Carolinian
and Deciduous Forest Trees
of Southern Ontario

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.


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 was accidentally brought to Canada in 1944, via wooden crates. White Elm are the most affected by the disease. Once a tree is infected, nothing can be done. Preventative pesticides can be used but are costly. Development of a disease resistant hybrid has been unsuccessful. White Elm trees once lined many streets in Southern Ontario. Today, only a few stately Elms can still be spotted, indicating some immunity to the disease. Elm trees produce abundant seeds every year, and the seeds easily germinate. This has helped Elm to survive Dutch Elm Disease.
Reference:

Dutch Elm Disease History

Habitat: Rock Elm prefers rocky areas. It likes to grow on exposed bedrock.

Landscape Use: 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.

Leaves: Rock Elm leaves are simple, with double teeth. The leaves are asymmetrical.

FALL COLOUR WATCH: yellow to brown 

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.

Fruit: Rock Elm seeds are nutlets contained within a thin wing (samara). 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.

Bark: Rock Elm bark is dark gray with flat topped ridges. On most trees (but not all), branches have corky layers.


Corky branches, University of Guelph Arboretum 

Size: A medium sized tree, Rock Elm can reach 25 metres and live to 175 years or more.

Genetics:  Rock Elm is diploid, with 28 chromosomes.

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


Rock Elm growing in pavement

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