Garden Industry Manufacturers Association director Neil Gow said: "We need the second six months of the year to be good because the first six months weren't terribly good."
But autumn can make more of a difference than Christmas, he said. "If we get the weather with us, then we can get people gardening. If there is too much focus on Christmas too early, then people will miss out on trade," he added.
"People shut their planterias early last year because they feared winter losses but Christmas shopping is being left later and later now so why have Christmas in store in July?"
Set up Christmas displays in the October half-term, he advised. "Some will put Christmas out early and they're the ones that will regret it. Autumn is a huge opportunity and this year we need that potential spend."
He said lawns, cleaning patios, clearing leaves and winter grow your own were all sales opportunities caused by the poor spring and summer weather.
HTA marketing director Andrew Maxted said while consumers were feeling the pinch, when buying Christmas gifts garden centre grottoes are relatively inexpensive days out.
Garden centres could exploit the trend towards buying local to bring in Christmas and October half-term trade, he added. "Garden centres are better placed for this Christmas than in the past."
But he agreed that there was a greater focus on autumn sales than for a long time with many consumers ignorant about bulb and hardy nursery stock planting opportunities. "Christmas is going to be big but make the most out of core gardening first," he said.