Rock Elm

Ulmus thomasii Sarg.

Ulmus thomasii (note new growth in foreground, older corky twigs in back.
Julie A. Lundgren

Dicotyledoneae (Dicots)
Ulmaceae (Elm Family)
State Protection
A plant listed as Rare by New York State. Removal or damage without the consent of the landowner is prohibited.
Federal Protection
Not Listed
State Conservation Status Rank
Vulnerable in New York - Vulnerable to disappearing from New York due to rarity or other factors (but not currently imperiled); typically 21 to 80 populations or locations in New York, few individuals, restricted range, few remaining acres (or miles of stream), and/or recent and widespread declines.
Global Conservation Status Rank
Secure globally - Common in the world; widespread and abundant (but may be rare in some parts of its range).


Did you know?

Ulmus thomasii is known as "Cork Elm" for the distinctive corky ridges on its twigs and branches. It has the hardest and heaviest wood of the elm species, and is desirable for use in furniture, tools, and fence posts. Another common name, "Rock Elm", may refer to the hardness of the wood or to its preferences for rocky ridgetop habitats (Little 1979).

State Ranking Justification

There are at least 35 existing sites, and about 50 historical sites, mostly known from the 1930s and before. Like our other elm species, Ulmus thomasii is threatened by Dutch Elm Disease.

Short-term Trends

In recent years about over 20 new populations have been discovered.

Long-term Trends

The long-term trend for this species is unknown, though it apparently has persisted in its limited geographical range in the state.

Conservation and Management


Dutch Elm disease is a threat to populations of Cork Elm. Larger trees may also be threatened by logging. The elm zigzag saw fly maybe a future threat.

Conservation Strategies and Management Practices

Care should be taken to avoid cutting the species in logging operations.



In New York, Cork Elm is most often found at dry sites with shallow soils over limestone bedrock, often on ridges or exposed ledges. It may grow with northern hardwood species oak woodlands and forest edges, or in pastures and savannahs (New York Natural Heritage Program 2008). Rocky slopes, limestone outcrops, rich woods, flood plains, stream banks (Flora of North America 1997). Rich upland woods (Gleason and Cronquist 1991). Rich woods and calcareous uplands (Fernald 1970).

Associated Ecological Communities

  • Alvar pavement grassland (guide)
    This community consists of exposed, flat limestone or dolostone pavement with grassy or mossy patches interspersed throughout. Some examples may be solely grassland with no pavement.
  • Alvar woodland (guide)
    A subset of the limestone woodland community restricted to the alvar region in Jefferson County, New York.
  • Appalachian oak-hickory forest* (guide)
    A hardwood forest that occurs on well-drained sites, usually on ridgetops, upper slopes, or south- and west-facing slopes. The soils are usually loams or sandy loams. This is a broadly defined forest community with several regional and edaphic variants. The dominant trees include red oak, white oak, and/or black oak. Mixed with the oaks, usually at lower densities, are pignut, shagbark, and/or sweet pignut hickory.
  • Calcareous red cedar barrens* (guide)
    A small-patch calcareous rocky summit community occurring on dry, south-facing to southwest-facing slopes and low summits. These sites are characterized by stunted, sparse woodlands with small grassland openings.
  • Calcareous talus slope woodland* (guide)
    An open or closed canopy community that occurs on talus slopes composed of calcareous bedrock such as limestone or dolomite. The soils are usually moist and loamy; there may be numerous rock outcrops.
  • Limestone woodland (guide)
    A woodland that occurs on shallow soils over limestone bedrock in non-alvar settings, and usually includes numerous rock outcrops. There are usually several codominant trees, although one species may become dominant in any one stand.
  • Maple-basswood rich mesic forest* (guide)
    A species rich hardwood forest that typically occurs on well-drained, moist soils of circumneutral pH. Rich herbs are predominant in the ground layer and are usually correlated with calcareous bedrock, although bedrock does not have to be exposed. The dominant trees are sugar maple, basswood, and white ash.
  • Northern white cedar rocky summit* (guide)
    A community that occurs on cool, dry, rocky ridgetops and summits where the bedrock is calcareous (such as limestone or dolomite), and the soils are more or less calcareous. The vegetation may be sparse or patchy, with numerous rock outcrops. The species have predominantly boreal distributions.
  • Pastureland
    Agricultural land permanently maintained (or recently abandoned) as a pasture area for livestock.
  • Red cedar rocky summit* (guide)
    A community that occurs on warm, dry, rocky ridgetops and summits where the bedrock is calcareous (such as limestone or dolomite, but also marble, amphibolite, and calcsilicate rock), and the soils are more or less calcareous. The vegetation may be sparse or patchy, with numerous lichen covered rock outcrops.
  • Successional red cedar woodland*
    A woodland community that commonly occurs on abandoned agricultural fields and pastures, usually at elevations less than 1000 ft (305 m). The dominant tree is eastern red cedar, which may occur widely spaced in young stands and may be rather dense in more mature stands.

* probable association but not confirmed.

