the arboretum across the road.

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