Rock elm
Rock elm, Meise.
Conservation status
Least Concern (IUCN 3.1)[1]
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Rosids
Order:	Rosales
Family:	Ulmaceae
Genus:	Ulmus
Subgenus:	U.subg. Oreoptelea
Section:	U.sect. Chaetoptelea
Species:	U. thomasii
Binomial name
Ulmus thomasii Sarg.
Natural range ofUlmus thomasii
Synonyms[2]
Ulmus racemosaThomas

Ulmus thomasii, therock elm^[3] orcork elm (ororme liège inQuébec), is adeciduous tree native primarily to theMidwestern United States. The tree ranges from southernOntario andQuebec, south toTennessee, west to northeasternKansas, and north toMinnesota.^[4]

The tree was named in 1902 for David Thomas, an Americancivil engineer who had first named and described the tree in 1831 asUlmus racemosa.^[5]

Ulmus thomasii grows as a tree from 15–30 m (50–100 ft) tall, and may live for up to 300 years. Where forest-grown, the crown is cylindrical and upright with short branches, and is narrower than most other elms.^[6] Rock elm is also unusual among North American elms in that it is oftenmonopodial.^[7] Thebark 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.^[8]

Theleaves are 5–10 cm (2–4 in) long and 2–5 cm (3⁄4–2 in) wide,oval toobovate with a round, symmetrical base and acuminate apex.^[9] The leaf surface is shiny dark green, turning bright yellow in autumn; the underside ispubescent. Theperfectapetalous,wind-pollinatedflowers are red-green and appear in racemes up to 40 mm (2 in) long two weeks before the leaves from March to May, depending on the tree's location. Thefruit is a broad ovatesamara13–25 mm (1⁄2–31⁄32 in) long covered with fine hair, notched at the tip, and maturing during May or June to form drooping clusters at the leaf bases.^[10]

AlthoughU. thomasii isprotandrous, levels of self-pollination remain high.^[11]

• 
  U. racemosa [:U. thomasii] diagnostic illustration (1865)
• 
  U. racemosa [:U. thomasii] diagnostic illustration (1900)
• 
  U. thomasii shoot and buds (1920)
• 
  U. thomasii new leaves, Meisse
• 
  U. thomasii August leaves on short shoot,Jamesville, New York
• 
  U. thomasii leaves, Sir Harold Hillier Gardens, England
• 
  U. thomasii bark, Meisse
• 
  U. thomasii young bark
• 
  U. thomasii corky twig, Meisse

Ulmus thomasii is moderately shade-tolerant.^[12] Its preferred habitat is moist but well-drained sandy loam, loam, or silt loam soil, mixed with otherhardwoods. However, it also grows on dry uplands, especially on rocky ridges andlimestone bluffs.

Like most North Americanelms,U. thomasii is very susceptible toDutch elm disease.

There are no knowncultivars ofUlmus thomasii, nor is it known to be any longer in commerce. It appeared in some US nursery catalogues in the early 20th century.^[13][14][15] It has, however, been planted as a street tree inCheyenne, Wyoming, where in tight lines it keeps a monopodial habit recallingJersey elm.^[16] The species is occasionally grown beyond its native range as a specimen tree inbotanical gardens andarboreta, for example in northwestern Europe, but not commonly cultivated in northern Europe, being unsuited to the region's more temperate, maritime climate. However, the tree was propagated and marketed in the UK by the Hillier & Sons nursery,Winchester, Hampshire, from 1965 to 1977, during which time 49 were sold.^[17][18]

Ulmus thomasii was crossed experimentally withJapanese elm (U. davidiana var.japonica) at theArnold Arboretum inMassachusetts, but no clones were released to commerce.^[11] Seedlings arising from crossings withSiberian elm (U. pumila) at the Lake States Forestry Experimental Station in the 1950s all perished,^[19] a classic case ofhybrid lethality.^[20]

The US National Champion, measuring 100 ft (30 m) high in 1989, grows inCass County, Michigan.^[21]

Thewood 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, notablyboatbuilding,furniture,agricultural tools, andmusical 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 less than2 mm (3⁄32 in) a year. Over 250 annual growth rings were once counted in a log24 cm (9+1⁄2 in) square being sawn forgunwales in an English boatyard, while a tree once grown atKew Gardens,London, attained a height of only 12 m (39 ft) in 50 years.^[22]

North America

• Arnold Arboretum,Massachusetts, US. Acc. no. 444-88.
• Brenton Arboretum,Dallas Center,Iowa, US. No acc. details available.
• Dominion Arboretum,Ottawa, Canada. No acc. details.
• Morton Arboretum,Illinois, US. Acc. no. 178-84, wild collected fromReedsville, Wisconsin; 843-2005 (Kelleys Island,Erie County,Ohio); 122-2006 (Dixon County,Nebraska).
• Nebraska Statewide Arboretum, US. No details available.

Europe

• Grange Farm Arboretum,Lincolnshire, UK. Acc. no. 706.
• National Botanic Garden of Belgium,Meise, Belgium. Acc. no. 19800105.
• National Botanic Gardens, Ireland,^[23]Glasnevin, Ireland. Location: A3 (155)
• Sir Harold Hillier Gardens,Hampshire, UK. Acc. no. 2008.0419, wild collected in Ontario, Canada, by Kristl Walek
• Wakehurst Place GardenWakehurst Place, UK. Acc. no. 1968-48603.

Wikimedia Commons has media related toUlmus thomasii.

• U. thomasii, Flora of North America, www.efloras.org
• Brian Pruka,Ulmus thomasii: 'The Hard Elm That's Hard to Find',Arnoldia, Volume 74, Issue 1, 2016

• IUCN Red List least concern species
• Ulmus
• Trees of Northern America
• Elm species and varieties

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