Slippery elm (Ulmus rubra) is one of the most well-known, and widely used, medicinal tree barks native to North America. You can read about why is slippery elm good for you in our slippery elm monograph. The name slippery elm refers to the texture of the inner bark, especially when moistened. The dried bark has historically been mixed with water and applied topically to treat wounds and skin irritations, and internally for sore throat, coughs, and gastrointestinal conditions. The bark contains a complex assortment of chemical and nutritional compounds including mucilage (hexoses, pentoses, methylpentoses), glucose, polyuronides, tannins, starches, fat, phytosterols and various nutrients (calcium, iron, zinc, magnesium, potassium) (1). In herbal practice, slippery elm is used as a demulcent, expectorant, emollient, diuretic, and as a nutritive dietary supplement (2).
North American Native American groups utilised a variety of tree barks in their pharmacopoeia (3). Slippery elm was one of a few (which includes sassafras (Sassafras albidum) and black cherry (Prunus serotina)) to be accepted by North American colonists and descendent communities. It has subsequently become a commonly traded bark in the herbal marketplace. In the nineteenth century, the de-barking of slippery elm in the eastern United States for medicine was apparently commonplace and in his published profiles of American Forest Trees in 1913 (19), Henry H. Gibson wrote:
“The inner bark has long been used for medicinal purposes. It is now ground fine and is kept for sale in drug stores, but formerly it was a household remedy which most families in the country provided and kept in store along with catnip, mandrake, sage, dogwood blossoms, and other rural remedies which were depended upon to rout diseases in the days when physicians were few… The supply is rapidly decreasing. The cut for lumber is the chief drain, but a not inconsiderable one is the peeling of trees for bark. This goes on all over the species’ range and much of it is done by boys with knives and hatchets. It is often hard to find slippery elms within miles of a town, because all have succumbed to bark hunters (page 391-2).”
The sustainability status of slippery elm
Despite the long history of exploitation to supply herbal markets, slippery elm is considered ‘secure’ or ‘un-ranked’ within its native range of the central and eastern United States (4). Slippery elm is a medium-sized tree (60 to 70 feet in height and 24 to 36-inch in diameter) of moderately fast growth that may live to be 200 years old (5). Bark harvesting is usually done on trees or branches greater than one inch in diameter to ensure a reasonable yield from the effort. A study of one-inch diameter wild slippery elms showed age to be 7 to 18 years old, depending on the severity of competition (Ibid). Published studies show a growth rate of around one-half to one-inch increase in diameter over an 8 to 10-year period (ibid).
Attempts to cultivate trees for bark are small, sporadic, and generally insufficient to supply markets with the entire volume demanded. The amount of cultivated material reported in AHPA surveys is a small percentage of the overall trade volume (e.g., generally less than 12,000 pounds) (6,7). Surveys by the American Herbal Products Association (AHPA) provide a glimpse into the volume of slippery elm bark traded in recent years: Between 1999 and 2010, the total quantity of wild harvested dried bark handled was 78,000-353,000 pounds annually (6). More recent trade data suggests a trend towards the inclusion of more cultivated materials, but this is still a relatively small proportion of the total reported trade (7).
There have been no published scientific studies examining proper slippery elm plantation establishment and management nor any to provide insights into any differences in bark yield, quality, and phytochemistry resulting from tree diameter. Any transition to cultivated sources has undoubtedly been hindered by the longer horizon involved until crop ‘maturity’ coupled with a relatively low price offered for raw bark. In recent years, the price paid per pound of slippery elm bark has ranged between $1.00-$5.00. There are reports that suggest that one dozen trees are needed to yield fifty pounds of fresh bark (8, 9); however, tree diameter and age are not provided in these reports, and both would clearly influence yields. The stripping of live trees for bark continues today in parts of Appalachia. Disappointingly, there have been ‘poaching’ or theft incidents reported during the past decade, especially on public lands and natural areas (e.g., 8, 9, 10,11).
Other threats to slippery elm
Aside from wild harvesting, there are additional threats to slippery elm in North America that continue to impact wild populations and therefore future herbal trade supplies. It has long been known that slippery elm may succumb to some of the same diseases as American elm (Ulmus Americana) – perhaps most notably the slippery elm Dutch elm disease (Ceratocystis ulmi). Increasingly, however, slippery elm is also being impacted or killed by ‘elm yellows’ or ‘elm phloem necrosis’ (Candidatus Phytoplasma ulmia, a wall-less bacterium called a phytoplasma). These two diseases are so virulent and widespread that slippery elm seldom reaches commercial size and volume in parts of its range (e.g. central Pennsylvania).
