Titchmarsh has a contract with B&Q and the TV gardener's BBC-branded books, which have his face on the cover and come in a similar format as Hessayon's 50-year-old series, have appeared in B&Q advertising and stores this spring.
Expert Books co-ordinator Gareth Pottle said: "Every year people try to copy the Doc but they don't get it right because Experts are done by the Doc and his PA. This is a clever piece of marketing by Titchmarsh and B&Q has got a lot of money to spend so it got a TV celebrity."
Hessayon's books have sold more than 500,000 in B&Qs in the past decade and sales rose 16 per cent - around 10,000 - to more than 50,000 in 2009 alone. Hessayon is the world's best-selling garden writer, having sold 50 million books over a 50-year career.
Figures from Neilsen Bookscan show Titchmarsh's six How to Garden books sold 40,000 in all in 2009, the year they were released. Hessayon's 22 titles sold 800 per cent more - 640,000 copies.
Pottle added: "Before the Alan Titchmarsh/B&Q link up the Titchmarsh How to Garden books weren't for sale at B&Q. After the link-up they are scheduled to be the only gardening books available in store. Despite Expert sales increasing at B&Q by 16 per cent in 2009, it has decided to replace the range with the Titchmarsh books."
Pottle said B&Q had sold Hessayon books for at least 10 years: "I could understand it getting rid of us if we were not selling hundreds of thousands. Over 10 years we've sold more than 500,000 and we were up 16 per cent last year, which is thousands."
B&Q is one the three biggest gardening outlets for Hessayon's books, alongside Homebase and independent garden centres. Titchmarsh's books retail at £6.99 while Hessayon's are £7.99.
Pottle added: "The Doc is 84 now but still doing well. He has retired three times that I know of. But a Veg and Herb follow up is due out in 2011 and there are plans for five more books."
Titchmarsh has written a new series of six books to add to last year's six How to Garden guides, on greenhouse gardening, flowering shrubs, growing fruit, climbers, wall shrubs and perennial garden plants (BBC Books, £6.99). B&Q declined to comment.