Ulmus thomasii (Rock Elm)

Plant Info
Also known as: Cork Elm
Family:Ulmaceae (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):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: indistinct Cluster type: raceme

[photo of flower cluster] 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: Leaf attachment: alternate Leaf type: simple

[scan of leaves] 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.

[photo of twig with leaf and flower buds] Young twigs are hairy, initially green turning reddish brown. Buds are somewhat hairy, pointed at the tip, with brown scales; flower buds are larger and more oval.

[photo of mature trunk] 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: Fruit type: seed without plume

[photo of fruit] 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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More photos

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?

Posted by: Allan Dietz - CHATFIELD
on: 2019-07-08 11:19:29

West side of root river near Chatfield

Posted by: Brian Schreiber - Big Lake Township
on: 2019-09-10 09:55:08

found one VERY similar but must not be. Trying to figure out WHICH Elm it is?! It is early identical to all described in this webpage but 3 things:
1) some veins fork near the end, not 100% of leaf veins go ALL THE WAY to leaf perimeter and
2) branches not cork like as shown here, more smooth, cherry tree appearance
3) not hairy on buds

Based on the main trunk bark it looks like a typical elm tree, not a cherry bark elm, except on the twigs and branches coming off the main trunk. any ideas?

Posted by: Jon Nicholson - New Hartford Twp., Winona County
on: 2022-10-04 16:13:39

We have 1 tree on our hillside (limestone bluffs in SE MN karst area, next to the Mississippi). About 30 feet tall. Double-tooth leaf serrations, leaves about 2"-3", with symmetrical bases. The lower branches are dead, but branches are healthy looking for the top 2/3 of height. I've wondered for years whether or not it has Dutch elm disease, but it seems to keep cranking along. We may have a second tree about 20 yards away, almost as large, but I have to get in there amid the buckthorn to find out for sure.

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