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Courtesy of  Silvics of North America

Ulmus thomasii Sarg.

Rock Elm

Ulmaceae -- Elm family

T. R. Crow

Rock elm (Ulmus thomasii), often called cork elm because of the irregular thick corky wings on older branches, is a medium-sized to large tree that grows best on moist loamy soils in southern Ontario, lower Michigan, and Wisconsin. It may also be found on dry uplands, especially rocky ridges and limestone bluffs. On good sites, rock elm may reach 30 m (100 ft) in height and 300 years of age. It is always associated with other hardwoods and is a valued lumber tree. The extremely hard, tough wood is used in general construction and as a veneer base. Many kinds of wildlife consume the abundant seed crops.


Native Range

Rock elm is most common to the Upper Mississippi Valley and lower Great Lakes region. The native range includes portions of New Hampshire, Vermont, New York, and extreme southern Quebec; west to Ontario, Michigan, northern Minnesota; south to southeastern South Dakota, northeastern Kansas, and northern Arkansas; and east to Tennessee, southwestern Virginia, and southwestern Pennsylvania. Rock elm also grows in northern New Jersey.

{The native range of Ulmus thomasii}
-The native range of rock elm.


The climatic conditions associated with the distribution of rock elm can be characterized as continental, with cold winters and warm summers. Within the species range, a maximum summer temperature of 38° C (100° F) and a minimum winter temperature of -34° C (-30° F) are common.

Annual precipitation is 640 mm (25 in) in the western part of the range and 1270 mm (50 in) in the extreme southern and eastern parts. At least half of this precipitation occurs during the growing season. Snowfall averages from 50 to 200 cm (20 to 80 in), depending on geographic location.

The frost-free period averages 100 days in the north and 200 days in the south. Rock elm grows best where the frost-free period is from 120 to 160 days.

Soils and Topography

Rock elm is most frequent in lower Michigan, Wisconsin, and southern Ontario, and it is regularly found on moist but well-drained sandy loam, loam, or silt loam soils in mixture with other hardwoods. In Wisconsin, rock elm is most frequent in the southern wet-mesic forest (7). Although rock elm often grows on rocky ridges, limestone outcroppings, and streambanks, the highest quality sawtimber is found on deeper loamy soils.

The major soil orders associated with the distribution of rock elm are the Mollisols, Alfisols, and the Spodosols. Most common are the Hapludalfs (Gray-Brown Podzolic soils) within the Udalfs suborder of the Alfisols. Soil pH ranges from slightly alkaline or neutral to strongly acid.

Associated Forest Cover

Rock elm is a minor component in two forest cover types: Sugar Maple-Beech-Yellow Birch (Society of American Foresters Type 25) and Black Ash-American Elm-Red Maple (Type 39). In addition to type species, other important associates include white ash (Fraxinus americana), black cherry (Prunus serotina), basswood (Tilia spp.), northern red oak (Quercus rubra), American elm (Ulmus americana), eastern hophornbeam (Ostrya virginiana), and eastern hemlock (Tsuga canadensis).

Some of the woody shrubs commonly associated with rock elm include prickly ash (Zanthoxylum americanum), beaked hazel (Corylus cornuta), blackberry and raspberry (Rubus spp.), dogwoods (Cornus spp.), gooseberry (Ribes spp.), Atlantic leatherwood (Dirca palustris), bittersweet (Celastrus scandens), Virginia creeper (Parthenocissus quinquefolia), grape (Vitis spp.), hawthorn (Crataegus spp.), American and redberry elder (Sambucus canadensis and S. pubens), and nannyberry (Viburnum lentago).

Life History

Reproduction and Early Growth

Flowering and Fruiting- Rock elm flowers appear 2 weeks before the leaves at any time from March to May, depending on locality and site. The perfect flowers are protandrous, that is, the male elements of the flower develop 2 to 4 days before the female elements are receptive (6). Female flowers are receptive for only a few days.

The hairy fruit has a broad wing from 13 to 25 mm (0.5 to 1 in) long and matures during May or June. Clean, fully ripened, unwinged seeds number from 11,000 to 19,800/kg (5,000 to 9,000/lb). Seeds germinate soon after they ripen.

