Ulmus thomasii aka Cork Elm

Ulmus thomasii, or rock elm, is a deciduous tree. It belongs to the elm family, Ulmaceae. Other names include cork elm, hickory elm, and white elm. Its native range spans southeastern Canada and the central and eastern United States. It grows in a temperate climate and is not native to Britain. Wild sites include rich woods, rocky slopes, and river cliffs. It also grows on gravelly uplands and low clay soils.

The tree reaches about 70–80 feet [21–24 metres] tall. Another account gives a height of 98 feet [30 metres]. Its crown is upright and narrower than that of American elm. The Chicago Botanic Garden describes its shape as oval. Growth is slow, even within its native range. Gray-brown bark has deep grooves and flat, crossing ridges. Slender, red-brown twigs follow a zigzag course. Some develop uneven cork ridges after one or two years. The buds are slender, oval, and red-brown.

Leaves grow in an alternate pattern along the twigs. Each leaf is simple, with an oval to elliptic shape. Leaves measure 2.5–4 inches long and have doubly toothed edges. The two sides of the leaf base are uneven. Their upper surface is smooth and dark green. The underside is paler and bears some soft hairs. Small flowers hang in short, dense clusters before leaves appear. Virginia Tech describes red-green flowers in early spring. Plants For A Future gives April as the flowering month. Chicago's archived entry instead lists July to August. Wind carries the pollen. The thin, winged fruits are samaras, about half an inch long. They have fine hairs and ripen in spring, with May reported for seeds.

Grow rock elm in full sun for its preferred exposure. It also tolerates light shade. Deep, fertile, moist soil suits it best. It accepts sand, loam, or clay where drainage is good. It tolerates poor soil and mildly acid through mildly alkaline conditions. Chicago lists USDA zones 5–9 and moderate watering. Plants For A Future reports poor growth in Britain despite cold hardiness. Dutch elm disease threatens this species, and beetles spread the disease.

Rock elm can form the canopy of a woodland garden. Its hard, heavy wood resists splitting and has some flexibility. Uses include furniture, farm tools, and wheel hubs. Plants For A Future lists leaves as edible, raw or cooked. It reports no known hazards and no known medicinal uses. These reports do not establish safety for pets.