"The problem with the heritage and botanic sector is that the pool of potential applicants has shrunk," he said. "With the demographics involved, the way society is evolving and the Government encouraging more people to go into secondary education and industries such as science, engineering and innovative design, gardening doesn't factor."
Deene Park head gardener Andrew Jones has suggested that career changers from the city may be undercutting gardeners' wages. "This is the kind of job people love doing. The downside is there are not the same financial rewards. A lot of people who have been in wealthy city jobs have decided they have had enough and want to become gardeners," he said.
"They can afford not to be paid particularly well because they have money from their previous employment. That can drive wages down if people are willing to accept lower salaries."
Calnan added: "People do come on quality-of-life second careers if they are bankers and the like. But I've not heard they have been pushing out employed people. They won't have the experience."
He said the trust had weathered the careers crisis through its careership programme. It now has 26 kitchen gardens and 300 allotments, with 700 allotments to come and 50-80 kitchen gardens to be restored, he pointed out.
"There doesn't seem to be a shortage of community groups coming forward that can't wait to get growing," said Calnan.
"People like the idea of being attached to trust sites. It gives a kudos and an opportunity to speak to our gardeners and get advice on how to grow that you wouldn't get on a local authority site."
Since 2006, the National Trust has carried out a flower count in Devon and Cornwall to gauge how gardens are responding to changes in climate.
Calnan said the count had a historical value. He suggested this year's late flowering "might be a short-term blip" and said recent controversy about the validity of climate change claims was "unfortunate" because "the wind has been taken out of the whole message".