People around the world need to embrace a "new deal" to recognise the economic, health and environmental benefits of plants, according to the outgoing director of Kew Gardens in London. A fifth of the planet's plant species are in danger of extinction within the next century, said Prof Stephen Hopper in an exclusive interview with the Guardian, and governments around the world needed to make firm plans to reverse the decline.

"We're at a crossroads in many ways, we now have half the remaining wild vegetation on the planet left that was around 200-300 years ago," said Hopper, who will leave his position as chief executive and chief scientist at Kew Gardens in a few weeks, after six years at the top. "It's really important for us to decide, as a global community, do we want to care for what remains and get into the business of repairing and restoring diversity? Or continue on the path of ever more incursions into wild places in the hope that human beings will be able to exploit resources and continue with a reasonable quality of life?"

Hopper said there were reasons to be optimistic, because people and institutions around the world had already demonstrated that conservation and restoration programmes could work in turning the tide against ecological degredation. All that was needed was to ramp up the speed and scale of the efforts, he said.

"All the indicators suggest that a new deal of some sort needs to occur with biodiversity and plant life in particular," said Hopper.

Plants form the bulk of our everyday experience and they are the things we most often manipulate for human benefit, said Hopper. "We can grow them, use them for food, medicine, shelter, culture, clothing – you name it. [Conservation] is something that has to grasp the hearts and minds of a significant majority of people in most nations. It's not a nice-to-do, it's a must-do."

There are an estimated 400,000 species of plants on the Earth, compared to around 10,000 species of birds and around about 7,000 species of mammals. To get a handle on the conservation status of the world's flora, scientists at Kew worked with colleagues at the IUCN and the Natural History Museum in London to assemble information on a sample of known species. Assessing all of the world's plant species would have been impossible but, by selecting key groups and several thousand species, the scientists got a robust handle on the state of plant diversity.

"One-third of the plant species we looked at, we didn't have enough data," said Hopper. "Of the two-thirds for which we did have data, one-fifth of them were threatened in some way. We finally have reasonable statistics at a global level to understand the scale of the problem and it's formidable. Some 80,000 species on the planet at some risk of going extinct over the next 50-100 years."

The pressures on plants come from a combination of human incursion into wild habitats, through agriculture and urbanisation, and also the direct effects of man-made climate change. The latter not only affects water availability but also plant life cycles. "If you're a plant that produces annually, the timing of when you germinate can vary – you might not germinate at the right time and grow fast enough to be around when your pollinator's emerging if it's an insect, and you get into a reproductive spiral downwards," said Hopper.

The physical changes brought by climate change were already showing how the environment was being pushed hard, he said.

Agricultural plants themselves have also become worryingly limited. Hopper cited a 2000 study by scientists in the department of nutrition at the University of California, which found that, in recent centuries, humans have focused efforts on relatively few plant species with the result that 80% of total dietary energy intake, globally, is obtained from twelve domesticated species: eight cereals (barley, maize, millet, rice, rye, sorghum, sugar cane and wheat) and four tubers (cassava, potato, sweet potato and yam).

There are ways to head off some of the problems. Projects such as Kew's Millenium Seed Bank collect seeds from tens of thousands of species of plants and store them in cold, low humidity conditions. As such, these banks can serve as an ecological backup in case things go wrong in nature.

There are also more direct moves afoot to restore damaged environments. In May, Kew Gardens partnered with botanic gardens in Chicago, Kings Park, New York and Missouri to restore degraded habitats. Over the next 20 years, the alliance will work with 100 places that have been damaged or destroyed across six continents.

Bruce Pavlik, head of restoration ecology at Kew Gardens said that healing the wounds that humans have inflicted on the planet will require a global, scientific effort. "Botanic gardens have already begun the healing process, building native forests and rich grasslands on degraded earth, but their efforts are small in scale and woefully underfunded," he said. "Forming a worldwide alliance [...] will spread the effort across six continents, training a new generation of practitioners, improving knowledge and techniques and producing restored ecosystems with high value to biodiversity and to human economies."

In 2009, South Korea announced re-afforestation and river care plans, with the idea of creating a million jobs within five years. "Some governments are starting to bite the bullet and starting, at scale, to make a difference," said Hopper.

Maintaining plant diversity would also be important for future ecological health. "People might say we don't need 400,000 species, we can just have a few plants and they'll provide the oxygen. There might some sense in that if the world was unchanging but the power of diversity - like cultural or intellectual diversity in humans - is all about a risk strategy in changing circumstances," said Hopper. "You simply reduce your options if you say you'll deplete the diversity down to the lowest possible level."

• Listen to the interview with Hopper in this week's Science podcast