They include David Austin Roses, Lowaters, Hillier, Seiont, Guernsey Clematis, Whetman Pinks, Wyevale Nurseries, Merediths, Nurseryfresh Plants and Mahood. A further 27 companies have registered their interest.
Organiser and HTA marketing director Andrew Maxted said: "We're considering opening additional space around the halls.
"One of the most encouraging things about the plant show is it is not just hardy nursery stock but also bedding and all other sectors.
Maxted said Garden Expo reducing to two days from three was a good idea and came on the advice of HTA members after a survey. He said Glee (20-22 September, Birmingham NEC) should consider cutting to two days as well.
The Garden Expo show, which is running alongside NPS in two halls to NPS's one, has 43 sign-ups. The HTA has a marketing agreement to promote Garden Expo.