Ulmus thomasii (Rock Elm)
|Also known as:||Cork Elm|
|Life cycle:||perennial woody|
|Habitat:||part shade, sun; hardwood forest, floodplains, river and stream banks|
|Bloom season:||March - May|
|Plant height:||50 to 100 feet|
|Wetland Indicator Status:||GP: FACU MW: FAC NCNE: FAC|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Loose, pendulous racemes up 1¼ inch long from lateral buds on 1-year-old branches, each with 5 to 13 flowers and appearing before leaves emerge. Flowers have no petals, the green to reddish, cone shaped calyx is only about 1/8 inch wide with 5 to 8 roundish, papery lobes that are hairy around the edges. In the center are 5 to 8 erect, white stamens that are longer than the calyx, the stamen tips initially reddish turning purplish black. A 2-parted, greenish-yellow, feathery style is typically recessed in the calyx tube. Flower stalks are up to 3/8 inch long and minutely hairy.
Leaves and stems:
Leaves are simple and alternate, oval to obovate (widest above the middle), 2 to 5 inches long, 1 to 3 inches wide, abruptly tapered to a pointed tip, the base rounded to almost straight across and nearly symmetrical, on a short, hairy stalk. Edges are double toothed, the veins straight and not forking towards the tip. The upper surface is medium to dark green, somewhat shiny, typically hairless and smooth or with stiff, short hairs making it rough to the touch; the lower surface is light green and softly hairy, without tufts of hairs in the vein axils.
Older branches are hairless, the bark turning gray, becoming thick and corky in the fourth year. Older bark is thick and somewhat spongy, vertical ridges are coarse and interlacing with deep furrows between. Trunks are up to 30 inches diameter at breast height.
Fruit is a winged seed called a samara: flat, egg shaped to somewhat oblong, ½ to 7/8 inch long, surfaces softly hairy with a dense fringe of hairs around the edge. The tip can be slightly cleft or often hooked off to one side.
Rock Elm is the least common of our three native elm species but is most common in lowland habitats and floodplains in the southern 1/3 of the state. Like our other native elms, it is susceptible to Dutch Elm Disease, so much so it has nearly disappeared from the landscape. It is similar in many ways to our other elms, but one only has to notice the thick, corky ridged branches to easily distinguish it.
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- Rock Elm tree
- Rock Elm tree
- corky branch
- cross-section of corky bark, with alternating light and dark bands
- corky upper branches
- twig with leaf scars
- reddish branch, corky ridges starting to form
Photos courtesy Peter M. Dziuk taken in Anoka and Hennepin counties.
Have you seen this plant in Minnesota, or have any other comments about it?