By Jonathan Amos
BBC News science reporter
Britain has made good progress in trying to preserve some of its rarest wild plants, but it has largely failed to halt widespread species decline.
That is one key message to come out of the new Vascular Plant Red Data List for Great Britain, published by a coalition of botanists.
The report represents the most comprehensive assessment to date of the state of the UK's flora.
Of 1,756 plant types, about 20% are currently threatened with extinction.
The analysis laments the near-disappearance from large areas of the country of arable "weeds", such as the prickly poppy.
This population of wild flowers that once proliferated in field margins has seen perhaps the steepest decline of all plant groups in the past 40 years.
They have been pushed out by highly intensive methods of crop production that give little opportunity for competing seeds to flourish on farmland.
"We've got two arable plants now that are listed as critically endangered - corn buttercup and shepherd's needle - which have never appeared on any Red List before. They've both shown a greater than 80% decline in recent years," said Chris Cheffings, the plants adviser at the Joint Nature Conservation Committee (JNCC).
"Yes, arable plants have suffered the most, but we're also extremely concerned about upland plants - such as mossy saxifrage, the dwarf willows and northern-rock cress - which have suffered because of overgrazing.
"Some upland plants have bounced back after the reduction in sheep grazing following foot-and-mouth - but others have not, and we need to act now if we're going to have any hope of them recovering," she told the BBC News website.
Of concern, too, are those plants that prosper on unimproved grassland. Many are said to be hanging on in small fragments, such as at roadsides, and even here they are under severe pressure.
Overall, the assessment is that 30% of Britain's flora have experienced a decline since the 1960s.
"The threats facing some species have been overlooked in the past - 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," commented Trevor Dines, a conservation officer with the Plantlife charity.
"But perhaps more alarming is how so many once widespread plants, such as butterfly orchids and corn spurrey, have suffered in the modern landscape.
"It's only by understanding which species are threatened and why that we can save them."
The Red List is essentially a statistical analysis of the data collected for the "bible" of UK botany known as The New Atlas Of The British And Irish Flora. Published in 2002, the atlas updated a volume put out 40 years previously.
The intervening decades saw colossal changes in the landscape that were largely brought about by the subsidy-driven practices of the European Common Agricultural Policy.
What the new assessment does is highlight the significant trends in the data, and this information will now be used to help set the priorities for future conservation policy.
Some botanists are hopeful that many plants currently classified as threatened can make strong returns under new agri-environment initiatives, such as the Entry-Level Stewardship scheme which pays farmers to restore habitats.
Conservation groups and agencies, too, are sponsoring more plant-friendly approaches to land management, including the re-flooding of drained wetlands and the re-introduction of gentle livestock grazing.
David Pearman is a past president of the Botanical Society of the British Isles and was a joint editor on the New Atlas.
"I'm quite positive, particularly about the lowland areas," he told the BBC News website.
"Take the pygmy rush on the Lizard, the early spider orchid on the Dorset coast; and on the Dorset heath, the great sundew, an insectivorous plant that thrives on open bogs. They've all turned the corner."
Chris Cheffings added: "Obviously, we do have to accept some change, particularly with climate change. There are certain things over which we simply have no control. We have a suite of Mediterranean-type plants, for example, which have extended their range in recent years.
"But where the numbers are so down; where you get clear anthropogenic effects - I just think those kinds of changes are just too great to sit back and do nothing."
There are more than 70 plant species on the UK's Biodiversity Action Plan. The collection is currently being reviewed and a revised list supported by conservation proposals should be published towards the end of 2006.