Rock Elm grows with a straight trunk 50 to 100 feet high and 1-1/2 to 3 feet in diameter. The crown is narrow and cylindrical with short drooping branches. The largest known Rock Elm in Minnesota is in Kandiyohi County measuring 113 feet high, 61 foot crown spread, 107 inches in circumference and scoring 235 points. The national champion is in St. Louis County MO measuring 79 feet high, 76 foot crown spread and 196 inches in circumference, scoring 294 points.
The bark is gray and deeply furrowed on mature trees. Twigs are stout, reddish brown with fine surface hair, particularly in the spring. Older twigs can have corky wings (hence the alternate name of "cork elm"). Buds are large, sharply pointed with reddish-brown overlapping scales and darker coloration along the scale margins. There is one terminal bud that is always a leaf bud and the lateral leaf-buds appress close to the twig at the top of a leaf scar. Flower buds are larger, with reddish-brown overlapping scales also.
The leaves are are in two rows on the twigs, alternate, elliptical, up to 3-1/2 inches long and up to 2 inches wide, abruptly tapering to a long point and like many elms, with a rounded but unequally sided base. The margins are doubly saw-toothed except near the base. There are many straight parallel lateral veins. Veins on the underside distinctly stand out from the surface. The upper surface is a shiny deep green while the underside is paler with soft surface hair, but the hair is not tufted along the veins. There will be variability in leaf shape and size. Leaves are bright yellow in the Autumn.
Flowers in the genus Ulmus are bisexual. They appear in clusters, which in this species elongate into pendulous cymes up to 5 cm long (about 2 inches), usually about 10 flowers per cluster. Flowering is about 2 weeks before the leaves appear. The flower calyx is reddish-brown and deeply lobed almost to the middle, into 7 or 8 lobes. Stamens number 5 to 8 with dark purple anthers. The stigma of the style is greenish with fine hair. Flower stalks are yellowish-green. The anthers mature about 2 to 4 days before the female parts of the flower are receptive. This helps with the interchange of pollen between flowers and prevents self pollination of the individual flower.
Seed: The fertile flowers form a flattened elliptic to oval single seeded samara, about 15 to 22 mm long. These are brown at maturity with a narrow papery wing that has fine marginal hair. The samara will usually have hair on the body of the seed cavity also giving the overall surface some fine hair. The seed is inflated but not thickened. The tip of the samara has a shallow notch. These mature rapidly in late spring to early summer and are dispersed by the wind in the immediate vicinity of the tree or can be carried by water some distance. They can germinate immediately, but the germination rate is poor. Seeds may be produced as early as 20 years of tree age, but 45 years is more normal for good sustained seed production.
Habitat: Rock Elm grows best in moist to dry loamy soils of rich woods, stream banks, limestone outcrops and in association with other hardwoods. It is shade tolerant only in the seedling stage.
Names: The genus Ulmus, is the Latin word for Elm. the species name, thomasii, is an honorary for David Thomas, American civil engineer who first described the tree in 1831 and named it Ulmus racemosa. It was renamed U. thomasii in 1902 by the author of the current plant classification - ‘Sarg.’ which is for Charles Sprague Sargent (1841-1927), American botanist, first director of the Arnold Arboretum and specialist on trees of the American forest.
Comparisons: Only the American Elm, U. americana, displays inflorescences that are somewhat pendulous, but those are much shorter and the calyx is only shallowly lobed and only the margin of the samara is hairy, not the seed cavity.
Above: A Rock Elm in fall color. Drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: 1st photo - The upper leaf surface - note the unequal base and the many straight parallel lateral veins. 2nd photo - the underside is pale in color with fine whitish hair throughout, not tufted.
Below: Two views of the pendulous flower clusters.
Below: 1st photo - The bark of mature trees is deeply furrowed into scaly ridges. 2nd photo - Terminal buds are leaf buds, pointed with reddish brown scales. 3rd photo - flower buds are back on the twig and larger.
Notes: Eloise Butler introduced the Rock Elm to the Garden in 1912 with several plants she acquired from Mr. Chase's Nursery in Colorado (surprising she had to go so far for a local tree). She referred to them as 'Cork' Elms. Rock Elm grows in the temperate zone on North America from the eastern side of the Plains States eastward, south as far as Arkansas, Tennessee and West Virginia. Absent in Maine and the Canadian Maritime Provinces. It is concentrated in the Upper Mississippi Valley and Lower Great Lakes regions. In Minnesota it is found mostly in counties on the eastern side of the state with scattered other occurrences in the western half. It is found in about 1/2 of the metro area including Anoka, Hennepin and Ramsey counties.
Uses: The wood is hard and tough. It was used for implements and tool handles and in the 19th century exported to England for use in ship construction.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"