North One and West Six garden centre managing director Joel Beckman said the market had been "flooded" with black-market Christmas trees selling from car parks and out of vans. He added: "The number of trees we sold this year was slightly less than last and we will order more cautiously for 2013 because of guys coming in on street corners."
He said garden centres sell good-quality trees, give advice and offer service, and "we need to defend our patch" to stop sales falling.
Beckman said there was a risk of street-bought Christmas trees being stolen goods and claimed some traders were operating without licences and not paying VAT or taxes.
Alton Garden Centre director Andy Bunker said the practice of "barrow boys" selling trees was leading to gradually declining sales of about three per cent a year in some garden centres. He said the economy was a factor behind traders selling seasonal items such as trees and fireworks and undercutting garden centres.
British Christmas Tree Growers Association chairman Harry Brightwell said the police called him in December about a stolen consignment of Scottish trees bound for Ikea.