The pollen count will increase as temperatures heat up and the worst conditions are expected on Sunday and Monday, according to forecasters.
Alexander Burkill, of the Met Office, said tree pollen would be more significant than grass pollen until the middle of May.
He said the tree pollen count will be high towards the end of the weekend, adding: "Today we've got a fair bit of cloud and the rain coming down, so it's going to go down to moderate.
Today and Monday in places, especially across England and especially in the south east, you're looking that it could get up to high."
More top news
The event, in its 28th year, is organised by and is the main fundraising event for The Sussex Beacon, a local HIV charity.
The casualty was cut free from the wreckage after the vehicle ended up on rocks near the obelisk at Portland Bill.
Sunday's weather for the east