Gardening is officially the new cookery, at least as far as television spin-off deals go. Alan Titchmarsh was yesterday catapulted into the Delia Smith league of celebrity publishing when he announced a £1m five-year book contract.

Titchmarsh, who becomes the most valuable gardening writer in the country through the deal, will write three gardening books to be published from next autumn.

BBC Worldwide, the corporation's commercial arm, which also publishes Smith's best-selling series How to Cook, signed the deal with Titchmarsh after several months of negotiations.

Around 10m people garden every day and half the country now does some gardening once a week. "Broadcasters and authors like Alan help to feed and develop this interest through accessible programming and quality books, and we see no signs of the demand slowing up," said Seamus Geoghegan, director of lifestyle publishing for BBC Worldwide.

Mr Geoghegan said the size of the deal demonstrated the scale of the BBC's commitment to Titchmarsh. "Alan is the face and voice of gardening in Britain and his ability to communicate that knowledge is unparalleled," said Mr Geoghegan.

Titchmarsh, the main presenter of Gardeners' World, has written more than 30 gardening books and two novels.