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.
Matthew Oates, the Trust's specialist on nature and wildlife experience, said he wanted to help children and adults have "epiphany moments".
"Mine was at the age of four when I saw a willow-warbler's nest," he said.
"But it's not just for children - we want adults who've gone to cities, with children and grandchildren, who can go out and experience nature and recall that in the end, we belong to it and it's part of us."
The Trust will mark Octavia Hill's legacy through the establishment of new walking routes at some of the properties that she helped safeguard.
She was one of three people who founded the organisation in 1894, initially through concerns that the burgeoning railway network would ruin some of the Lake District's beauty spots.
Later she developed the concept of a "green belt" around London.
At the same news conference, National Trust chairman Sir Simon Jenkins clarified the organisation's position on wind power following a newspaper report saying it was "deeply sceptical" of the technology.
"We are strongly pro-renewables," he said, "but we are also concerned with the countryside and worried about things that appear to intrude.
"It's a question of trying to achieve balance."
Other issues of current concern, Mr Jenkins and Ms Reynolds said, were proposals to reform planning in England, and the high-speed rail link project that will link London to Birmingham and further north.
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