With only 15% of annual sales completed by the end of March, the GCA says sales figures for April, May and June will make or break the season.
Sidmouth Garden Centre owner Ian Barlow said: "One month doesn't make a season. We still have a long way to go and we're chasing two poor years in 2007 and 2008. It shows we're very reliant on the weather."
Meanwhile, the GCA is to ask the BBC to once again supply lists of plants that will be used on Gardeners' World, to help garden centres know what to stock before each programme is aired.
GCA representative Pat Adams said: "Producers change so often that we lose continuity. And in the past, the list has not always been particularly useful for garden centres - it went off the boil when Monty Don took over because he didn't recommend using centres. But we plan to go back and ask for a list."
March sales results for 40 garden centres across the UK
Outdoor plant sales up 15.65%
Seeds and bulbs up 20.66%
Garden sundries up 15.31%
Food hall up 15%
Clothing up 21%
March overall up 30.73%
Centres under £2m turnover up 45%, £2m-£4m up 35%, over £4m up 28%
Year-to-date up 11.29%.