Overall sales were up seven per cent on May 2014, with catering above the average and sundries and seeds and bulbs slightly below.
GCA chief executive Iain Wylie said a north-south divide narrowed as the month went on, with the south east and east doing best in the end, and sales increases only 3-4 per cent different around the country.
Year-to-date figures show a 5-7 per cent rise in sales.
Orchard Park Garden Centre said: "May has proven to be an interesting month; we would generally attach April and May together as all too often we see the gain of one lost against the loss of the other! May 2015 has shown overall turnover up slightly against 2014. Particular areas of note: Dry goods were up one per cent. Nursery sales were up 2.7 per cent overall but we saw a drop of 11 per cent on shrubs and an increase on herbaceous of 15 per cent, bedding sales were up by two per cent."None of the above figures really come as much of a surprise to us as we had a very good April with the following notable figures. Dry goods were up 12 per cent, nursery sales were up 19 per cent overall with shrub sales up 22 per cent; herbaceous sales up 10 per cent and bedding sales up 28 per cent.
"Although May was affected by the weather particularly the wind in this part of the world it has been reassuring that gains we saw made in April were not completely eroded away by a bad May."
Old Railway Line Garden centre's Katie Cleary said: "It's been a really good month and most departments are up." Sales were up 16 per cent on last year overall and it was "pretty good" last year. Plants have been "amazing" - they are normally quite steady but were up 15 per cent.
British Retail Consortium figures showed footfall in May was 1.0 per cent lower than a year ago, down from the 0.8 per cent fall in April. This was below the three-month average of -0.5.
Both High Streets and Shopping Centres reported a decline, falling 1.5 per cent and 2.0 per cent respectively. Footfall in out-of-town locations fared the best with a 1.4 per cent increase year-on-year.
Meanwhile, garden centres are receiving their GCA annual inspections.
GCA chief executive Iain Wylie said: "The inspectors are reporting back so far that they are delighted with what they are seeing and that standards are especially good this year, particularly at the top end.