On the £450m cuts announced in this summer's Budget, Dowbiggin said: "The Government is cutting funding for adults and that's hurting us because the demand we get from industry is from career changers.
"In September a raft of people will come to Capel Manor but we may have to say they can't come because of cuts in Government funding. There will be people we can't take.
"They can get jobs easily after we train them. The Government thinks businesses should take these people on and train them but in horticulture there are not the margins to do that."
There has been a one-third cut in funding from 1,800 places to 1,200. Dowbiggin said the areas hit hardest are "the very ones where there is a skills shortage".
The gardening courses hit are the ones that 16- to 18-year-olds do not want to do, he explained, so "professional nurseries will feel the skills shortage".
Dowbiggin said he is keen to air his views before the Government spending review in November. Courses such as the RHS level 2 and 3 for skilled and craftsman gardening could be hit.
Landex chief executive Chris Moody said: "The recently announced 3.9 per cent cut in the adult skills budget comes on top of the 24 per cent cut announced earlier in the year. This reduction in funding is targeted at further education provision for adult students.
"This will have a dramatic effect upon those colleges, like Capel Manor, that enrol significant numbers of adult students. Removal of this funding at such a late stage may force some colleges to close courses that recruit significant numbers of adult students, which is traditionally the case in horticulture as it attracts people looking to retrain for a second career.
"Most adults who are over 24 and studying a course at level 3 or 4 will be less affected, as since 2013 they have been required to fund their education and training through the student loan company. Funding for students aged 16-18, while not a protected area, has not suffered cuts to allocated budgets for 2015-16.
"Not only are these substantial funding cuts but colleges have been given very little notice. Like universities, land-based colleges have a recruitment cycle, which spans at least the six months before courses start. This will have a significant effect on budget projections.
"The Government's priority for vocational training lies with apprenticeships, where the prime minister has restated the target to generate three million over the life of this parliament. However, this will present a major challenge for the horticulture industry where apprenticeship starts in all sectors fell steadily from 2,630 in 2011-12 to 2,240 in 2013-14."
- Meanwhile, Capel Manor's National Gardening Centre will relaunch in September with the aim of attracting big London-based business dual sponsors to add to horticulture trade sponsors.