After some garden centres reported early May as a quarter down on early May 2016, when the season had a short peak, he said: "It will be virtually impossible to get May back realistically to anywhere near 2016.
"In the plantarea it will be 20-25% down, however with a good Chelsea week and bank holiday hopefully will eat away at that figure, which should see the year still around 10-15% up.
He added: "I do have a good feeling about June this year so all in all quite happy. This figure is mirrored throughout the whole centre."
However, in the north of Britain garden centre sales have been better, with centres reporting sales up on May 2016.
The Met Office said: "For May to July, warmer-than-average conditions are more probable than below-average.
"The probability UK average temperatures will fall into the warmest of our five categories is 35 per cent. The probability of the coldest categories is 10 per cent. There is an enhanced likelihood of more frequent sunny spells and winds from a continental flow in May."
* Weekend newspaper reports suggest Wyevale Garden Centres came close to breaching its banking covenants at the start of 2017. A lender reportedly refused to waive its terms, so owner Terra Firma had to spend around £25m buying out the bank’s debt. Justin King, formerly of Sainsbury, has been chairman of Wyevale since August 2016, while Roger Mclaughlan from Toys’R’Us has been chief executive since March 2016. Wyevale has not commented on the reports.