Almost a fifth of Britain's plant species are struggling to survive the threats posed by agriculture, overgrazing and the use of herbicides and fertilisers, says a report published today.
The Vascular Plant Red Data List for Great Britain is the result of a two-year survey of British flora. Its analysis shows that out of 1,756 species and subspecies, 345 (19.6%) are threatened.
The report also notes that efforts to save rare species have not been matched by initiatives to safeguard more familiar plants which are now under threat. Among once-common species in serious decline are corn spurrey, purple milk-vetch, basil thyme, lesser butterfly and frog orchids and tubular water dropwort.
Also on the list is henbane, the malodorous herb with which Dr Crippen infamously dispatched his wife in 1910.
Work on the project began in January 2003 when botanists from across the UK were invited to join a working group set up to revise the list of endangered wild plants.
They were charged with drawing up a new catalogue of threatened species and classifying them as "extinct", "critically endangered", "vulnerable" or of "least concern".
The study was co-ordinated by the government's wildlife advisory body, the Joint Nature Conservation Committee, and involved groups such as Scottish National Heritage, the Biological Records Centre, the Botanical Society for the British Isles, the Countryside Council for Wales, English Nature, the Natural History Museum, the conservation charity Plantlife and the Royal Botanic Garden in Edinburgh.
In contrast to previous red lists, the new survey analysed the decline of each species instead of simply listing those that occur in a small number of sites.
It also treated all species - including hybrids - equally, regardless of whether they were native or archaeophyte (introduced before AD1500).
The survey found that some species of grassland plants are disappearing, with many clinging on only in small roadside groups.
Arable plants, too, have almost disappeared from large areas of the country while upland plants are declining because of overgrazing.
"The diversity of our countryside is being constantly degraded as habitats are fragmented and the associated plants are lost," the report says. Although reversing the decline will be "a massive challenge", it is not impossible.
"[Regeneration] ... can only happen through changing the policies for countryside management. However, with the improved understanding presented in this report, we can be optimistic that we know where to focus our efforts."
Simon Leach, a botanical adviser at English Nature, said: "We've been rather good at stopping rare plants from becoming extinct, but less good, perhaps, at stopping common plants from becoming less common.
"We are hoping that agri-environment schemes and other landscape-scale initiatives will help to arrest and reverse the decline of many of these declining species. The new red list points to those that need the most urgent action."
Trevor Dines, a conservation officer at Plantlife, said there was no time for complacency about more common species. "It's horrifying that there are only 11 plants of western juniper left in Britain, for example, yet it has never been listed as a threatened species before."
Among plants not before classified as at risk but now threatened are:
Ranunculus arvensis - corn buttercup (critically endangered). A very attractive annual buttercup with small yellow flowers, it came to Britain with Roman farmers.
Papaver argemone - prickly poppy (vulnerable). A small, brightly flowered poppy which has not yet made the jump to waysides and road verges as the common poppy has.
Silene noctiflora - night-flowering catchfly (vulnerable). Its creamy-white flowers are semi-closed during the day, but at night reopen fully and emit a strong scent to attract insects.
Astragalus danicus - purple milk-vetch (endangered). Found along the east coast from Scotland to Lincoln and East Anglia.
Euphrasia anglica - eyebright (endangered). Its tiny white flowers are blotched with yellow and purple like a bruised eye. Compresses and tinctures from it were used to treat many eye disorders.
Monotropa hypopitys - yellow bird's-nest (endangered). An odd-looking perennial herb that lives on dead and decaying plant material in the soil.
Polystichum lonchitis - holly fern (vulnerable). An evergreen alpine fern that is small but long-lived. Small populations in Wales and the Lake District; more frequent in the Scottish uplands and western Scotland.
Cuscuta epithymum - dodder (vulnerable). An annual, rootless, twining and scrambling herb which is a parasite, attaching itself to its host by small suckers that remove water and nutrients. Gentianella campestris - field gentian (vulnerable). A biennial or annual herb with attractive, purply-blue flowers. It is locally common in northern England and Scotland, but absent from most of south and central Britain.