Longacres' Mike Ainley said June was "pretty much the same as everywhere else - a June to forget".
He added that "as soon as the sun comes out everything has gone ballistic, unfortunately for us it could have come earlier in the year".
Ainley said summer products such as pools and cool mats for pets were selling well and he expected July to end uip on last year.
He said Brexit was not making an impact on smaller purchases but could make one on a £5,000 sale. But Euro football and the weather had hit sales.
Alton Garden Centre director Andy Bunker said June was "disappointing" but July has seen sales improve with the weather, though July was generally a quiet month.
He said stay at home holidays were benefitting the industry: "If people are nesting, they're going to do the garden up."
"Off the back of last year I'm really happy we're two per cent up in my section."
Aylett's Adam Wigglesworth said June had disappointed too but he was optimistic for the future. He said June was down two per cent.
GCA figures show sales 1.2 per cent down in June.
Sales are around two per cent up year to date.
Restaurants made up 18.9 per cent of turnover, up 1.1 per cent greater than a year ago. Food halls and farm shops reached 6.2 per cent.
Outdoor plants sales are up but total share is down from 17.1 per cent to 16.7 per cent. Garden sundries have fallen from 15 per cent to 14.3 per cent. Clothing is 2.8 per cent of sales.