Hessayon, 79, is to write what he believes will be his last book next year.
The new book by the best-selling author - 50 million copies of his books have been sold in 22 languages - entitled The Orchid Expert, will be out in early 2008. He has also written another one and has one more planned. But that will be it, he says.
He adds: "Now that retirement is approaching, I felt it was time to give my favourite flower the 'Expert' treatment in order to remove some of its mystery.
"Orchids are the most beautiful of plants but they are also the most mysterious. The present species of orchids we get is usually one of the high street seven - Cambria, Cymbidium, Dendrobium, Miltoniopsis, Oncidium, Paphiopedilum or Phalaenopsis. There are lots and lots of others, and these are described, together with names of suppliers for people who want to treat orchid-growing as a hobby."
Hessayon hardly ever speaks to the press and has never appeared on TV. He says he has become "reclusive" in recent years. Nevertheless, the Manchester-born botanist has been at recent Garden Writers' Guild awards and was at Peter Seabrook's 30th anniversary party of Sun garden-writing earlier this year.
In a rare interview he says: "I've written one, which I can't name, to be published in 2009 and have another bubbling around in my head. There will be certainly one more book and maybe two, then that will be it. I am thinking about retirement but it's not stopping me writing.
"Retirement is very easy. It's appealing and will continue to appeal. I'm 80 in a couple of months' time, though there are those two books on the stocks.
"I've been thinking about retirement for 20 years. After these other two I'll think about retirement again.
"I'm just like a smoker who thinks just one more cigarette and then I'll stop. I'll write another and then I'll stop. But Gladstone wrote a book at 90 so maybe 80 is too young."
Hessayon adds: "We've got into the official list of best sellers - until the end of October we had seven of the top eight gardening bestsellers this year so I'd hate to stop right now."
Hessayon doesn't like garden makeover shows fronted by the likes of Alan Titchmarsh and Diarmuid Gavin.
"I'm happy with the presenters who tell you what to do in their shows. But the programmes where they had £500 to create a garden in 48 hours did a bloody lot of harm.
"People were expected to make a garden in less than 48 hours 'before your wife gets back'. But in that time you could do nothing. You couldn't even get the soil ready for a lawn. So I didn't think much of that. The gameshow makeover garden in 48 hours was not very amusing."
Hessayon says his secret to success is "not worrying about the critics and reviews. I'm writing for the man in the semi-detached. I don't give opinions. I just give facts.
"People come out with books and want to be inspiring or attractive. I don't try to be any of those things. I just try to put across information. We'll never win design awards.
"The best compliment I have ever had was when someone said to me: 'With all due respect, I could have written that.' That really is good.
"More and more people want to (write about) more than that - to show off where they've been and what they like. I've sold 50 million books in 50 years. But even if you read them all, you would never know what I like. That's not my job. I'm just telling you the basic things and (other authors) choose what they do.
"You think about my gardening books and you think it's all about what I believe, but I just give the answers.
"I've never been on TV and never done the usual book signings or kissing of babies. I expect people to think I'm the reclusive man who pulls the strings. I very much like to be in the background. I come from an era when people like Percy Thrower were not too worried about their latest series."
Hessayon says he now has no goals left. "At 80 you run out of ambitions. I never had great ambitions. But I've been damn lucky. When you've been at the top of the bestseller lists for 50 years, you run out of ambitions."
1950s: BSc in Botany from the University of Leeds; later lectures on
horticulture in Ghana and Manchester
1954: PhD in soil ecology
1955: Chief scientist with Pan Britannica Industries (PBI)
1955-60: PBI technical manager
1959: Be Your Own Gardening Expert was published
1959 to date: Publishes over 50 Expert guides
1960-64: PBI technical director
1964-72: PBI managing director
1972-93: PBI chairman
1993: RHS gold Veitch memorial medal and National Book Awards Lifetime
2007: Receives an OBE