Of the 390,900 plant species on Earth, 21 percent of them are at risk of extinction, according to a report published Tuesday by Britain's Royal Botanic Gardens, Kew.
The report listed the biggest threat to plant species as destruction of habitats for farming, with deforestation for timber, and construction of buildings and infrastructure coming in second and third.
Meanwhile, climate change is expected to grow as a bigger factor in the near future. The growth of coffee beans, for example, has already been affected by rising temperatures.
Among the tallied plants, some 2,034 new species were discovered in 2015.
This is just scratching the surface, and there are "thousands out there that we don't know about," said director of science at Kew, Kathy Willis, who led the study.
Invasive species, numbering 4,979, are also closely monitored by the researchers. These species cause damage to the environment and are usually hard and costly to get rid of. The global cost of invasive species is estimated at nearly five percent of the world's economy, according to the report. Pests and diseases were listed as major problems as well.
Now, with the global challenges of population growth, land-use change, plant diseases and pests, there is "an increasing urgency to find and conserve crop wild relatives," said the report.
"Having access to this large and diverse genetic pool is essential if we are to furnish crops with the valuable traits that enable resilience to climate change, pests and diseases, and ultimately underpin global food security," it also said.
The plant number estimated by researchers at Kew does not include algae, mosses, liverworts and hornworts.