Charity Stop Hate UK listens to people's experiences and gives confidential and independent support.
Applications for the additional 70 homes and access to the site were rejected in the summer.
Volunteers had worked for four days on the garden but the two men damaged it on Monday.
Reading Borough Council is evaluating quotes for the structural work to the Grade II listed arch.
This year's event saw around 100,000 people attend the festival in Richfield Avenue.
Developer wants Planning Inspectorate to decide on its proposed four 11-storey blocks on Napier Court.
The area close to the stadium was previously a landfill site between 1972 and 1985.
Hard ground created by drought and five heatwaves has increased the risk of flash flooding.
The Reading Council sub-committee rejected the application at a meeting on Thursday.
