Rock Elm
Ulmus thomasii

Location in Ontario

Rock Elm grows throughout the Deciduous Forest Region, and further north. Rock Elm is the rarest of the Elm trees in Ontario. It prefers rocky areas. It likes to grow on exposed bedrock.


Rock Elm, University of Guelph Arboretum

Genus Ulmus
The leaves of Elm are asymmetrical. The overall form of Elm trees is an umbrella shape, White Elm with the most pronounced. Elms are monoecious with perfect flowers in dense clusters. They appear in very early spring. Seeds are nutlets contained within a thin wing (samara). Nutlets are deeply notched and fall off in May. In Ontario, this genus includes Rock, Slippery and White Elm. Dutch Elm Disease has killed many elms   (see Tree Diseases).

TREE FACT: Elms flower very early in the spring, second only to Red and Silver Maples.

Landscape Use
A medium sized tree, Rock Elm can reach 25 metres and live to 175 years or more. If an Elm tree is planted, care must be taken to avoid Dutch Elm Disease. Rock Elm is a medium sized tree that has unusual corky branches and an interesting bark pattern.


Corky branches, University of Guelph Arboretum 

Leaves
Rock Elm leaves are simple, with double teeth. The leaves are asymmetrical. In fall, leaves turn yellow to brown.
Bark
Rock Elm bark is dark gray with flat topped ridges. On most trees (but not all), branches have corky layers.
Flowers
Rock Elm is monoecious with perfect flowers. Flowers are tiny and light coloured, and found in dense clusters. They appear in very early spring before the leaves. Flowers are wind pollinated.
Fruit
Rock Elm seeds are nutlets contained within a thin wing (samara). Fruit is formed soon after flowering. Nutlets are deeply notched and fall off in May. Rock Elm produces fruit after 20 years, with good seed crops every 3 or 4 years.

Wood
Rock Elm wood is the hardest and heaviest of the Ontario Elms. Due to its hardness, Elm was favoured for wheels, hockey sticks and boat frames.
Specific gravity: 0.63
Janka Hardness:
1320 lb
Wood Comparison Chart

Genetics
Rock Elm is diploid, with 28 chromosomes.


Rock Elm growing in pavement

Link to United States Forest Service Silvics Manual for Rock Elm.

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