Hessayon, 86, retired last year, promising that the Experts would live on.
Seabrook said: "When you get into the Expert books there's a lot of detail and you interfere with them at your risk. The Doc was very good indeed and his books are very reliable, but horticulture life is moving so fast - pest and disease control especially. You have to watch the chemicals being recommended like a hawk. That series of books are about the only ones that do offer advice on man-made plant-protection materials."
Transworld Publishers director Susanna Wadeson said: "Hessayon has retired but the Expert series is on-going and sales are up on last year. The books are reprinted regularly and Expert Publications has a planned programme of revised and updated editions. The New Fruit Expert will be next, scheduled for next spring. We will also be bringing out new titles in due course."
Gardening book sales were worth £5.6m in 2013, down five per cent on 2012 and 62 per cent on 2003, when sales topped £14.5m. Volume (-18 per cent) and value (-16 per cent) sales dropped in the first quarter of 2014.
Hessayon is the genre's best-selling author, with 28 of his titles selling almost 10,000 copies in all, for a sales value of £73,000, according to TCM Top 5,000 data. The 2014 edition of New Vegetable & Herb Expert, the first overhaul since 1997, has sold 2,500 copies since March, up from the 1,700 copies the old edition sold in the first quarter of 2013. Only seven gardening books have sold more than 1,000 copies in 2014 so far.