Paving over gardens with impermeable surfaces has and will continue to undermine urban resilience, says Prof Alistair Griffiths, the director of science and collections at the Royal Horticultural Society (RHS). Water can't get through concrete, asphalt and paving, which contributes to surface flooding and overwhelms the sewer system, leading to pollution runoff. Loss of vegetation also contributes to global heating. We've got these increased extremes of heat and if you lose green space, you lose that cooling effect, he adds.