Growers have said the trust is ignoring its own peat-free policy, announced in 2001, which demanded that by 2003 all plants grown in or sold at trust properties would be raised in peat-free compost except in "exceptional circumstances", such as raised peat beds and ericaceous plantings integral to the history of the site.
Stratford-upon-Avon-based Fibrex Nurseries managing director Richard Godard-Key said: "We still supply to the National Trust - it doesn't ask, and probably couldn't get the plants otherwise."
He added: "We have trialled peat-free growing media but there isn't a good, reliable one. The companies haven't done enough research on them - they still break down, and sometimes they're even still hot when they're delivered.
"You can get away with it for some herbs and shrubs, and other growers do manage it, but for pot plants you have to be able to control things like fertilisers, which peat-free doesn't allow."
Hertford-based Daisy Roots nursery owner Anne Godfrey said: "The National Trust used to insist on peat-free plants but there simply weren't enough nurseries that were growing that way, so it has relented."
She said she exhibits peat-grown plants at plant fairs at National Trust properties. She added: "We don't use pesticides and try to encourage biological controls like ladybirds, but peat use isn't something we've looked into. Growing plants is difficult enough and our customers haven't asked about it."
Trust head of parks and gardens Mike Calnan said: "Our peat policy hasn't changed since we went peat-free."
He said the trust would crack down on "policing the policy tightly. I'd like to know which properties they are and investigate. The policy is absolutely clear. If people are breaking it, we need to find out."
He suggested the growers may be supplying let properties or that trust-employed contractors "forgot we're peat-free".
National Trust southern territory parks and gardens adviser Mike Buffin said: "We operate a peat-free policy on my patch. All the gardeners know about it. I wouldn't be too happy if we were breaking that. But we can't police every garden."
Buffin said the policy was in place to protect wetlands from environmental damage caused by peat extraction and to protect carbon sinks.