Organiser Aztec Events has booked Don to open the three-day flower show on June 20.
The show, in its second year, will feature 150 floral and gardens exhibitors.
On June 21 Don's co-presenter Joe Swift will be joined by ITV’s Love Your Garden presenter Frances Tophill in the Experts Theatre.
Organisers expect visitor numbers to reach 40,000 this year.
Ground Force television duo Charlie Dimmock and Tommy Walsh will appear on June 22.
Meanwhile, Gardeners' World has returned to BBC2 at 8.30pm on Fridays for a 31-episode series. Programme two on 14 March will feature Don on box blight and Carol Klein on cyclamen.