Compared to the same time in 2014 sales in catering departments were up 14.46 per cent.
During the month sales of houseplants were up 13.55 per cent and furniture and barbecues did well at 10.3 per cent up.
Garden centres also reported a rise of 9.54 per cent in pets and aquatics departments.
Food hall/farm shop sales were up 8.18 per cent. Clothing sales were 9.08 per cent up compared to the same time last year.
Overall sales are about seven per cent up year to date.
Iain Wylie, GCA chief executive, said: "It's great news that catering sales were on the up during May.
Garden centres have so much more to offer than just plants these days and people often make a family day out of visiting their local centre as there's something for everyone.
"They can browse the plants, pick up gifts and clothes, let the kids play in the play area and have a hearty lunch or dinner."
Simon Bourne, garden centre manager at Perrywood Garden Centre and Nurseries in Essex, said: "May was a really great month for us and was in fact our best ever month in the garden centre and in the coffee shop. Sales were up 18 per cent for the garden centre on last year and up 24 per cent in the coffee shop. Plant sales in May were fantastic and were up 21 per cent on last year.
"The higher than average rain fall and lower temperatures has actually helped this time around. The rain has fallen in short sharp spells that have kept plants looking fresh but dry spells in between have allowed gardeners to get out and do things.
"The lower temperatures have meant that people haven't headed to the coast/park/pub beer garden and although it has been colder we have had quite a bit of sunshine which has been perfect for spending a day doing work in the garden.
"The Chelsea week always gives us a lift and this year was very strong and the week after that was half term which again was exceptionally busy. There seems to be confidence from customers to spend more this season with larger plants as well as leisure lines selling well."
Katie Eckley, manager at The Old Railway Line Garden Centre in Wales, said: "May was very good for us, the weather was bright enough to keep bringing people out, although those few late chilly evenings were a bit worrying.
"Overall we were 16 per cent up on last year. The restaurant is still seeing growth at 10% up for May. It had a very busy half term with our 'Wild Wednesday' event running, bringing in over 100 children and the famous Hay Festival runs for 10 days bringing in lots of tourists, keeping the place full of families.
"Outdoor living is doing extremely well for us, it was over 80 per cent up for May compared to last year. We just hope we can keep it up as suppliers stock levels are getting low now. Outdoor plants had a cracking month at 15 per cent up, we are very happy with this level of growth for a department that we haven't made any bigger, just worked it harder and turned stock over quickly.
"Houseplants are doing well for the year so far but May was about on par with last year."
Monkton Elm Garden & Pet Centre's new Elm Tree restaurant saw positive growth in its first full month of trading since opening in April.
Managing director Mike Lind said: "Catering sales for May were up 48.6 per cent on 2014, with good performances in both hot and cold food sales from the new deli island and open-kitchen carvery. Hot drinks sales performed exceptionally well with our new coffee lounge sales confirming that if you offer good quality tea and coffee customers will visit."