GCA chief executive Iain Wylie said after a successful first four months of the year, May sales "depend where you are".
He says at his Cheshire base there has been a "bubble" of good weather throughout the month, but added: "The weather had an impact over vast swathes of the country in May, though it came good for Chelsea Flower Show."
He says the south and Midlands had "miserable" weather at times in May which impacted on early to mid-May sales.
In April, furniture and barbecue sales were up 37.46% compared to the same month in 2016 with outdoor plant sales close behind at 33.22%.
Wylie said: "We love nothing better than when the sun is shining. Lots of householders have been busy getting their gardens ready in order to enjoy the sunshine. They’ve been purchasing garden furniture and barbecues, so they can make the most of their outside spaces as possible and this has been reflected in the figures recorded by our members.
- Garden sundries sales up 27.65%.
- Hard landscaping up 20.88%.
- Clothing up 17.36%.
- Food halls and farm shops up 13.74%.
- Catering sales up 7.40%.
- Pets and aquatics sales up 6.68%,
- Seeds and bulbs up 3.27%
- Houseplants up 9.70%.
"Smaller garden centres showed a greater increase in plants as they make up a greater percentage of their sales," Wylie added.
"The results are reasonably consistent across the country, but we expect it will be a completely different story with May’s results."
Sales showed an average overall for the month of 21.86% and an average overall year to date increase of 11.07%.
Burston Nurseries managing director James Alcaraz says sales have been "very buoyant" with plants selling out until late frosts which gave "breathing space to kick off again", which all shows demand for box bedding is still strong.
BDO's High Street Sales Tracker of 85 non-food medium-sized retailers with 10,000 stores found like-for-like retail sales were down -1.3% in May.
Performance by sector:
Garden retailers says May bank holiday saw very good sales, with Alton Garden Centre director Andy Bunker saying: "The bank holiday was great. I’m sure like many the plantaria in particular will be showing a lot of records broken."
Buckingham Garden Centre's Chris Day said: "Sales were ok on bank holiday Monday and plants were nicely up, bedding especially, and alpines, which is quite encouraging. Leisure - garden furniture and barbecues were up nicely and so was aquatics but things like lawncare went back a bit because of the weather.
"Bedding is 4-5% up on last year and because of the early start to the bedding season in April, there are shortages there and we're having to look around other suppliers to keep the momentum going."