By Richard Black
Environment correspondent, BBC News website
The Royal Botanic Gardens at Kew has issued a "position paper" saying that man-made global warming is changing the outlook for plants and trees worldwide.
Kew says climate change threatens plants all over the world
It says four species on its own lands are flowering earlier each year.
Kew's conservation scientists say other human activities such as the expansion of cities are also hurting plants, some which poorer societies depend on.
It advises gardeners in Britain to adopt water-conserving techniques such as mulching and composting.
"We've made this statement I think probably because it is very noticeable in the southeast of England," said Tony Kirkham, head of the arboretum at Kew.
"We're seeing many tree collections and many gardens being affected by a third year of drought, and it's likely to get worse," he told the BBC News website.
Winter rains could make up for the dry summers, at least for trees, he said, but recent winters have been dry as well.
"Many of the trees are simply tired."
Many Kew-based scientists work in other areas of the world, particularly in developing countries, documenting and conserving plants and trees.
One such project is the Millennium Seed Bank, which aims to collect seed from threatened species throughout the world.
And it is in the developing world that plants and trees are most under threat, according to Seed Bank head Paul Smith.
"In February 2001, we were working in the Northern Cape, South Africa, and we came across one species which is down to just six individuals - and that's the only place in the world that species (Cylindrophyllum hallii) lives," he said.
"We are seeing extinctions at about 1,000 times the natural rate. What's behind them isn't always clear - it can be climate, overgrazing, overpopulation - but the fact is they are happening, and the reasons are likely to be anthropogenic."
Another anthropogenic factor noted by Kew is deforestation. While every effort should be made to slow deforestation, it says, greenhouse gas emissions should be reduced too, with governments, businesses and individuals taking responsibility.
"We support the majority view that there is cause for concern and action is needed," reads its statement.
"All of us, governments and individuals need to cut carbon emissions in order to limit warming to a 2C increase or less."
There is evidence from observation and computer modelling that a rise of 2C is almost inevitable.
Recently, Kew has seen an increase in comments and requests on climate issues from the public, with people documenting changes they are seeing and asking for advice on managing their gardens.
Its advice to British gardeners is to
- water carefully, in the right places and at each end of the day
- retain moisture as much as practical using mulches and organic material
- minimise water loss from plants with shade and windbreaks
- group potted plants together to improve local humidity
And Kew's own practices are changing. "We are thinking more about tree selection, asking whether the trees we plant are suitable for a dry warm climate and a long summer," said Tony Kirkham.
It is publishing evidence that oak, rowan, box and cow parsley are all flowering between nine and 15 days earlier in the year than they were a few decades ago.
Kew has until now refrained from issuing a position statement on climate change; now that it has, will the gardeners of Britain take more notice of climate issues?
"Certainly it should bring home to people the reality of it, that it's something that is happening," said Dr Smith.
"Of course, David Attenborough coming out several months ago might also do that. But I see our role not in the mitigation arena, but in adaptation - how are we going to adapt to climate change?"
"And that relates to back gardens as much as to conservationists working with global biodiversity."