Leading experts in health and green spaces met last week to look at the mounting evidence of the importance of parks and open spaces to people's well-being.
The Prescribing Green Space conference heard how the medical community was increasingly coming around to the powers of green spaces in treating conditions such as diabetes and chronic stress. Yet many people had not visited a park for at least a year.
Writer Tim Gill, who launched a report called Sowing the Seeds this week on how to reconnect London's children with nature, said one in seven families had not visited a park in the past year.
Youngsters, he told the conference, should be "indicator species" to point to the quality of parks, green spaces and public realm.
"Children should be to parks what the salmon is to a river or a house sparrow to an urban woodland habitat - an indicator species on the quality of the environment. The more children playing, climbing trees and making dens, the better. Park managers could use this to show the importance of their spaces."
Macmillan Cancer Support physical activity manager Jo Foster said patients who exercised could reduce the risk of recurrence of breast cancer by up to 40 per cent, 30 per cent for prostate cancer and 50 per cent for bowel cancer.
"People are beginning to sit up and look at this level of evidence," she added. "If there was a drug with those kinds of percentages, people would be clambering for it."
GP Dr William Bird, keynote speaker at the conference, said as part of their pay deal, GPs now had to fill in forms on how active patients were. They also had to suggest ways to get fitter. Park walks, for example, could be more help than medicines.
Nicci Carruthers, chief executive of charity Thrive, agreed. She told more than 100 parks and green space professionals that the charity's gardening therapy improved health and confidence and was more successful than a Government scheme at getting people back to work.
The Government target for employing people through its Pathways to Work scheme was 25 per cent. Thrive boasted a rate of 28 per cent and "a lot of the Government's delivery partners want to know how we do that".
The total NHS budget, of which 10 per cent goes on treating people with diabetes - £110bn.