Westonbirt, the National Arboretum occupies a 600 acre site three miles southwest of Tetbury within the Cotswold Hills, and contains over 15,000 trees and bushes (from around 2,500 different species) in two wooded areas, separated by the Downs, a strip of grassland. To the east is the Old Arboretum, also known as Down Plantation, consisting entirely of planted trees, some dating from the foundation of the arboretum in the mid-19th century, while to the west stretches the larger Silk Wood, an area of primarily ancient woodland to which other trees have been added, from the 1880s onwards.
The arboretum was established in 1829, as part of the Westonbirt estate, owned at that time by Robert Stayner Holford (1808 1892), MP and plant enthusiast. The Holford family had since the 1660s resided in a grand mansion (Westonbirt House) a short distance east, a structure rebuilt on three occasions and currently housing a girls boarding school. The arboretum, later further developed by the son George Holford, passed to the state in 1956 and has become a very popular attraction, with over half a million visitors each year. Although obviously managed, and crossed by many paths, the woods do still have the feel and character of a natural forest, especially the western section. The busiest times are during the autumn colour season, which generally peaks in late October, and again in the spring when the new growth leaves of some species are equally as colourful.
Westonbirt Arboretum lies along the A433, the main road southwest of Tetbury. The driveway leads to a large parking area and the fee station, from where a short walk over the grasslands of the Downs leads to the visitor facilities, principally a cafe, restaurant, gift shop, learning centre and childrens' playground. Paths enter the adjacent Old Arboretum, where circular walks range in distance from half a mile to 2.5 miles; especially popular in autumn are the Colour Circle and the Acer Glade, populated with maples and other species whose leaves take on especially rich shades of red and orange. The pathways are a mix of wide, straight rides or avenues, and narrower, more winding trails.
Silk Wood is over three times the size of the Old Arboretum, crossed by a similar network of paths and tracks, and is generally less visited, especially the western reaches which are mostly ancient woodland, without planted trees. The main north-south route through the wood (Green Lane/Broad Drive), is a public right-of-way and therefore can be walked without payment, starting either from the main road to the south, or via other paths from the village of Leighterton to the north. People entering the arboretum via this route are meant not to stray from the designated path. One recent installation (2016) is the Treetop Walk, a thousand foot-long, wheelchair-accessible corridor through the trees up to 40 feet above the ground, though still well short of the canopy above. The walkway forms part of the main loop through the eastern half of Silk Wood, of 1.6 miles. Specific collections include groves of Japanese maple, oak, ash and cherry, while one other point of interest is the oldest tree in the woods, a 2000 year old small-leaved lime; not a single, large specimen but rather a rough circle of smaller trees, centred on a long vanished original.