Photo: JDMcGreg, CC BY-SA 3.0 · Wikimedia Commons

Rock Elm

Ulmus thomasii

Also called cork elm.

The elm with the hardest, strongest wood, a tall tree of limestone ridges and rich woods with corky-winged branches and seed that wildlife relish.

RareWildlife value

Type: Large tree
Mature height: 70–90 ft
USDA zones: 2–9
Growth rate: Slow
Sun: Full, Part
Soil moisture: Dry, Average
Grows wild in: Rocky ground, dry uplands, rich moist slopes and stream banks
Native range: 23 states, north to Minnesota, south to Arkansas and west to South Dakota
List all 23
Arkansas, Connecticut, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Nebraska, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, South Dakota, Tennessee, Vermont, West Virginia and Wisconsin
Also recorded in 1 other state, outside its native range. See what is native in your state

Hardiness zones Find your zone by ZIP code

Rock elm, also called cork elm for the thick, corky wings on its older branches, is a tall elm with a narrow, upright crown. It is most common in lower Michigan, Wisconsin and southern Ontario, on moist loam among other hardwoods and on rocky ridges and limestone bluffs. Its wood has long been valued for its strength, and much of the old growth was exported for ship timbers. Dutch elm disease has cut it back across its range. Illinois lists it as endangered, and Vermont and New York list it as threatened. Plant it as the long-term shade tree of a limestone lot, or among other hardwoods in a woodlot.

Size, growth and what to expect

Rock elm grows slowly. Seedlings planted out among competing growth in northern Wisconsin averaged 20 inches after ten years. Our growth estimator allows about a foot a year on an average site. It reaches 70 to 90 feet with a narrow, upright crown and lives 250 to 300 years. Few trees recover as well after decades in shade.

The flowers open from March to May, about two weeks before the leaves, and the hairy samaras ripen in May or June. Trees bear viable seed from about 20 years old, with good crops every 3 or 4 years. Chipmunks, mice and squirrels relish the nutty seeds and often eat most of a crop. The leaves feed mourning cloak caterpillars.

It grows wild from New Hampshire, Vermont and southern Quebec west to Minnesota and South Dakota, and south to Kansas, Arkansas and Tennessee. In Wisconsin the number of live rock elms fell by more than half from 2004 to 2018.

How to grow Rock Elm

Rock elm grows best in full sun or light shade on a deep, moist loam that drains well. It also takes dry, rocky and limestone soils, from slightly alkaline to strongly acid. Plant it 30 feet from a wall and more than 50 feet from overhead wires, and set trees 30 feet apart.

Saplings ship bare-root or potted. Plant them dormant, from leaf fall until bud swell, in a hole wider than the roots and no deeper. Set the root flare at the soil line, put the native soil back in, and keep the mulch clear of the trunk. Pick the samaras off the tree in May or June, since chipmunks and mice take the ones that fall. The seed needs no chilling. Sow it at once, a quarter inch deep, and keep the bed moist until the seedlings are up. A seedling often makes only one pair of leaves in its first summer.

Water weekly through the first two summers. Deer, rabbits and squirrels eat the buds, so guard young trees for the first few years. Keep one leader as it grows.

Each flower carries both male and female parts, and rock elm is self-fertile, so a lone tree can set seed. Dutch elm disease, carried by elm bark beetles, is the great threat. It kills large, mature trees and often spares saplings, and it is expected to greatly reduce rock elm. Frost often kills the flowers, so some years bring no seed.

Heights and zones are typical ranges for the species in cultivation; local conditions and seed source shift them.

Sources (13)