Mountain Ash is a name used for several unrelated trees. It may refer to:

• Eucalyptus regnans, the tallest of all flowering plants
• Fraxinus texensis, an Ash tree species in Texas
• Trees in the genus Sorbus in North America (mainly U.S.A.) . In Ireland and Britain it is used exclusively for Sorbus aucuparia which is also commonly known as Rowan. The tree species Sorbus americana is commonly known as the American Mountain-ash.
• Mountain Ash, Rhondda Cynon Taff, a town in South Wales, United Kingdom

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