We've all experienced it. You visit a garden centre, find a plant that catches your eye, take it home full of optimism, give it a good watering, perhaps even add some compost, and for a few weeks everything seems promising. Then it begins to lose its colour. The leaves start to yellow, growth slows, and before long you're wondering what you've done wrong. Most gardeners have had that experience at some point, and it's easy to assume the fault lies with us. In reality, that's