Gregory, Plants for Europe's Graham Spencer, Seiont's Neil Alcock and Bunkpflantzen's Sole Bunk will speak on propagation on 28 September at Shifnal in Shropshire, followed by a visit to Boningale and speeches from New Places Nurseries' Mike Norris and Bulrush's Neil Bragg.
Day two will hear Will Burch of Osberton and Arno Engels of de Boomkwekerijj on adding value to rhododendrons, as well as Biotecture, Mobilane and Boningale on green walls.
Hillview's Boyd Douglas Davies will speak on what garden centres want, while Claire Austin will talk about specialist plants for retail sales.
Is the shrub dead? is the subject of a panel with McKenna, Adam Dunnett of Wyevale Nurseries, Norris and Neville Stein. John Ravenscroft will give the after dinner speech.
Tech support is the theme of day three with NFU's Amy Gray, ADAS' David Talbot, Fargro's David Hide and Melcourt's Catherine Dawson.