Thousands of volunteers will plant 100,000 trees and improve 75 miles of hedgerows.
The Woodland Trust said bigger and thicker hedgerows will help bees and provide shade and shelter for livestock.
Hannah Wilcox-Brooke from Keep Wales Tidy added it was ambitious but could have a “real impact”.
The Long Forest Project was launched at the National Botanic Gardens of Wales, Llanarthney, on Wednesday.
Jeremy Langford, Woodland Trust director, said: “There’s no particular protection on hedges or hedgerows trees, they are rather taken for granted and unloved and that’s something we aim to address.
“We want to involve lots of people in helping landowners to care for and plant new hedges.
“If you drive around the countryside or even look out your window, most of the trees you will see are in hedges and in fields rather than in woods.
“You will also see a lot of very gappy hedges where you can trace the lines across the landscape where hedges used to be but aren’t any more.”