Seed Production and Dissemination- Trees 20 years old produce viable seeds, but maximum yields are from trees 45 to 125 years old. Good crops occur every 3 or 4 years. Ripe seeds are dispersed as the leaves become fully expanded, which is usually 2 or 3 weeks later than the time of seed drop for American elm (9).

Although the thin, hair-fringed, winged samaras seem adapted to wind dispersal, seeds are generally carried no more than 40 to 45 m (100 to 150 ft) from the parent tree. The fact that rock elm grows as scattered individuals, often several miles from the nearest seed source, suggests that birds and small mammals play a role in dissemination. The large but very light seeds are also buoyant and water can carry them long distances. As a result, seeds often are concentrated along the banks of streams and lakes.

Seedling Development- Rock elm seeds germinate within a week or two after dispersal if moisture conditions are favorable. In a germination test, 90 to 100 percent of mature seeds were viable (2). Viability was not significantly different between seeds from different trees, between seeds with wings or without wings, or between seeds with seed coats or without seed coats. When germinated in a petri dish, radicles of the viable seeds emerged within 2 or 3 days and were 2.5 to 3.8 cm (1 to 1.5 in) long by the end of the fifth day Germination is epigeal. The cotyledons began to open about the fifth or sixth day. Under favorable conditions, rock elm seedlings are from 5 to 8 cm (2 to 3 in) tall by the end of the first summer.

Despite its high seed viability, rock elm regenerates poorly (2,9). Germination tests failed when mineral soil and equal volumes of peat moss, sand, and mineral soil were used for planting media, but 70 to 80 percent emergence was obtained in flats using peat moss. Another factor affecting seedling establishment is the persistence of dormant terminal buds. Emergent seedlings rarely develop more than a single pair of true leaves during the first growing season due to this dormancy. Observations on more than 200 seedlings indicated that only 1 percent broke dormancy long enough to develop an additional internode and a second pair of true leaves.

This species appears to be shade tolerant during the seedling stage (10). However, under field conditions with competition, 1.5-0 nursery stock averaged only 27 cm (10.6 in) in height 5 years after planting and only 52 cm (20.4 in) 10 years after planting in northern Wisconsin. In the same study, survival ranged from 85 percent at the end of the 1st year to 32 percent at the end of the 10th year.

Vegetative Reproduction- Rock elm regenerates vegetatively from root suckers and stump sprouts (10), but vegetative reproduction in the field is uncommon.

Most elms are considered difficult to root by means of cuttings. However, leaf-bud cuttings, consisting of leaf blade, axillary bud, and a shield of stem tissue, treated with a growth hormone and held under constant mist on a rooting medium of sand or mica, produced satisfactory results for several species of elm including rock elm (5).

Sapling and Pole Stages to Maturity

Growth and Yield- Few species have rock elm's capacity for recovering from prolonged suppression. An analysis of 153 trees indicated that a large percentage had survived under suppression for 50 years or more. This capacity makes it difficult to correlate diameter and age (9):

Crown class

D.b.h. class Dominant Codominant Intermediate Suppressed

2.5 cm or 1 in 14 22 30 48
7.6 cm or 3 in 26 50 64 99
12.7 cm or 5 in 39 72 97 --
17.8 cm or 7 in 51 93 -- --
22.9 cm or 9 in 63 -- -- --

The average number of rings per 2.5 cm (1 in) of radius by crown class was about 50 for suppressed, 30 to 40 for intermediate, 20 to 30 for codominants, and 10 to 20 for dominants.

On average to better sites, mature rock elm may reach 27 in (90 ft) in total height and 61 cm (24 in) in d.b.h. (12). In virgin hardwood stands in the east and north, 27 to 30 in (90 to 100 ft) heights and 91 to 152 cm (36 to 60 in) in d.b.h. have been recorded (10). Much smaller trees occur along river bluffs, on limestone outcrops, or other sites with thin soil mantles. Rock elm may live 250 to 300 years.

Rooting Habit- No information available.

Reaction to Competition- Rock elm is considered shade tolerant in the seedling-sapling stage and often recovers successfully after long periods of suppression at these stages. As the tree grows older, however, it apparently becomes more light demanding. Overall, the species is classed as intermediate in tolerance to shade (10).

