 The Royal Horticultural Society (RHS) throws open the gates of the Chelsea Flower Show today with a message for Britain's gardeners: the humble urban backyard can help save the planet.  It's something that keen horticulturalists have always suspected, but for the first time, the RHS has brought together all the published evidence in a scientific review entitled Gardening Matters: Urban Gardens.  Collating the evidence reveals four key areas in which gardens make a difference. They help control urban temperatures, mitigating the effects of extreme heat and cold. They help prevent flooding by absorbing rainwater that would otherwise overload drainage systems. They support human health by easing stress and providing physical exercise. and they have effectively become some of Britain's nature reserves, supporting a range of wildlife including birds, mammals and invertebrates.  There is growing evidence that some declining species, once common in low-intensity farmland, are now more abundant in urban areas. These include the song thrush, the common frog and hedgehogs. One study found that in Sheffield's domestic gardens, the density of birds was six times that of the nation as a whole.  Dr Tijana Blanusa, who led the RHS  