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.
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 "slippery elm" and "red elm." 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, likewise 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.
Below: Bark on older trunks becomes deeply furrowed. Twig buds are very dark with lighter color scale margins and with fine hair.
Above: The base of each leaf has a pair of thin narrow stipules. Below: The upper half of the leaf edge has a double sawtooth while near the in-equilateral base several of the lateral veins have forked edges. The veins are prominent on the underside.
Below: There is very fine white hair on the underside leaf veins. Flower buds are round and arranged in two rows on the twigs.
Below: Flowers are bi-sexual. The stamens have reddish anthers fading to black after maturity.
Below: Seeds mature quickly, forming a papery wing to the samara that has a small notch at the tip. The wing is without hair and not inflated.
Slippery Elm is indigenous to the Wildflower Garden, Eloise Butler catalogued it (as Red Elm) on April 29, 1907. There is a large specimen at the north end of the Woodland Garden. The species is widespread in Minnesota, absent only in the counties of original prairie area in the west and southwest of the state. In North America it is found from the Central Plains eastward in the U.S. and in Ottawa and Quebec in Canada.
Uses: The wood of Slippery Elm is coarse and considered inferior to American Elm (which is not considered an important lumber tree either as the wood is used for small wood projects). The two species' wood is frequently mixed together. Uses include wooden boxes, crates, barrels, etc. The mucilaginous substance in the inner bark has been used in folk medicine to treat coughs and diarrhea. The inner bark can be chewed or used as a powder, capsule or decoction. When powdered it was used in throat lozenges, having been listed in the U. S. Pharmacopoeia from 1820 to 1936 and in the National Formulary from 1936 to 1965. Native Americans taught the colonists to use it as a poultice and medication for fevers and diarrhea. The inner bark when soaked in warm water, produces the mucilage that can be used to soften skin and protect it from chapping and to hasten healing of skin conditions. (Ref. #26)
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"