Associated Species

  • Acer saccharum (sugar maple)
  • Asplenium rhizophyllum (walking fern)
  • Asplenium trichomanes
  • Carex backii (Back's sedge)
  • Carex hitchcockiana (Hitchcock's sedge)
  • Carex willdenowii (Willdenow's sedge)
  • Carya cordiformis (bitternut hickory)
  • Corydalis sempervirens
  • Corylus cornuta ssp. cornuta (beaked hazelnut)
  • Deschampsia flexuosa
  • Dryopteris marginalis (marginal wood fern)
  • Fraxinus americana (white ash)
  • Geranium robertianum (herb-Robert)
  • Hepatica acutiloba (sharp-lobed hepatica)
  • Juglans cinerea (butternut)
  • Juniperus virginiana
  • Laportea canadensis (wood-nettle)
  • Ostrya virginiana (hop hornbeam, ironwood)
  • Phlox divaricata
  • Pinus strobus (white pine)
  • Quercus macrocarpa (bur oak)
  • Quercus muhlenbergii
  • Ranunculus abortivus (kidney-leaved butter-cup, kidney-leaved crow-foot)
  • Solidago caesia
  • Staphylea trifolia (bladdernut)
  • Tilia americana
  • Toxicodendron radicans
  • Tsuga canadensis (eastern hemlock)
  • Ulmus americana (American elm)
  • Zanthoxylum americanum (prickly-ash)


New York State Distribution

Ulmus thomasii is known from scattered locations in most of northern New York. It is rarely found in large stands but seems to occur sporadically in small numbers.

Global Distribution

Cork Elm is found from Quebec and Vermont in the northeast, south and west through New England along the Appalachians as far as Arkansas, and in the Midwest and Plains states as far north as the Dakotas and Ontario. It is most common in the Great Lakes region.

Identification Comments

General Description

Ulmus thomasii is a medium-sized tree, commonly reaching up to 70 to 80 feet in height, and occasionally up to 100 feet (Burns and Honkala 1990), and may live for up to 300 years. It has a strongly upright form and a narrow crown, markedly different from the spreading shape of American Elm (Ulmus americana). The bark of the trunk is furrowed with flattened, spongy ridges, similar to that of American elm. Young twigs are covered in short hairs, and have reddish buds much like those of American Elm, but twigs a year or more old become covered in the distinctive corky ridges that give the plant its name. The leaves are alternate, with doubly-toothed margins and asymmetrical bases, and are smooth to only slightly pubescent. They also tend to be somewhat shiney and papery in feel, unlike those of American Elm or Slippery Elm. The flowers are small and lack petals, occur in racemes up to 4 cm long and appear in early spring before the leaves. The fruit are flattened, round samaras, notched at the top, and covered with soft hairs.

Best Life Stage for Proper Identification

Mature Cork Elm can be identified at any time of year.

Similar Species

Ulmus thomasii is the only elm species native to New York which has corky wings on the older twigs and branches. Not every twig develops the corky wings, however, so it may be necessary to look at several. The other two common elm species in New York, Ulmus americana and U. rubra, also both have smooth fruit, more pubescent, softer leaves (unlike Cork Elm's papery leaves), and are not typically found on the dry, limestone ridges and outcrops favored by Ulmus thomasii.

Cork Elm is very similar to Ulmus alata, a southern species which in New York is known only from cultivation. Ulmus alata has smaller leaves, the largest 4-7 cm long, and both young and old branches may have corky bark.

Best Time to See

This woody plant may be identified year-round using the very unique and characteristic corky bark. Fruits may be present late April through May.

  • Vegetative
  • Flowering
  • Fruiting

The time of year you would expect to find Rock Elm vegetative, flowering, and fruiting in New York.

Rock Elm Images


Rock Elm
Ulmus thomasii Sarg.

  • Kingdom Plantae
    • Phylum Anthophyta
      • Class Dicotyledoneae (Dicots)
        • Order Urticales
          • Family Ulmaceae (Elm Family)

Additional Common Names

  • Cork Elm
  • Winged Elm

Additional Resources


Barnes, B. V. and W. H. Wagner, Jr. 1981. Michigan trees: a guide to the trees of Michigan and the Great Lakes region. Ann Arbor: University of Michigan Press. 384 p.

Burns, R. M., and B. H. Honkala, eds. 1990. Silvics of North America, vol. 2: Hardwoods. U.S. Department of Agriculture, Agriculture Handbook 654, Washington, DC. Accessed 2004.

Edinger, Gregory J., D.J. Evans, Shane Gebauer, Timothy G. Howard, David M. Hunt, and Adele M. Olivero (editors). 2002. Ecological Communities of New York State. Second Edition. A revised and expanded edition of Carol Reschke's Ecological Communities of New York State. (Draft for review). New York Natural Heritage Program, New York State Department of Environmental Conservation. Albany, NY. 136 pp.

Fernald, M.L. 1950. Gray's manual of botany. 8th edition. D. Van Nostrand, New York. 1632 pp.

Gleason, Henry A. and A. Cronquist. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. The New York Botanical Garden, Bronx, New York. 910 pp.

Holmgren, Noel. 1998. The Illustrated Companion to Gleason and Cronquist's Manual. Illustrations of the Vascular Plants of Northeastern United States and Adjacent Canada. The New York Botanical Garden, Bronx, New York.

Little, E.L., Jr. 1979. Checklist of United States trees (native and naturalized). Agriculture Handbook No. 541. U.S. Forest Service, Washington, D.C. 375 pp.

New York Natural Heritage Program. 2010. Biotics database. New York Natural Heritage Program. New York State Department of Environmental Conservation. Albany, NY.

New York Natural Heritage Program. 2024. New York Natural Heritage Program Databases. Albany, NY.

Weldy, T. and D. Werier. 2010. New York flora atlas. [S.M. Landry, K.N. Campbell, and L.D. Mabe (original application development), Florida Center for Community Design and Research University of South Florida]. New York Flora Association, Albany, New York

Weldy, Troy W. and David Werier. 2005. New York Flora Atlas. [S.M. Landry, K.N. Campbell, and L.D. Mabe (original application development), Florida Center for Community Design and Research. University of South Florida]. New York Flora Association, Albany, NY. Available on the web at (


About This Guide

Information for this guide was last updated on: December 30, 2008

Please cite this page as:
New York Natural Heritage Program. 2024. Online Conservation Guide for Ulmus thomasii. Available from: Accessed April 17, 2024.