Commenting on Gardening Which? revelations that online retailer Gardening Direct was selling carrot and beetroot plugs at £1.09 each, Titchmarsh said people might need an easy way in to gardening but this case was a "bridge too far".
Speaking at the opening of the RHS Hyde Hall plant centre, Titchmarsh told HW: "At that kind of cost I think that's taking the Mickey. People are going to feel that they have been duped. I don't think it does the industry any good - it really doesn't.
"When companies do things like that I feel embarrassed on behalf of the industry. You just shouldn't do that - it's not on."
Gardening Direct has since withdrawn the plugs from sale with marketing manager Mark Sherwood saying the company did not wish to mislead gardeners or give poor value for money.
But some industry figures have questioned Titchmarsh's comments. Hayloft Plants owner Derek Jarman said: "I suspect the Gardening Direct price is fair and a true reflection of the cost of the product. If people want them it is up to the industry to supply them. [Titchmarsh] has got to be seen as being on the side of the consumer but I suspect he hasn't looked at the costing of the products."
Sun gardening correspondent Steve Bradley echoed that view: "If you have been gardening as long as Alan Titchmarsh it can be hard to view things from an amateur's point of view. Some plugs might be overpriced, but having said that it is worth bearing in mind the Gardening Which? test that said seeds were a rip off."
Alan Titchmarsh officially opened the £3.5m development of RHS Hyde Hall last week. The society hopes to increase visitor numbers to 200,000 a year by 2020.
Around 100 people including children from Collingwood Primary School saw the celebrity gardener cut the ribbon to open the new plant centre, car park, access road and landscaping.
Titchmarsh said: "This site is a real inspiration. The soils and exposure mean what grows here will grow for you. The outdoors is often the first to suffer from Government cuts. But gardening has a role to play in health and law and order."