Scottish ministers have been criticised for accepting £5m from Shell to help fund a tree-planting programme as part of its climate strategy.

The oil company said it would give the government agency Forestry and Land Scotland (FLS) the money to plant or regenerate 1m trees in Scotland to help offset about 20% of its petrol and diesel sales at UK service stations.

The Scottish Green party said Shell’s pledge raised challenging questions about the Scottish government’s commitments to tackling the climate emergency, after Nicola Sturgeon, the first minister, promised the country would cut its carbon emissions to net zero by 2045.

“This is not meaningful action to address the climate crisis, it is greenwashing, pure and simple,” said Andy Wightman, the Scottish Greens’ land reform spokesman.

He said the Scottish National party had allowed BP and Heathrow airport to sponsor its conference in Aberdeen, which finished on Tuesday with a renewed commitment from Sturgeon to lead the world on tackling the climate emergency.

“Governments which are really serious about the climate crisis should have nothing to do with private offset schemes run by oil majors,” he said.

The Guardian’s recent investigation into the role of oil companies in the climate crisis found Shell was planning to increase production by 38% over the next 12 years, increasing crude oil production by more than 50% and gas output by a quarter.

Experts have warned this will sabotage attempts to cut global CO2 emissions, which scientists say need to be reduced by 45% by 2030 in order to limit the increase in global heating to 1.5C.

Sturgeon accepted the 2045 target after the committee on climate change, the government advisory body, said Scotland’s greater capacity for new forestry meant it could reach net zero earlier than the rest of the UK. The UK’s overall net-zero target is 2050.

Ministers have yet to fix a revised forestry objective to help meet that commitment, but Scotland has repeatedly failed to hit its current annual targets, set at 10,000 hectares (24,700 acres) a year from 2010 onwards.

Official data shows that from 2010 to 2017, the planting rate averaged 6,800 hectares a year. Of that, only 820 hectares was planted annually by Forest Enterprise, the government-funded agency that preceded FLS, and the rest by private companies or charities. The overall target was met for the first time last year, when 11,200 hectares were planted.

Shell said it would spend £10m in the UK over the next year by buying carbon credits to offset its customers’ fuel purchases. The £5m offered to FLS would be spread over five years.

Wightman said there were significant flaws in this scheme. It would not involve Shell cutting its oil production and there was a danger this offsetting would be counted twice, since Shell would include it in its data and the Scottish government would also include it.

Shell’s initiative only affects customers using its new loyalty card, who it said bought about 20% of its fuel, and Shell would fund the donation free of charge. Fuel bought by business account holders would also be automatically included.

An SNP spokesman said the government had “smashed” its target for planting trees and was determined to meet its 2025 goal four years early. “Forestry and Land Scotland will use this funding to plant more trees and restore more peatland, helping to meet our climate change targets,” he said.

“Meanwhile, the Scottish Greens abstained in voting for the world’s most ambitious climate change legislation and aligned with the Tories to block measures in the forestry bill to allow compulsory purchase of land for creating more woodlands.”