It says ash dieback disease is spreading and wants landowners to plant different species near to existing ash trees to replace them when they die.
Its pilot planting scheme is offering 1,000 subsidised "disease recovery packs", each with 45 native trees.
The project will focus on areas most at risk, including Norfolk, Suffolk, Kent, East Sussex and Northumberland.
The Woodland Trust is using data to map 280 million trees across England and Wales to assess the potential impact of tree diseases, including ash dieback.
The organisation says the mapping technology has shown the importance of trees outside woods in contributing to wildlife corridors in hedges and along roadside verges.
About 12 million ash trees have been identified outside woodlands.
Source: Forestry Commission
Austin Brady, Woodland Trust director of conservation, said: "The difficulty with these trees in the wider landscape is there is no obligation on people to replace them if they die, so it's a one-way ticket for many of these trees.
"In lots of hedgerows, field corners and roadsides, it's difficult to imagine how these trees will get replaced.
"By the time people really notice the problem, we've almost left it too late to do something about it."
A Department for Environment, Food and Rural Affairs spokesman said there was a "duty to protect" trees and woodland, and added that the government had invested Â£4.2m since 2012 to combat ash dieback.
All the trees the Woodland Trust provides are grown in the UK from fully traceable seeds gathered in the UK and Ireland to prevent the importation of diseased trees.
Landowners should prepare for a future in which ash trees have disappeared, the Woodland Trust has warned.