More than two thirds of the world's plants could be protected by focusing on just 17 per cent of the Earth's land surface.
Researchers used a computer program to identify the smallest set of regions that could contain the largest numbers of plant species.
Scientists say that challenging "tactical decisions" had to be made to secure the most critical land within each region for conservation.
"To achieve these goals, we need to protect more land, on average, than we currently do and much more in key places such as Madagascar, New Guinea and Ecuador," said research leader Professor Stuart Pimm, from Duke University in the US.
Plant species are not haphazardly distributed around the planet, according to the findings in Science journal.
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Areas including Central America, the Caribbean, the Northern Andes and parts of Africa and Asia have much higher concentrations of vulnerable endemic species found nowhere else.
"Species endemic to small geographical ranges are at a much higher risk of being threatened or endangered than those with large ranges," said Dr Lucas Joppa, a conservation scientist based at Microsoft Research's Computational Science Laboratory in Cambridge, UK.
"We combined regions to maximise the numbers of species in the minimal area. With that information, we can more accurately evaluate each region's relative importance for conservation, and assess international priorities accordingly."
The scientists analysed data on more than 100,000 different species of flowering plants stored at the Royal Botanic Gardens at Kew in west London.
"We also mapped where the greatest numbers of small-ranged birds, mammals and amphibians occur and found that they are broadly in the same places we show to be priorities for plants," said US co-author Dr Clinton Jenkins, from North Carolina State University.
"So preserving these lands for plants will benefit many animals, too."
Two of the most ambitious aims of the 2010 Convention on Biological Diversity are to protect at least 17% of the Earth's land and 60% of its plants.
