Colleges are experiencing a "nosedive" of interest from career changers who are being put off by massive increases in course fees.
The trend is in marked contrast to increased interest from younger learners, particularly in the South East.
Government cuts to college funding for those aged 24 and above have left colleges no choice but to double or even triple course fees from the 2013-14 academic year for level 3 students.
The Department for Business, Innovation & Skills now offers student loans to cover the extra cost. But college chiefs say students are concerned about paying these back, particularly when starting wages in the industry are often low.
At London's Capel Manor College alone the number of successful adult applicants has gone from 260 last year to 22 this year, losing the college £400,000 income.
Chief executive Steve Dowbiggin said the focus on attracting young people to horticulture is "great" but risks marginalising career changers and adults - the "life-blood of the industry".
He added: "Of every 100 people working now in horticulture, 50 or 60 will be career changers. I think this is going to hit the industry hard. For us it's like spinning plates. We have to increase the younger entry but we have to keep the adult entry plate spinning as well."
At BCA - formerly Berkshire College of Agriculture - course fees have tripled from £3,000 to £9,000 for level 3 diplomas in landscape and environmental studies and floristry.
Head of department Chris Lambert said: "The 24-plus market has really dropped away, which is a real shame because they are often what the industry wants.
"We had a 30-year-old turn around at the last moment saying: 'I don't want that commitment for £9,000'. We spend a lot of time funding 16-year-olds who don't know if this is the industry for them whereas the older ones are more focused."
Older students at Warwickshire and Pershore Colleges will pay £8,000 per year from this September, up from £2,500 last year. But assistant principal of land-based studies Julia Page said few of its students fall into this category for horticulture.
Reaseheath College in Nantwich, Cheshire, has seen 24-plus numbers drop 50 per cent.
Curriculum area manager for horticulture and floristry Iain Clarke said: "We used to attract a lot of mature career changers but that's now taking a nosedive because of 24-plus loans. We went down from 15 to 20 a year to seven or eight.
"There is an effect on that level and a knock on effect on the higher education level. A lot of people had been made redundant or were wanting a career change. They are trying to self-fund but trying to live at the same time."
But Writtle College horticulture curriculum area manager Tom Cole said 24-plus numbers have gone up by 50 per cent, from 10 to 15, this year. The cost of Writtle's level 3 courses has doubled, but at £4,000 they remain less than some other colleges. Four of the new starters are career changers who previously worked in the City.
"There are quite a few people who are wanting to get out of the rat race," said Cole. "There have been more TV programmes about gardening and landscaping. It's very buoyant in horticulture and landscaping at the moment."
Most colleges reported an increase - of as much as 25 per cent - in younger students choosing horticulture courses both at further and higher education level. Department heads credited hard work giving talks in schools and offering taster sessions and outreach.
Loans system - Applications still increasing
A representative for the Department for Business, Innovation & Skills said national data shows the rate of applications has continued to increase since the loans system was launched in April.
"The latest figures covered the period up to the end of July and show we have had applications from nearly a quarter of the 80,000 learners that we expect the first-year budget of £129m could support."