A further 500 trees and shrubs, which were planted along the route of the Norwich Northern Distributor Road, have died, council bosses have confirmed.
Work to replace them will start next month, but it means more than 3,500 trees and shrubs planted next to the road since 2016 have perished.
Some 6,000 trees were chopped down to make way for the £205m road, which stretches from the A47 at Postwick to the A1067 Fakenham Road.
But the council pledged to plant five for every one removed - 30,000 in total.
However, in 2019, the council admitted 3,000 of the trees and shrubs which had been planted up to that point had died, which it blamed on the hot summer the previous year.
New ones were planted, but the council has confirmed a further 546 trees need to be replanted over the next two months, at a cost of £10,000.
Martin Wilby, the council's cabinet members for highways and transport, said: "It is very regrettable that some trees have not survived, and others are showing signs of decline.
"The development consent order requires the county council to replace any trees which die within the first five years following the completion of the landscape planting, completed in 2019.
"Norfolk County Council reports back to the planning authority Broadland District Council, who are monitoring compliance to
the development consent order.
"A maintenance and replacement contract exists with Norse, which identifies trees on Broadland Northway which have not survived.
"A substantial programme of replanting was undertaken last year which was largely successful.
"A low failure rate has been observed after last year’s replanting and those trees which have failed will be replaced in November/December 2021."
Liberal Democrat county councillor Steffan Aquarone said: "Tree planting is a small but important part of our local response to the climate emergency.
"But it's pure tokenism when it comes to offsetting enormous roadbuilding projects.
"It will take decades before for newly-planted trees can offer any meaningful carbon offset or habitat replacement, and even that depends on a sufficient proportion surviving, which, as we're seeing in the case of the NDR, is not the case."
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here