At the end of April sales were running at 18.88 per cent ahead of the same period in 2013.
Outdoor furniture and barbecues were up 23 per cent but outdoor plant sales fell four per cent during the month.
Alton Garden Centre director Andy Bunker said: "May was disappointing, however speaking to a few people last week at the GIMA golf day last week, all had very similar thinsg to say, trying to keep what we have made per centage-wise in the first four months - but it’s going to be tough.
"I was plus 60 per cent up to the end of April and I'm now plus 23 per cent to end of May, and June will be tough after the month last year was very good. All-in-all I can see it dropping back at this stage to my target as is every year of plus 10 per cent."
The British Retail Consortium meanwhile says flatter food sales led to May UK retail sales slowing.
UK retail sales were up 0.5 per cent on a like-for-like basis from May 2013, when they had increased 1.8 per cent on the preceding year. On a total basis, sales were up 2.0 per cent, against a 3.4 per cent rise in May 2013.
For more details see Horticulture Week magazine 13 June.