Ulmus thomasii, the rock elm or cork elm, is a deciduous tree native primarily to the Midwestern United States. There are no known cultivars of Ulmus thomasii, nor is it known to be any longer in commerce.
Ulmus thomasii grows as a tree from 15â30 m (50â100 ft) tall, and may live for up to 300 years. Where forest-grown, the crown is cylindrical and upright with short branches, and is narrower than most other elms. Rock elm is also unusual among North American elms in that it is often monopodial. The bark is grey-brown and deeply furrowed into scaly, flattened ridges. Many older branches have 3â4 irregular thick corky wings. It is for this reason the rock elm is sometimes called the cork elm. The leaves are 5â10 cm (2â4 in) long and 2â5 cm (3â4â2 in) wide, oval to obovate with a round, symmetrical base and acuminate apex. The leaf surface is shiny dark green, turning bright yellow in autumn.