Trees and Shrubs of the Eloise Butler Wildflower Garden

Slippery Elm is a medium sized native long-lived deciduous tree, with a open, somewhat flat-topped crown composed of many spreading to ascending branches, with less drooping at the ends than American Elm. Trees grow 40 to 80+ feet with a diameter of 1 to 3 feet. The largest known Slippery Elm in Minnesota (2019) is in Hennepin county with a height of 86 feet, a spread of 91 feet and a circumference of 238 inches, which is quite close to the national champion (2019) in Jefferson County KY that is 90 feet high, a spread of 82.5 feet and is 282 inches in circumference.

The bark is a darker brown to reddish-brown, and with age developing deep irregular furrows (not diamond shape). The inner bark is fragrant and mucilaginous, rust-red in color (containing or secreting mucilage which is a viscous or gelatinous polysaccharide substance produced by plants).

Twigs are gray to brownish gray, more stout, slightly zigzag, often with mottling, usually with hair on the young twigs, and are mucilaginous when chewed. Buds are ovate, dark, nearly black or chestnut brown, usually with rusty colored hair on the scale faces. Flower buds are similar in color but round.

The leaves are in 2 rows, alternate, simple, thin, ovate to obovate, 4 to 6 inches long and 2 to 3 inches wide with a short stalk. The margins are double-saw-toothed in upper half to 3/4ths of the leaf, grading to less distinct single sawtooth near the base. The bases are oblique with unequal sides (in-equilateral). Tips are pointed, the lateral veins are parallel with at least 4 of the lateral veins near the base showing forks; the upper surface is dark green and slightly rough (nature's sandpaper) - more so than on American Elm - while the underside is paler and usually with fine hair. Veins on the underside distinctly stand out from the surface with tufts of white hair at the vein intersections. There will be variability in leaf shape and size. A number of leaves will be strongly folded upward along the main leaf vein. There are a pair of linear narrow stipules at the base of the leaf which dry up and fall away early. Fall color is yellow.

Flowers in the genus Ulmus are bisexual. In Slippery Elm they appear clustered along the twigs in small dense clusters, not pendulous, of 8 to 20 in early spring before the leaves. Each flower is only 1/8 inch across on a short 1 to 2 mm stalk. They have a reddish-green calyx of 5 to 9 shallow lobes that are reddish with hairy margins, 5 to 9 stamens with reddish anthers and with the stigmas exserted. Tips of the flower tubes are reddish-pink. They are cross pollinated by the wind.

Seed: The fertile flowers form a flattened nearly round single seeded samara, yellow to cream in color, about 12 to 18 mm long (up to 3/4 inch) wide. These are brown at maturity with a broad papery wing that has smooth edges - no hair. The samara may have hair on the body of the seed cavity. The seed is thickened but not inflated. The tip of the samara has a small notch. These mature rapidly in late spring to early summer and are dispersed by the wind. They can germinate immediately. Seeds may be produced as early as 15 years of tree age, but 40 years is more normal.

Habitat: Slippery Elm likes full sun, fertile soil, but many soil types, and moist to mesic conditions. The root system is shallow and spreading but in dry soils a tap root will develop. It is more shade tolerant than American Elm. It reproduces from seed and stump sprouts or by layering. Unfortunately the tree is somewhat susceptible to the Dutch Elm disease. The species is known to commonly cross with Siberian elm, U. pumila.

Names: The genus Ulmus, is the Latin word for Elm. the species name, rubra, means 'red', referring to the to the rusty-red inner bark, which is also the source of the "slippery elm" and "red elm" names. The author name for the plant classification, from 1793, ‘Muhl’ is for Gotthilf Heinrich Ernst Muhlenberg (1753-1815) American Botanist who produced several catalogues of plants after retiring as a Lutheran pastor.

Comparisons: The most confusing comparison is with U. americana, the American Elm. There the winter buds have less hair, just on the scale edges and the samaras of the seeds have hairy edges and the flowers are stalked. The Siberian Elm, U. pumila, like the Slippery Elm has samaras without hairy edges, and flower buds are also round, with reddish-black scales but the leaves have a mostly symmetrical base with a single saw-toothed margin and are much smaller, only up to 1 inch wide.