The unique conservation achievement will be part of the development of the pounds 80m Millennium Seed Bank of the Royal Botanic Gardens, Kew.
A group of 200 enthusiastic botanists, professional and amateur, have spent the past two years striding up mountains, abseiling down cliffs, wading through streams and crouching in ditches seeking seeds from the 1,442 wild plants regarded as properly native to Britain.
They have brought the number collected to 1,190 so far, and Steve Alton, co-ordinator of the project, is "quietly confident" that the 252 remaining will be gathered this summer.
Plant species already in the bag range from Britain's commonest - either the stinging nettle, or annual meadow grass, that grows up through cracks in the pavement - to the rarest, the lady's slipper orchid, fabled for its beauty and now down to one carefully guarded clump left in the wild.
Still waiting to be collected are some of the more obscure denizens of the countryside, from few-flowered fumitory and glaucous glasswort, to tasteless water-pepper, confused fescue and nettle-leaved goosefoot.
The Millennium Seed Bank is being built at Kew's outstation at Wakehurst Place, West Sussex, with an initial grant of pounds 30m of national lottery funding, and other large donations from the Wellcome Trust and the Orange mobile phone network. It will open next year.
The most ambitious facility of its kind in the world, it will collect the seeds of the world's wild plants, of which perhaps a quarter will face extinction in 50 years' time. It aims to have seeds from 10 per cent of the world's 240,000 plant species by 2010, and 25 per cent by 2025.
All will be carefully dried and stored at 20 degrees below zero, with many capable of germination, Kew's scientists believe, hundreds of years into the future. The underground storage vault is designed to last at least 500 years.
But the seed bank's first project is to collect and store the seeds of the entire British national flora - something no country has done before.
The scheme, begun in 1997, had a head start as seeds from nearly 600 British species were already represented in the present seed bank at Wakehurst Place. The remaining 800-plus have been the targets of a small army of 200 dedicated wild flower enthusiasts, more than half of them volunteers. They are mainly members of county wildlife trusts and the Botanical Society of the British Isles, not a few of them pensioners, and they have visited every type of landscape.
Some plants, however, are beyond even the most determined pensioner's reach: seeds of the rock whitebeam, one of Britain's rarest trees, were collected by a member of the Mourne Mountain Rescue Team, abseiling down a cliff face in Co Londonderry.
Mr Alton, 33, the former conservation officer with the Nottinghamshire Wildlife Trust who co-ordinates the seed bank's UK Flora Programme, eggs on his volunteers with newsletters. But he points out that collecting seeds is sometimes much harder than gathering wild flowers. "After the flowers die, some plants tend to disappear in the surrounding foliage. You often have to grapple around on your hands and knees."
Although the collection of the seeds of the entire British flora probably means that no existing wild plant will now become extinct in the UK, Mr Alton is keen that people do not see it as as a solution to plant conservation problems. "There's no substitute for managing nature reserves. We're here as a backup, in case of disaster."
The Very Rare
Lady's slipper orchid
Cypripedium calceolus is probably Britain's rarest plant, down to a single clump in the wild. Kew's orchid unit has already managed to propagate plants from its seed
The Very Common
Urtica dioica is probably Britain's most common plant, found virtually everywhere from Land's End to John o'Groats, although annual meadow grass may run it close.Reuse content