"Making the outdoors comfortable is very important for society if we are not all to retreat into our own space," he said, pointing out that green walls offer advantages to passers-by as well as building occupiers.
"Solar canyons" reflect heat from curtain walling, meaning that even shaded areas can be uncomfortably warm, he said. "And you feel the urban heat-island effect mostly at night, as heat is radiated by buildings, rather than by day."
Plants are good at getting rid of heat because they have a lot of surface area on different levels so will also absorb dust, Watts said.
He added: "Shading is one thing plants can do that would otherwise be done by hard materials. But when it's damp it feels warmer than when it's dry so it's better to have your evaporation up high."
Architects do not always want their buildings draped in plants, he added. "But something like Wisteria will provide adaptable shading in the summer but not in winter, while a green roof might let you do without air conditioning."
He added that moss has virtues that are often overlooked. "It is small, controllable, doesn't mind drying out and will grow on vertical surfaces," he said. "And as it's spongy, it will also absorb noise."
However, he cautioned: "Most buildings and plants don't get on that well - you have to create barriers. Anti-foulant paint on roofs will kill plants. And some biodiversity isn't very nice. Mosquitoes and rats might like the swales in your sustainable urban drainage systems."