Alternatives to the plastic guards, which protect saplings from animal grazing and other damage, include using “crates” built from locally-felled diseased trees, using existing shrubs to protect young trees and cardboard tubes.
The trust, which has committed to planting or establishing 20 million trees by 2030 as part of efforts to cut its emissions to net zero, said it already routinely collects and reuses the plastic guards.
But the time and expense of reusing the plastic guards, which have commonly been used in tree planting since the 1970s, on a large scale is becoming more of a problem as tree-planting ramps up to tackle climate change, the trust said.
And some of the guards, which have been reused for more than 30 years, are coming to the end of their lifespan.
So the conservation charity, which has planted 60,000 trees this year, is trialling new methods that are more in keeping with nature to protect newly planted saplings as they get established in the landscape.
In one method, crates or small fenced areas constructed from locally-felled wood are spaced across the landscape with saplings planted inside them surrounded by blackthorn or thorny plants that will grow and continue to defend against animal damage after the crates disintegrate.
Scattering the planting across the landscape means pockets of saplings will grow and set seed to naturally create joined-up woodland over time.
At Coniston Hall, in the Lake District, around 4,000 saplings have been planted over winter in 46 crates created from untreated diseased larch wood, which is not considered to be a disease risk to the new trees, with a further 3,000 planted “naked” on Bleathwaite.
The crates contain between 25 and 120 trees surrounded by thorny shrubs.
In the South Downs, robust crates have been created using sweet chestnut from the surrounding landscape, and contain one to 20 saplings .
At Ennerdale in the Lake District, around 20,000 native trees have been planted along the River Liza, combined with conifer cuttings to enable more mixed woodland to develop.
There has been a dramatic shift away from plastic tree guards on the project, the trust said, and some trees have been planted within the felled conifer brash to protect them from grazing.
At Hardcastle Crags, West Yorkshire, and in Somerset, cardboard tubes have been trialled with mixed success, as they biodegrade quickly and leave saplings still vulnerable to animal damage.
Cardboard and wool tubes are not yet proven cost effective alternatives to plastic with the large-scale planting the trust is undertaking, the charity said.
Other approaches include allowing trees to regenerate naturally and planting in shrubland where vegetation such as gorse and hawthorn can protect young trees.
The National Trust said that while it was difficult to move away from plastic guards entirely, it hoped the crates and other methods could be used at scale across its landscapes.
John Deakin, head of trees and woodlands at the trust, said: “We have made a massive commitment to plant or establish 20 million trees on our land in the next 10 years – and have already made a great start.
“However, we are mindful of the potential unintended consequence of this and how you protect these trees. We are exploring every possible option to minimise this impact.”
He said trees must be allowed to reach maturity to achieve their maximum potential for storing carbon, providing homes for nature and public access to woodlands.
And he said: “In time, we hope to allow trees to self-propagate – there is no substitute for the ability of trees to naturally regenerate.
“In the meantime, however, we must explore everything we possibly can to minimise the environmental impact of what we are trying to achieve.”