Gardeners in London are being asked to make seven pledges to help reduce the impacts of climate change.
The London Wildlife Trust said there are 3,267,174 gardens in London, which cover about 93,757 acres (379 sq km).
The environmental organisation said gardens can absorb carbon, soak up flood water and can cool cities.
The seven pledges in its "Garden for a Living London" campaign include planting drought resistant plants and adding a green roof to your shed.
"The fact there are over three million gardens in Greater London shows just how important a resource they are," a London wildlife Trust spokesman said.
"In urban areas especially, gardens will help reduce the impacts of climate change.
"They provide shade, absorb carbon, soak up flood water, retain water and help to cool the city.
"They also provide vital habitat for wildlife, and a strong network of wildlife and climate friendly gardens stretching across the capital will provide room for species to move freely and adapt to our changing climate."
The seven pledges are:
• Plant drought resistant plants
• Plant a mixed hedgerow
• Plant a broad leaved tree
• Make a pond
• Use mulch
• Add a green roof to your shed
• Wild up your decking