Damaging Agents- Nearly all native North American elm species are susceptible to Dutch elm disease (Ceratocystis ulmi) (6,13) and isolates of C. ulmi have been obtained from rock elm logs (5). It is likely that Dutch elm disease will greatly reduce the number of rock elm..

A seed-borne fungus (Gleosporium ulmicolum) has been reported for rock elm but few of the fungi that are able to invade the fruits and seeds of North American hardwoods are thought to be pathogens that reduce germination or weaken seedlings (1).

Although rock elm has not been listed as a particular host for specific insects, undoubtedly it is host to the various borers, defoliators, and sucking insects that attack American elm.

Throughout the range of rock elm, killing frosts are common during the flowering period and subfreezing temperatures may prevent seed development in some years.

Special Uses

The seeds and buds of rock elm are eaten by deer, rabbits, squirrels, and a variety of birds. Small mammals such as chipmunks, ground squirrels, and mice apparently relish the filbertlike flavor of rock elm seed and frequently eat the major part of the crop.

Rock elm wood has long been valued for its exceptional strength and superior quality (3,8). For this reason rock elm has been drastically overcut in many localities. The wood is stronger, harder, and stiffer than any of the other commercial species of elms. It is highly shock resistant and has excellent bending qualities which make it good for bent parts of furniture, crates and containers, and a base for veneer. Much of the old-growth was exported for ship timbers. Currently, the highest quality sawtimber is found in north-central Wisconsin, lower Michigan, and southeastern Ontario.


In a study of compatibility and crossability in Ulmus (11), the form of dichogamy (protandry or protogyny) correlated with the compatibility between the different species. Ulmus thomasii is a protandry species and is compatible with two other protandry species-U. pumila and U. laevis. Ulmus thomasii is also self-fertile.

Literature Cited

  1. Ames, R. W. 1952. Gleosporium ulmicolum reported on fruit of rock elm and variegated English elm. Plant Disease Reporter 36:301.
  2. Arisumi, T., and J. M. Harrison. 1961. The germination of rock elm seeds. American Nurseryman 114:10.
  3. Baudendistel, M. E., and B. H. Paul. 1944. Southern hard elms as substitutes for rock elm. Southern Lumberman 169:211-215.
  4. Brasier, C. M., and J. N. Gibbs. 1973. Origin of the Dutch elm disease epidemic in Britain. Nature 242:607-609.
  5. Bretz, T. W. 1949. Leaf-bud cuttings as a means of propagating disease-resistant elms. Plant Disease Reporter 33:434-436.
  6. Britwum, S. P. K. 1961. Artificial hybridization in the genus Ulmus. In Proceedings, Eighth Northeastern Forest Tree Improvement Conference, New Haven, Connecticut, August 18-19, 1960. p. 43-47. USDA Forest Service, Northeastern Forest Experiment Station, Broomall, PA.
  7. Curtis, J. T. 1959. The vegetation of Wisconsin. University of Wisconsin Press, Madison. 657 p.
  8. Dohr, A. W. 1953. Southern hard elm strength properties compare favorably with rock elm. Southern Lumberman 187:187-188.
  9. Dore, W. G. 1965. Ever tried rock elm (Ulmus thomasii) seeds for eating?. Canadian Audubon 27:90-91.
  10. Fowells, H. A., comp. 1965. Silvics of forest trees of the United States. U.S. Department of Agriculture, Agriculture Handbook 271. Washington, DC. 763 p.
  11. Hans, A. S. 1981. Compatibility and crossability studies in Ulmus. Silvae Genetica 30:149-152.
  12. Hess, L. W., and D. B. Dunn. 1967. Evidence of ecological isolation between Ulmus thomasii Sarg. and Ulmus rubra Muhl. Transactions Missouri Academy of Science 1:31-36.
  13. Smalley, E. B. 1981. Dutch elm disease resistant elm varieties. In Proceedings, Society of American Foresters Wisconsin-Michigan Section Meeting, September 18-19, 1980, Marquette, MI. p. 58-63.
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