In 1789 William Paxton bought an estate in Wales. Two centuries later it could become a pounds 35m botanical garden

Less than one-sixth of the 550 detailed proposals considered for funding by the Millennium Commission squeezed through to the final round. Between now and September, commissioners will be travelling all over the country to visit the sites of these 83 selected schemes, asking awkward questions, listening beadily to the answers and acquiring in the process yet more reams of paperwork and plans.

In this last lap of the race towards the pots of gold that will be handed out in September is a bold and visionary scheme for a national botanic garden for Wales. The principality somehow shuffled all the way through the generally philanthropic 19th century without ever acquiring a botanic garden of its own to match those of England and Scotland.

The chosen site is Middleton, between Swansea and Carmarthen, where in 1789 William Paxton spent the fortune he had made in India building a huge neo-classical house and laying out a park. To the east and south of the house he dammed streams to make four lakes, connected by waterfalls and fancy cascades. He built bath houses and bridges, planted trees and constructed a folly, locally called Paxton's Tower, to commemorate Nelson's heroic deeds in the Battle of Trafalgar.

The folly still stands, but the house disappeared more than 50 years ago, burnt in a fire in 1931 and never rebuilt. The lakes silted up, brambles crept over the paths through the plantations and the extraordinary double-walled garden, where one great rectangle is enclosed within another, even larger one, fell into disrepair. Only a few old fruit trees cling on and in spring there are carpets of daffodils once bred by an enthusiastic head gardener. The estate, covering 568 acres, was acquired by Dyfed County Council.

The proposal, which has been brought together by the acting project director, William Wilkins, encompasses much more than the restoration of this late 18th-century landscape park. The idea is to fuse past and present, to use the matrix of the 18th century to support a garden that will be entirely of the 21st century. Where once Middleton Hall stood on its hill, there will be a great oval glasshouse designed by the architect, Sir Norman Foster. Below it, a series of curved terraces following the contours of the hill will be connected by formal cascades splashing their way down to Llyn Uchaf, one of the string of long lakes originally created by Paxton.

The glasshouse, the descending terraces and the old walled garden (the outer section planted as an orchard with Welsh varieties of fruit) will form the central core of the botanic garden. Flowing out from this are less intensively managed areas: a habitat for threatened Welsh plants, a trials ground, a rock garden, space for a collection of trees and shrubs from the temperate regions of Asia.

"It's an ideal site," says Mr Wilkins. As an architect, he relishes the potential of the Middleton acres as a setting for some fine modern buildings, and as an artist he responds to its magic as a landscape. "It is virtually free of pollution. There is plenty of rain and no shortage of space for plant collections to expand into. And it's only a short hop from the M4."

To a landscape historian, Middleton is slightly disappointing because relatively little evidence or planting remains from Paxton's era. But this is an advantage in terms of its potential as a new botanic garden. The planting can be planned from scratch and, since ideas on conservation have moved on a long way since the great botanic gardens at Kew, Edinburgh and Oxford were founded, it will be made up entirely of trees and shrubs that have been propagated from wild populations.

Early botanic gardens were collections of curiosities, with trees and shrubs growing as isolated specimens. More useful, in conservation terms, is to conserve viable populations of plants, with all the genetic diversity of wild colonies. Recently, too, the Botanic Gardens Conservation Secretariat has been trying to co-ordinate the work of botanic gardens all over the world, so that valuable resources are not duplicated. With their help, the new "Botanic Garden for Wales" will be able to fill some gaps, making studies of ferns and mosses, which grow superbly well in this soft western air, and developing extensive collections of aquatic and waterside plants along the margins of the lakes.

The cost of this new national botanic garden is a cool pounds 35m, half of which will come from the Millennium Fund if the scheme gets through this final, nail-biting stage of the selection process. If it does, expatriates like me will also be reaching for their chequebooks. Gardd Botanic Genedlaethol ar gyfer Cymru. Cymru am byth!