His proposal followed the launch of the RHS Greening Grey Britain report at Chelsea. It warns that plant-free gardens have tripled to 4.6 million in 10 years, risking the health of people and wildlife through pollution, heatwaves and flooding.
More than five million front gardens are now devoid of plants, an RHS-commissioned Mori poll shows. The RHS is sharing a range of ideas for greening up at www.rhs.org.uk/ggb.
But Alister Scott, professor of environment and spatial planning at Birmingham City University, said: "Most non-permeable driveways require planning permission, whether new or non-replacement. If people are encouraged to keep their gardens green they can deliver environmental benefits and flood management services to the local authority that could be valued, leading to a pro-rata reduction in council tax.
"However, if people have decking and driveways they are placing an increased burden on drainage systems and this should lead to an increase in water charges, acting as an incentive to use green space productively."