Because of a dispute with the local council over car parking, the society planned to only allow members to have access to the site in St Albans, Hertfordshire. But the society will now open its gardens for 28 days. It will also negotiate with the council to open for a longer period.
RNRS chief executive Roz Hamilton said: "We will exercise the 28-day rule and will open from 12 June to 31 July from 10am to 5pm.
"We've been permitted by the council to use the overflow car park. The planning application is being looked at favourably. It's excellent news - we're very chuffed."
The RNRS said it expected 10,000 visitors, mainly from overseas, in the peak-season opening period.
The problems were caused after a dispute with the council about car parking. The society had been unable to use its usual car parking facilities because they were sold to the Butterfly World project.
The society typically opens the gardens throughout the summer. But without car parking this was impractical. It was told that it would have to seek planning permission if it wanted to use a neighbouring field for car parking for more than 28 days.
The RNRS initially decided it would simply close the gardens to the public. But, after requests from the public, it has relented. It will open for 28 days and hope that an agreement to extend can be reached with the council.
A representative said: 'We realised that people would have been very disappointed and many people would have come anyway. We hope to resolve our differences with the council as soon as possible."