Book excerpt: A greenhouse can be a garden's greatest asset: its nucleus and powerhouse. There are many ways of using it and a gardener can adopt one or all of them. In spring, you can start young plants from seed; raise tender perennials to fill gaps in the garden; give young vegetable plants a head start; sow early lettuce while the soil is too sticky to work outdoors. During spring and summer, you can raise greenhouse crops like tomatoes, cucumbers, aubergines and chillis. You'll be able to overwinter tender plants such as bananas and cannas. And the greenhouse can be a showcase full of beautiful, unusual and exotic plants. Trained at Kew and for many years Glasshouse Supervisor at the RHS Garden at Wisley, Anne Swithinbank is the expert on greenhouse gardening, and in this book she comprehensively describes the techniques that will allow you to greatly expand the growing capabilities of your garden.