The Royal Horticultural Society has called for compensation from the government after major roadworks close to its flagship visitor attraction cost it £6m. 

The charity said it had recorded 350,000 fewer visitors to RHS Garden Wisley because of major work on the nearby A3/M25 junction, which has included closure of a stretch of the M25 over several weekends. The A3 will close again from 17 to 20 January for further works. 

The RHS forecast its losses will rise to £11m when the roadworks, which were started by National Highways in 2022, end in 2026.

An online survey conducted in October 2024 on behalf of the RHS by the independent insight and analytics agency Trueology found nearly 80 per cent of members who visited Wisley less frequently in the previous 12 months attributed this to the roadworks.

The research also found 63 per cent of non-visiting members in this period said they decided not to visit Wisley for the same reason.

“Since September 2022 to date there has been an overall 25 per cent reduction in visitors at RHS Wisley, severely impacting gate admissions and secondary income,” the charity said.

Total garden visitors at the charity declined to 2.95m in 2023/24 from 3.1m in 2022/23, according to the charity’s latest accounts, for the year to the end of January 2024.

“RHS Garden Wisley accounted for the bulk of this decline, with the other four gardens holding broadly steady,” the accounts said.

“RHS Gardens and Horticulture income comprising gate receipts and income from affiliated societies was £7.3m (2022/23: £8m), down £0.6m (8 per cent), as a result of lower numbers of paying adults.”

It said the disruption also contributed to lower than normal attendance at its annual festive lights event.

The charity said its financial losses meant it has had to delay the development of new arboretums at its gardens and the planting of 4,000 trees to investigate climate resilience for the next century.

The RHS said it had also had to cut back on its community outreach work and was looking at training 10 per cent fewer work-based student horticulturists over the next two years.

The TV presenter Alan Titchmarsh and the RHS have partnered to launch a petition calling for the government to recognise RHS Wisley as a special case for compensation for the “devastating losses” caused by the roadworks.

Clare Matterson, director general of the RHS, said: “The highways compensation laws are complicated and unlikely to enable the RHS to recoup these devastating losses. 

“If there was ever a special case for compensation, surely RHS Wisley stands out as a national treasure that needs to be upheld and prized and our charitable work as vital to be protected. ”

Matterson said while the RHS was grateful for the new road and the positive difference it was beginning to make after “months of disruption”, it continued to be a flawed solution that increases car miles around Junction 10 by about one million kilometres annually.

“We continue to believe circular routing could have been avoided, saving these increased car miles, by creating slip roads off the A3,” Matterson said.

“At the time of granting consent to the scheme, the secretary of state reviewing the planning inspectors’ decision assessed that the RHS had a case, but that we were overstating the heritage and economic harm and that it would be short-lived and insubstantial. 

“Today we can now evidence that the harm is exactly as we predicted.”  

The RHS and National Highways have been contacted for comment.