Only about 50,000 trees will have been planted two years into a project which set a target for a million to have taken root in Norfolk by 2025.
Council leaders have admitted they are disappointed by the progress on Norfolk County Council's One Million Trees for Norfolk scheme.
But they say the coronavirus pandemic affected how many trees could be planted, particularly because community groups were not able to gather to get digging.
The idea was that planting trees will help tackle climate change, provide safe habitats for wildlife and boost people's health and wellbeing.
The first trees in the project went into the ground towards the end of last year, with the council keen to work with landowners, community groups, district and parish councils to get trees in the ground.
But, at a recent meeting of the county council's scrutiny committee, where the council's environmental policy came under the spotlight, figures revealed the progress on hitting the target.
The statistics showed 24,884 trees were successfully planted in 2020/21, with about 27,000 more due to have gone in by April next year.
While the report by officers said the scheme was "progressing well", Brian Watkins, leader of the Liberal Democrat group, disagreed.
He said: "I do not see it that way and I think plenty of other people feel the same way.
"We will have had about 50,000 planted in the county against a one million target over five years.
"We really do have to speed this up very quickly. I know we have had the pandemic and that has held us back, but even with regard to that, this is still painfully slow progress."
Andy Grant, the Conservative-controlled council's cabinet member for environment and waste said: "We are disappointed the numbers are not higher than originally planned."
He said Covid-19 had meant it had been difficult to bring community groups together to carry out plantings.
He said: "We will get the one million target over the next three years and will see new policies coming forward which will see a considerable uplift."
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