Speaking at the RHS 2008 show season launch last week, RHS head of shows Bob Sweet said that it was worrying to see the likes of parrot's feather available in garden centres.
He said the RHS was working with the industry to make sure nurseries don't sell these types of plants. "We're asking more questions than ever before."
But HTA consultant David Gilchrist said while it was possible "one or two" garden centres were selling those plants, HTA members had been advised "on numerous occasions" not to sell them.
"But there are a lot of centres that aren't members of the HTA or the GCA and we have no control over them."
GCA administrator Gillie Westwood said the organisation's members did not sell the plants Sweet had highlighted.
A paper was developed in association with Plantlife and Defra to identify which plants should not be sold and to offer alternatives. "Our members have been informed about the problem and to our knowledge none of them sell the plants in question," she said.
Paul Cooling, of leading plant retailer Coolings Nurseries, said he was unaware of any garden centres selling those plants and would be "very surprised" if any were.
"We haven't sold any of those things for years. It's one of those things you blindly follow the lead of those supplying the plant until you find out they're actually not very good so we stopped selling them."