Dr William Bird, who founded the hugely successful Green Gyms programme, says most medical professionals are starting to grasp the health benefits of nature and green space.
Bird - a GP, chief executive of Intelligent Health and board member of the Parks Alliance - agreed that general practitioners now "get it", though many still see the topic as a distraction from the pressing financial problems the NHS faces.
"Ten years ago I would have said no (they don't understand); five years ago a few of the leading ones; but the evidence that is coming through now is so strong that actually I don't think you'll get any doctor now saying this is a waste of time."
Practitioner understanding is set to improve further with the publication of the Oxford Textbook of Nature and Public Health, which Bird is co-editing. It is due to be published in 2017.
"You'll get a few cynics but I think most of them would say I agree it's good for you; some of them will then go forward and say well we need to put some money into it. It has to be seen as a saving, not a cost; it becomes an investment."
What form such an investment would take is the question. Bird is a fan of recommending gardening to patients but this is of limited value if they can't easily access a garden.
"At some allotments someone has to die for a person to get off the waiting list; having one's own garden is out of the question for most people. Then you've got working in parks doing conservation work, that's great. We've got horticultural therapy for mental health problems and learning disabilities but what I would like to see is mass communal gardens close to where people live. Then, if they don't own a garden, it doesn't matter.
"Communal gardens would be planted by the community. Then you've got the benefits of green space around, say, a housing estate and you've got people who don't have to go far to get their fingers dirty in the soil. You've got children appreciating it and understanding it."
Bird believes the health sector could be persuaded to put money into this model as there is overwhelming evidence of the health and societal benefits. For example, a Chicago study showed domestic violence was halved when people on a housing estate were surrounded by greenery.
The GP was speaking last week at the launch of a horticultural therapy garden for mental health patients at Maudsley Hospital in South London.
The formerly grim concrete space was only used for smoking but has been transformed into a garden where staff and patients at the South London and Maudsley NHS Foundation Trust (SLaM) can relax, socialise and tend vegetables. Trees for Cities designed the garden, using cool colours such as violets, blues and greens.
Raised planters are filled with herbs and vegetables while apple trees on dwarf rootstock line the garden wall. Over time thornless blackberries and hydrangeas will grow for added privacy. Trees for Cities patron Chris Collins built the garden with the help of RDC Landscapes.
The launch is a first for Trees for Cities, which has until now focused on edible playgrounds and tree planting, but chief executive David Elliott said it hopes to create many more hospital therapy gardens in the future.
Dr Bird said the modern lifestyle - including the "steel and glass" environment of a modern hospital - is bad for humans, which as hunter-gatherers have perfectly evolved to live in a natural environment. The stress of being removed from that habitat shows itself in everything from mental health problems to chronic inflammation, he said.
"When you put someone into nature, it can do as much good as a lot of medication if not better ... People who go outdoors, within two minutes their blood pressure drops, the brain changes, sociability improves, agitation drops and we're able to be more productive.
"What we have to now work out is what's the best way of making sure NHS starts to see this - not just for inside hospitals but outside in the community."
David Norman, director of facilities at Maudsley Hospital, said mental health providers are very focused on providing good quality psychiatric care for their patients and "tend to overcompensate and spend a lot of time working on the physical design of the buildings".
"We do a lot of thinking and put a lot of support into the clinical team to provide good quality care; perhaps what we're not so good at is thinking about utilising the benefits of the environment, and thinking about the opportunities of bringing the outside into the overall hospital environment."
He wants SLaM to look at its other green spaces that could be developed around the premises and hopes the project will be replicated around the country. Funding for the project came from the SLaM and the Greater London Authority's London Trees and Woodlands Community Grant scheme.