Don hailed the Westminster allotment when it was launched as an "inspirational example" to new growers. But the Trust has admitted it is "guilty as charged" of neglect of the allotment, which is at its London HQ.
Don said: "It is a shame, rather foolish of them perhaps. In a way we all love these stories. But the National Trust are promoting allotments and are getting huge numbers of new members as a result-hundreds of thousands directly on the back of that so they're clearly doing something right."
The Trust built the showpiece allotment to launch its Food Glorious food campaign, which aims to build 1,000 allotments at Trust sites by 2012. Around 300 have been built so far.
Don, who is promoting the Ideal Home Show at Earl's Court in London until April 5 and features a David Domoney-designed allotment garden, added: "We all know with allotments you have to persevere when the weather is bad. Allotments are a long haul. It's brilliant if people take them on. They might go through sticky season. I've had bad years in my garden. With allotments there is no end of the story-no beginning or end."
National Society of Allotment & Leisure gardeners secretary Donna McDaid said: "There are so many people crying out for allotments and this was supposed to be a model site. The National Trust needs to lead by example and this is hardly a good example.
"If they could not be bothered to look after the site themselves, they could have just advertised for people to have their own allotment there and they would have been deluged.
"It does not say much for the rest of their campaign and although it is a good thing, 1,000 new plots is actually a drop in the ocean of what is needed."
But National Trust spokesman Ivo Dawney said of the allotment: "You are quite right - we are guilty as charged. Despite huge success with our allotment campaign across the country with 300 new plots and another 750 in the pipeline, the Westminster one has failed."