GCA chief executive Iain Wylie said: "It's great to see core categories, such as outdoor plants, soaring and we have the Great British weather to thank for it after staying unusually dry and warm.
"The good weather continued into July so we hope to see sales in these categories continue to rise."
Richard Cumming, managing director at Orchard Park Garden Centre in Dorset, which has recently become a GCA member, said outdoor plants, composts and pots had shown significant sales growth along with giftware and the coffee shop. "Good weather makes us a good destination and we can benefit from that in so many ways."
Russell Winteridge, general manager of Barton Grange, said: "Sales of plants were particularly strong in June and we also sold more planted containers than we have in recent years. Elsewhere we had strong sales in houseplants, home décor and giftware, and across all our food offerings in the restaurants and the Farm Shop."
Gary Carvosso, managing director at Coolings Nurseries in Kent, said: "2015 has been an excellent year for plant sales at Coolings Nurseries running second only to 2011 in which we had near perfect weather.
"Hardy plants led by the perennials had a superb month with a 14 per cent increase on last June with bedding up 23 per cent. Overall growth this year on plant sales is seven per cent.
"After a slow start to the season, plants and associated core garden product sales have held up well, which is fundamentally good news for the industry. This also makes a change to catering leading the departmental increases for some time."