Visitors can tour the gardens between 1.30pm and 5.30pm on both days.
The arboretum was planted by Lord Clive of India’s son, Edward, with cascades of rhododendrons and azaleas amongst specimen trees and pools.
There are views of Sir William Chambers’ clock towers, with lake and hills beyond.
Walcot also has borders and rare shrubs, a walled kitchen garden, dovecote, meat safe, ice house and mile-long lakes.
Teas will be served in that hall's magnificent ballroom and there will also be a plant stall.
All proceeds will go to the National Gardens Society charity.
Admission is £5 and pre-booking is available but not essential at ngs.org.uk/view-garden/9487