Marshall, speaking to the gardening press for the first time since last summer, said 2010 is "probably the latest spring on record". But he added: "In our view gardeners realise things are very late but they will happen".
He hit out a recently published Gardening Which? survey that rated GCG as the poorest garden centre of 53 retailers studied. Coolings came top.
Marshall said: "It's sad. The staff worked jolly hard and our 892,000 garden club members can't be wrong. They voted with their feet."
He added that an online education system was now underway and that GCG spent £20m on revamping centres last year. "Morale has never been higher and we are excited about the future."
Marshall said the Which? survey was out of date and failed to acknowledge the name change from Wyevale to GCG, which includes Bridgemere and Blooms. The data was collected online in May and June last year.
Garden Centre Association (GCA) chief executive Gillie Westwood said: "The findings of Gardening Which? do not surprise me. The report's top garden centres are independent (with the exception of Marks & Spencer and Bridgemere) and a number of them are GCA Garden Centres of Excellence. This proves that the independent sector offers the best all-round service - something we strongly believe.
"Fantastic news for Coolings - they should be justifiably proud."
Gardening which? Customer scores
RHS Wisley Plant Centre 82%
Aylett Nurseries 79%
Chessington Garden Centre 79%
Bridgemere (GCG) 75%
Hillier Nurseries 69%
Barton Grange 64%
Van Hage 63%
Blooms (GCG) 60%
Robert Dyas 56%
Otter Nurseries 55%
Country Gardens 54%
Wyevale (GCG) 49%