Money raised from the industry in 2014 will be used to market gardening to 30-45-year-olds, encouraging them to use outdoor space for relaxation and enjoyment, rather than "hard work and dirty fingernails".
Project manager David Arnold said: "'Levy' is too strong a word. We want to make this as equitable as possible, not just cap in hand. Some people have more funds than others."
He added that 25 breeders, producers, retailers, manufacturers and suppliers met in October and they agreed the campaign is a good idea.
"We have a very interesting, exciting and creative pool of ideas that came from the industry planning workshop held last month and these are forming the basis of our campaign planning, and the consumer research we are carrying out soon to ensure we are on message to the target audience."
HTA chief executive Carol Paris suggested that an associated consumer magazine could help the industry.