The National Trust is planning a campaign this year to improve peoples' links with nature and wildlife.
The Trust's director general Fiona Reynolds said children needed freedom to discover nature for themselves.
More children go to hospital having fallen out of bed than having fallen out of a tree, she said.
The trust is marking the centenary of the death of Octavia Hill, one of its founders, who fought to preserve public open spaces in London and elsewhere.
"It's about wanting to give children a sense of freedom to discover," said Ms Reynolds at a news conference in London.
"The campaign will help children to get outdoors and connect them with nature, including things that can be done at trust properties, to try and stimulate a nation of nature-lovers."
In recent times, a number of agencies have warned that children in the UK and other developed nations are spending ever less time in natural surroundings.
A 2009 report for Natural England showed that only 10% of children now experience woodland play, as opposed to 40% of their parents' generation.
This retreat has even led to suggestions of a new quasi-medical syndrome, nature-deficit disorder.
Ponds and dens
The ingredients of the campaign will be made clearer in April but are likely to include an expansion of activities such as den-making and pond-dipping, which children can do now at some Trust properties.
Walker on Hampstead Heath London's Hampstead Heath is among the areas…
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