|Rock Elm, Meise.|
|Natural range of Ulmus thomasii|
Ulmus thomasii, the Rock Elm or Cork Elm, is a deciduous tree native primarily to the Midwestern United States. The tree ranges from southern Ontario and Quebec, south to Tennessee, west to northeastern Kansas, and north to Minnesota.
Ulmus thomasii grows as a tree from 50–100 feet (15–30 m) tall, and may live up to 300 years. Where forest-grown, the crown is cylindrical and upright with short branches, and is narrower than most other elms. Rock elm is also unusual among North American elms in that it is often monopodial. The bark is grey-brown and deeply furrowed into scaly, flattened ridges. Many older branches have 3-4 irregular thick corky wings. It is for this reason the rock elm is sometimes called the cork elm.
The leaves are 5 – 10 cm long and 2 – 5 cm wide, oval to obovate with a round, symmetrical base and acuminate apex. The leaf surface is shiny dark green, turning bright yellow in autumn; the underside is pubescent. The perfect apetalous, wind pollinated flowers are red-green and appear in racemes < 40 mm long two weeks before the leaves from March to May, depending on the tree's location. The fruit is a broad ovate samara 13 – 25 mm long covered with fine hair, notched at the tip, and maturing during May or June to form drooping clusters at the leaf bases.
Ulmus thomasii is moderately shade-tolerant. Its preferred habitat is moist but well-drained sandy loam, loam, or silt loam soil, mixed with other hardwoods. However, it also grows on dry uplands, especially on rocky ridges and limestone bluffs.
Pests and diseases
There are no known cultivars of Ulmus thomasii, nor is it known to be in commerce. The species is occasionally grown beyond its native range as a specimen tree in botanical gardens and arboreta, for example in northwestern Europe, but not commonly cultivated in northern Europe, being unsuited to the region's more temperate, maritime climate.
Ulmus thomasii was crossed experimentally with Japanese Elm (Ulmus davidiana var. japonica) at the Arnold Arboretum in Massachusetts, but no clones were released to commerce. Seedlings arising from crossings with Siberian elm (U. pumila) at the Lake States Forestry Experimental Station in the 1950s all perished, a classic case of hybrid lethality.
- North America
- Arnold Arboretum, Massachusetts. Acc. no. 444-88.
- Brenton Arboretum, Dallas Center, Iowa. No acc. details available.
- Dominion Arboretum, Ottawa, Ontario, Canada. No acc. details.
- Morton Arboretum, Illinois. Acc. no. 178-84, wild collected from Reedsville, Wisconsin; 843-2005 (Kelleys Island, Erie County, Ohio); 122-2006 (Dixon County, Nebraska).
- Nebraska Statewide Arboretum. No details available.
- Grange Farm Arboretum, Sutton St. James, Spalding, Lincs. UK. Acc. no. 706.
- National Botanic Garden of Belgium, Meise, Belgium. Acc. no. 19800105.
- National Botanic Gardens, Ireland, Glasnevin, Dublin, Ireland. Location: A3 (155)
- Wakehurst Place Garden Wakehurst Place, UK. Acc. no. 1968-48603.
The wood of the rock elm is the hardest and heaviest of all elms, and where forest-grown remains comparatively free of knots and other defects. It is also very strong and takes a high polish, and consequently was once in great demand in America and Europe for a wide range of uses, notably shipbuilding, furniture, agricultural tools, and musical instruments.
Much of the timber's strength is derived from the tight grain arising from the tree's very slow rate of growth, the trunk typically increasing in diameter by < 2 mm a year. Over 250 annual growth rings were once counted in a log 24 cm square being sawn for gunwales in an English boatyard, while a tree once grown at Kew Gardens, London, attained a height of only 12 m in 50 years.
- "Ulmus Thomasii Range Map". United States Geological Survey. Retrieved 2008-03-02.
- This name had been used in 1800 for a different species of elm, hence the need for the later renaming that honored Thomas.
- Photographs of mature Rock Elm showing narrow profile: Virginia Tech Department of Forest Resources , Natural Resources of Canada, tidcf.nrcan.gc.ca  
- Bean, W. J. (1981). Trees and shrubs hardy in Great Britain, 7th edition. Murray, London.
- Photograph of corky ridges of Rock Elm branches, Michigan State University Plant Encyclopedia 
- White, J & More, D. (2003). Trees of Britain & Northern Europe. Cassell's, London.
- Hans, A. S. (1981). Compatibility and Crossability Studies in Ulmus. Silvae Genetica 30, 4 - 5 (1981).
- Sholtz, H. F. (1957). Rock Elm (Ulmus thomasii). Lake States Forest Experimental Station Paper 47:16.
- Elwes, H. J. & Henry, A. (1913). The Trees of Great Britain & Ireland. Vol. VII. pp 1848-1929. Private publication.