Scientists say they are racing against time to name and describe new plants, before species go extinct.
Plants and fungi hold promise as future medicines, fuels and foods, according to the Royal Botanic Gardens, Kew.
But opportunities are being lost to use this "treasure chest of incredible diversity" as species vanish due to habitat destruction and climate change.
New estimates suggest two-fifths of the world's plants are at risk of extinction.
The assessment of the State of the World's Plants and Fungi is based on research from more than 200 scientists in 42 countries.
The report was released on the day of a United Nations summit, which will press for action from world leaders to address biodiversity loss.
The report revealed that only a small proportion of existing plant species are used as foods and biofuels.
More than 7,000 edible plants hold potential for future crops, yet only a handful are used to feed a growing world population.
And some 2,500 plants exist that could provide energy for millions worldwide, while only six crops - maize, sugarcane, soybean, palm oil, rapeseed and wheat - generate the vast majority of biofuels.