There is just a "short window", with the next bank holiday crucial, to make up for lost early season sales, the HTA's council heard this week.
Regional garden centre and grower representatives say that although business has been good for several weeks, they are not wholly confident the season will extend, despite it's late start.
An HTA representative said: "Council members talked about getting more interest through events, year-round. Even if sales kick in, it's not a bit late. The window is short and compact."
Meanwhile, speaking at the RHS Chelsea Flower Show, Thompson & Morgan managing director Paul Hansord said: "Top-ups from garden centres are very good. People have been buying seed, but just later. The season will extend, no question from what we're doing with top-ups. We hoped April would be like March and May would be like April, though it won't be quite as good as that."
Hillier nursery director Kevin Hobbs said: "We've been very busy in the last few weeks, with the sunny weekends that's all it takes. We hope that June will be May."
He said he met his May budget by 20 May.
Kings Seeds MD Les Day said: "Trade is one month behind, and the garden centre pictorial packet seed trade been slow this year. It's picked up a lot in the last few weeks, but one of the problems for later sales is there's not guaranteed weather in September and October anymore."
Hayloft Plants' Derek Jarman said: "The season will exten. It has got to. It's got to run until the end of June for the sanity of quite a few garden centres and growers. The only issue is if garden centres lose confidence."
Scotsdale MD Caroline Owen said: "I hope the season will extend. Business is good when the sun shines. May's good so far and the bank holiday was was 50 per cent up. I hope the season will extend."
Bord na Mona business operations manager Steve Harper said: "Things have definitely picked up over last three weeks. The last two weeks have roared off. It's now a case of hoping the weather keeps going. Volume is now three or four times three or four weeks ago. Garden centres are ordering big style, especially the Tuesday after the bank holiday. The end of May bank holiday is now important.
Sales for the week to 19 May were five per cent down on the same week in 2012, according to figures supplied to Horticulture Week. Books were down eight per cent, clothing was up three per cent, craft was down four per cent, food was down one per cent, furniture was down three per cent, sundries was down five per cent, gifts were down 10 per cent, houseplants was up two per cent, outdoor plants was down tow per cent, pets was down three per cent and catering was down three per cent.