Slippery Elm (Ulmus rubra)

Slippery elm (Ulmus rubra), also called red elm or moose elm, is the second most common North American elm after American elm. It is most famous outside arboricultural circles for its medicinal inner bark, used by Indigenous peoples and herbalists for centuries.

Identification

Mature size: 50–80 feet tall, with a relatively narrow crown spread (40–60 feet).

Form: Single trunk with a broad, somewhat flat-topped or rounded crown. Less vase-shaped than American elm — the branches are more horizontal.

Leaves: 4–8 inches long (the largest of common North American elms), oval with sharply double-serrated margins and an asymmetrical base. Very rough on the upper surface — distinctly sandpapery to the touch, more so than American elm. Underside hairy. Bright yellow fall color.

Bark: Reddish-brown to gray-brown, with vertical furrows that are less deep than American elm. Inner bark is the distinctive feature — when sliced, fresh inner bark is reddish and slimy/mucilaginous when wet. This gives the species both its common name and its medicinal use.

Twigs and buds: Twigs are gray-brown and rough; flower buds are larger and more rounded than American elm, often covered with reddish-orange hairs.

Flowers and fruit: Greenish-red flowers in early spring before leaves; samaras with the seed centered (vs. American elm's offset seed) and lacking marginal hairs — another key field distinction from American elm.

Native range and habitat

Native to most of the eastern and central United States, from Maine to North Dakota and south to Florida and Texas. Prefers moist, deep soils along streams and on lower slopes — it is somewhat more shade-tolerant than American elm and can grow as an understory tree under oaks and hickories.

Susceptibility to Dutch Elm Disease

Slippery elm is moderately to highly susceptible to DED — somewhat less devastated than American elm, but still significantly affected. The species suffers heavily in areas with high disease pressure but persists better in mixed-forest settings where individual trees are not concentrated together.

Slippery elm's somewhat lower urban-tree popularity (compared to American elm) means its losses have been less culturally visible, but ecologically the species has declined substantially across its range.

The famous inner bark

Slippery elm inner bark is the most famous tree-medicine product in North American herbalism:

  • Mucilage: When mixed with water, the inner bark forms a thick, soothing gel due to high polysaccharide content
  • Traditional uses: Sore throat lozenges, cough syrups, digestive remedies, wound dressings, and as an emergency food source
  • Indigenous use: Many Eastern Woodlands tribes used slippery elm bark medicinally; it was also harvested for utilitarian purposes (cordage, baskets)
  • Commercial harvest: Slippery elm bark is still sold as throat lozenges and herbal supplements; the FDA recognizes it as Generally Recognized as Safe (GRAS) for use as an oral demulcent

The harvest pressure has historically been a conservation concern — taking bark from a living tree (girdling) usually kills the tree, so commercial harvest has typically targeted DED-killed trees or relied on cultivated stock.

Other uses

Slippery elm wood is similar to American elm — hard, interlocked, difficult to split. Used historically for:

  • Wagon-wheel hubs and spokes
  • Boat construction
  • Ship pulleys (the interlocking grain made it durable under load)
  • Furniture, especially chair seats

Hybridization concern

Slippery elm freely hybridizes with the invasive Siberian elm (U. pumila) where their ranges overlap in the Midwest. Genetic studies show substantial introgression of Siberian elm genes into native slippery elm populations — a conservation concern for the integrity of native gene pools.

Related pages

References

  • Cooley, J. H., & Van Sambeek, J. W. (1990). "Ulmus rubra Muhl., slippery elm." In Silvics of North America: Hardwoods (Agric. Handb. 654), USDA Forest Service.
  • Brinker, F. (2000). "Slippery elm bark: a review." British Journal of Phytotherapy, 5(4), 175–179.
  • Zalapa, J. E., Brunet, J., & Guries, R. P. (2009). "Patterns of hybridization and introgression between invasive Ulmus pumila (Ulmaceae) and native U. rubra." American Journal of Botany, 96(6), 1116–1128.