Labour is pledging to plant two billion new trees in England by 2040 if it wins the general election.
The party is also promising 10 new national parks during its first term, as part of its "plan for nature".
The Tories want to plant 30 million trees a year, while the Lib Dems have pledged 60 million if they win power.
Friends of the Earth has hailed Labour's plan - which would mean more than 270,000 trees being planted a day - as the most ambitious of the three.
The Green Party has pledged to plant 700 million trees by 2030, and Brexit Party leader Nigel Farage has also said he would support a wide-scale tree planting programme.
Labour claims its plan will lead to 20,000 new green jobs in forestry management and timber trades, contributing to its overall goal of creating a million green jobs if it wins the race to Downing Street.
Speaking at the launch of the policy, party leader Jeremy Corbyn said: "This election is our last chance to tackle the climate and environment emergency. Labour is on your side and on the side of the environment."
Labour says it will make £2.5bn available to plant the trees in a number of sites, including urban parks, farmland and schools.
And along with the new national parks, the party would invest an extra £75m a year into new and existing park authorities to manage them.
Areas being considered for the parks include the Malvern Hills, Chiltern Hills, Lincolnshire Wolds and North Pennines.
Labour says it is wants 75% of the population in England to live within half-an-hour of a national park or an area of outstanding natural beauty.
These moves - alongside the investment into restoring habitats, such as woodlands, grasslands and meadows - could help store up to 47 million tonnes of CO2 each year by 2050, and allow endangered species to recover.
'Kick-start the recovery'
The party said it would also reverse the 20% cuts the Conservatives had made to the annual national park authorities budget since 2010, increasing funding by 50% "in order to enable the parks to provide natural solutions to the climate and environment emergency".
Our Reality Check team has asked Labour where its figures came from - but looking at statistics provided by the government in March, the Conservatives cut the authorities' budget by closer to 10% in cash terms (comparing the numbers directly) or 25% in real terms (when inflation is taken into account).
Commenting on Labour's announcement, Friends of the Earth tree campaigner Guy Shrubsole said: "This is by far the most ambitious tree-planting pledge we've seen from a political party.
"Tree cover in the UK needs to double as part of the fight against climate breakdown and this means adding three billion new trees, and fast.
"If sustained, Labour's promised tree-planting rates would achieve this by 2050. While parties have been racing to make bigger trees pledges, it's crucial to remember that trees will only help fix the climate crisis if emissions cuts happen at the same time."
Although Labour's plans are just for England, the party said it would work with devolved governments in Scotland, Wales and Northern Ireland to develop more nature recovery works and provide funds towards extra national parks.