Country Life called Trewithen One of the outstanding West Country houses of the 18th century. Its fine Georgian elevations, which took 40 years to complete, perfectly complement perhaps the most beautiful woodland gardens in the British Isles. The story starts with what many would regard as a disaster, the compulsory felling of 300 beeches to provide WWI trench props.

However George Johnstone used this opportunity to create his masterpiece, the great glade which spreads out in front of the house. This contains magnolias, camellias, including the original Donation, (all examples of which worldwide stem from this plant at Trewithen), and rhododendrons, especially the lovely variety named by Johnstone after his wife, Alison. There are many rare trees and shrubs found here, though not in many other places in the British Isles, some of which are introductions by the famous plant hunters E. H. Wilson and George Forrest.