The FAITH Plant Centre, in Wood Green, will close on 29 August after disability charity Livability abandoned the scheme.
Livability representative Cathy McKenzie said £1m is needed to carry on giving horticultural training to 46 people with learning, mental and physical difficulties.
She said: "In this economic climate it is increasingly difficult to get funds - people are giving less than they used to. And there are more and more charities - 14 are founded every week.
"It's not financially viable for us to be involved with the FAITH Plant Centre. We're trying to cap our losses. We were expecting it to be sustainable long-term but on 29 August our involvement goes unless we get sustainable investment of £300,000 a year. We also need to spend £300,000 to £500,000 on essentials like electrics and a boiler, so that's £1m before we can go forward."
She added: "We've been fundraising like mad as soon as we realised there would be an issue - it's a very special project to us ... This is our only training place for young horticulture and we did fund-raise effectively for five years before we got into difficulties."
Livability employs three permanent staff at the centre along with four contractors, three of whom are employed by Haringey Council.
Horticultural trainer Clare Ritson said: "The centre is a lifeline to many disadvantaged people here. Livability gave us just three months to find other funders or to persuade the council to keep us open."
She suggested that Livability did not have the skills to raise money for a horticulture project as it was not the charity's core business.
Staff at the centre have collected more than 500 signatures to save it. FAITH operates as a garden centre and provides Haringey Council with autumn and spring bedding.
Council assistant head of parks John Morris said: "Livability is having problems with the competitive external funding market and is not able to bring about resource to maintain the site.
"We're considering the options. We recognise the value of the centre and we recognise Livability's position. I've written an internal options paper that shows we could achieve efficiencies."
The council will next meet on the issue on 18 July.