Westonbirt, The National Arboretum is an arboretum in Gloucestershire, England, about 3 miles (5 km) southwest of the town of Tetbury. Managed by the Forestry Commission, it is perhaps the most important and widely known arboretum in the United Kingdom.
Planted in the heyday of Victorian plant hunting in the mid-19th century as part of the Westonbirt House estate, the arboretum forms part of a site which is listed Grade I on the Register of Historic Parks and Gardens of special historic interest.
There is evidence of coppicing at the site from 1292. First use of the name "Weston Birt" was in 1309. This was taken from Weston, a settlement to the west of Bowldown Road, and Birt from then lords of the manor, the Bret family.
The arboretum was established in 1829 by Robert Stayner Holford and was later extended by his son George Lindsay Holford. After the death of George in 1926, ownership of the arboretum passed to his nephew the fourth Earl of Morley, and eventually to the Forestry Commission in 1956. The Holford family's mansion, Westonbirt House, became a girls' boarding school in 1927 when it was separated from the arboretum. Westonbirt Arboretum backs onto the Highgrove Estate of Prince Charles, the Prince of Wales.
Westonbirt Arboretum comprises some 18,000 trees and shrubs, over an area of approximately 600 acres (2.4 km²). Its 17 miles (27 km) of marked paths are popular with visitors, and provide access to a wide variety of rare plants. There are two main areas to explore. The Old Arboretum is a carefully designed landscape offering beautiful vistas, stately avenues, and a host of rare and exotic trees from across the globe dating back to the 1850s. Silk Wood is a very different experience: although it also contains many exotic plantings, at its heart is a traditional working woodland, dating back to the 13th century. Dogs are welcome in Silk Wood but not allowed in The Old Arboretum.
Throughout the arboretum, each specimen tree is labeled, either on the trunk or one of the low-hanging branches. Blue labels indicate Westonbirt's "champion trees", the tallest or largest of their kind in Britain; in 2011 there were 79 of these.
From 2003 to 2005 the arboretum hosted Britain's first designer-led garden festival: Westonbirt Festival of the Garden.
In 2011, Treefest was launched. Following several successful years of the Festival of the Tree event, Westonbirt Arboretum refreshed the popular August bank holiday event with camping, music and more activities celebrating trees and nature. 
Christmas and the spectacle of bare, sculpted trees in winter is celebrated at the Enchanted Christmas event. From the end of November and throughout December, an evening illuminated trail runs throughout the Old Arboretum, highlighting the beauty of Westonbirt's trees in winter.
- "Westonbirt, The National Arboretum". Forestry Commission. Retrieved 9 November 2017.
- Historic England. "Westonbirt (1000457)". National Heritage List for England. Retrieved 9 November 2017.
- "Westonbirt's timeline". Forestry Commission. Retrieved October 25, 2011.
1292 - earliest evidence for coppicing Silk Wood; 1309 - first recorded use of name Weston Birt – Weston derived from location of settlement west of Roman Bowldown Road, Birt from the Bret family, then lords of the manor
-  Westonbirt Festival of the Garden
-  Treefest
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