With spring in the air, a good gardener's thoughts turn to mulching and potting on. Sacks of peat compost will be bought and used. But the mining of peat bogs is both unsustainable and ecologically destructive. So what can a gardener do? The answer is to use green waste-derived compost. Quentin Cooper finds out how to grow a greener garden. Producer Jonathan Rides E-MAIL: material.world@bbc.co.uk