"It's absolute nonsense," he insists. "She's sold nearly 100 million. I have tremendous admiration for her. She knows her market and she won't be insulted into changing her style."
He could easily be talking about himself. Like the queen of breathless prose, he has resisted pressure to alter his winning formula since he published the slender, 36-page Be Your Own Gardening Expert at one shilling and sixpence in 1959. He is Britain's best-selling non-fiction author and the most widely read gardening writer in the world, yet few can put a face to his name.
The 16th title in the Expert series, The Bulb Expert, is published on Thursday. The initial print run of 150,000 has already been snapped up by retailers and it is being reprinted.
Few gardeners do not have at least one of Dr Hessayon's works. Crammed with well-organised information, they now run to 128 pages, but boast the same designer-resistant layout as the early volumes, packed with charts and line drawings. Today's price, pounds 4.99, still represents good value.
"My books are to help people, not to inspire them with great pictures," he says. "I look on them as first-aid manuals. I don't think gardening is a religion or a holy experience. It's an enjoyable hobby, making something around the house you can be proud of."
The books' covers are an accurate pointer to the down-to-earth, anti- fashionable philosophy that runs through them. The latest is defiantly garish, dominated by a large painting of a scarlet and yellow mottled lily.
The first book's cover showed a suburban gardener in a maroon pullover puffing a pipe. Now called The Garden Expert, thatwork has run to 20 editions and sold 5 million copies. Its current cover portrays a Middle England paradise, with tiger-striped lawn, beds of colourful annuals, kidney-shaped pond, dinky greenhouse and a tall conifer planted a little too near the garage.
Naff? Dr Hessayon does not use the word or accept the concept: "There's nothing wrong with liking red salvia and white alyssum. To call that common is ridiculous. If that's what you want, do it."
He refuses to impose his own horticultural taste: "You don't have to tell readers whether you like it. One of the proudest claims I make about my books is that I don't think anyone would ever know what I like. I give them the full range, tell them where it will grow, when it will flower and what colour it is, and let them choose."
He has the same no-nonsense approach to the design and style of the books: "They aren't meant to impress. The photographs aren't meant to be the greatest example of that flower, but to show the kind of result an ordinary gardener might expect. And the words aren't deathless prose. I began as a journalist and journalism is writing 300 words on x, not sitting waiting for the muse. It's bricklaying with words.
"I hate it when a designer comes to me and says: 'You probably don't know this but you should have more white spaces, fewer typefaces, stop using drawings and use more photos'. It offends me because I know all of that and I choose to ignore it.
"With a reference book, people want it absolutely crammed. These aren't coffee-table books."
Dr Hessayon (the name is Armenian in origin) was born in Manchester in 1928 and read botany at Leeds University. In 1950 he went to work for a newspaper in Missouri, where he married the proprietor's daughter Joan - today a successful writer of historical romances.
Returning to England as a lecturer in horticulture, he switched careers in 1955 and became a chemist with Pan Britannica Industries (PBI), which makes garden chemicals. Struck by the lack of accessible literature about gardening, he proposed that the company should publish a basic manual. Unconvinced, the firm made him risk some of his own money before they let him go ahead.
He based his approach on cigarette cards, which he collected as a child. "You had a picture of small size, limited in area, and on the back was a limited number of words. Whatever you had to say had to fill that space. So you had, say, 68 words to describe Napoleon or the Battle of Waterloo - that's very tight."
The book was an instant success. Soon after publication it was noticed by a reviewer in the Times who said gardeners should not be without it. "Wagon-loads of mail arrived the next week and the whole company came to a grinding halt. It sold out and the next book, on house plants, did even better."
Dr Hessayon's business career was also flourishing. He became managing director of PBI in 1964 and chairman from 1972 to 1993, when the company was sold to a Japanese conglomerate. The books are now published under their own "Expert'' imprint by Transworld.
With more time to spend on the books, he produces two a year. He also has time to put his advice into practice by building his ideal garden on 26 acres near the border of Essex and Suffolk, where he and Joan moved this year. He has scarcely begun work on it yet and, like most gardens this summer, it is brown and parched. However, he has made his plans to divide the area into specialised sections. "It's like a beautiful naked woman you have to clothe," he enthuses.
But when this paradise is complete he does not plan to open it to the public. He values his privacy too much. "I don't want to be a public figure,'' he explains. That is why you will not see or hear David Hessayon plugging his book on television gardening programmes or radio talk shows, or signing copies in bookshops.
Occasionally, though, his distinctive name gives him away. Buying a bed in a local shop, he handed over his Barclaycard. The salesman studied it for a while, then said: "I don't want to be rude, but, with the royalties you get, surely you can afford a better bed than this?"Reuse content