As the more recent of these two elm diseases, elm yellows remain a bit of a mystery. It was first described in the Midwestern United States in the 1930s, but it may have been present much earlier (12). In fact, it may have been introduced into North America in the 1800s (13, 14) but it was only first described in Ohio by Swingle (15). Its initial presence and spread were probably mistaken with Dutch elm disease, as the two diseases can be confused symptomatically.
In addition to slippery elm, natural infections of elm yellows are known to occur in four other North American elm species: American elm (U. americana), cedar elm (U. crassiflolia), September elm (U. serotina), and winged elm (U. alata). Elm species of European or Asiatic origin and hybrids between these and native species are largely regarded as tolerant or immune to elm yellows and such trees survive in localities where epidemics have killed all large native elms.
Elm yellows now occur throughout the eastern half of North America and can appear as localised or regional epidemics. In North America, the disease is transmitted by the white-banded elm leafhopper (Scaphoideus luteolus) and other small insects (16) that feed on the sap. Slippery elm trees frequently die within a year or two after symptoms appear (17), although more tolerant individuals may only become stunted and may develop chlorosis and witches’ brooms. Diseased trees never recover. Foliar symptoms of infected trees usually appear from mid-July to mid-September in the United States and include yellowing, drooping, and premature leaf drop. Symptoms may resemble those caused by water stress or nutrient deficiencies and generally affect the entire crown. An odour of wintergreen oil (methyl salicylate) is often noticed when the moist inner bark of an infected elm is exposed; placing freshly exposed inner bark into an airtight container often serves to concentrate the wintergreen scent. Dying slippery elm may sometimes also emit an odour similar to maple syrup (18).
Trees diagnosed as having elm yellows should be removed promptly because there is no practical control available. From a bark harvesting perspective, it may already be too late to salvage the bark from infected trees by the time symptoms become obvious. The reason is that the bark may be of lower quality, especially if the smell is ‘off’ (i.e., smells like wintergreen or maple syrup) and/or is discoloured. This underscores the need for proactive monitoring of wild or cultivated trees, and timely harvesting of tree bark once elm yellows is detected in the region or area. Epidemics of elm yellows, although locally impactful, do not spread rapidly; an outbreak can persist within a region for many years without advancing to adjacent areas.
The future of slippery elm
In coming decades, there will be an urgent need for herbalists, harvesters, traders, and academics to work together to observe, gather, and share information about elm yellows throughout eastern North America. There is limited awareness or effort being directed at this topic, with the dire possibility that slippery elm may soon be reduced to seedling/sapling stature across its range. If or when this happens, the ability to procure raw materials will be severely curtailed with a likely spike in demand (and price) resulting in increased harvest pressure on remaining large healthy trees. Grassroots cooperative efforts might include proactive monitoring networks, seed distribution from possible resistant trees if they are found, and cooperation between buyers and harvesters to examine questions more closely regarding differences in bark quality as a function of degree of disease infection symptoms.
Slippery elm is a tree with a long tradition of herbal use and exploitation in North America. Despite historical harvest pressures, recent “headline” theft incidents, and a lack of widespread cultivation, the species is presently considered ‘secure’ within much of its range. This status may change in coming years with the spread of diseases such as elm yellows and highlights a real need for the herbal community to proactively work together to avoid any possible supply crisis, and loss of this important species from the North American flora. The future of slippery elm may in fact be slipping away if there is no collective and urgent action to conserve and make wise use of, this important North American medicinal tree.
To read about how to sustainably harvest slippery elm, see the “easy slippery elm sustainable harvest guide” by Eric P. Burkhart.
References
- Braun, L. and M. Cohen. 2010. Herbs and Natural Supplements: An Evidence-based Guide, Third Edition. Elsevier, Australia.
- Wood, M. 2009. The Earthwise Herbal: A Complete Guide to New World Medicinal Plants. North Atlantic Books. Berkeley, CA.
- Moerman, D.E. 1998. Native American Ethnobotany. Timber Press, Inc. Portland, OR.
- NatureServe. 2023. Comprehensive Report Species – Ulmus rubra. Available at: http://explorer.natureserve.org
- Cooley, J.H. and J. W. Van Sambeek. 2016. Ulmus rubra, Slippery elm. In: Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654 (1990) (Tech Coord.: Burns, R.M. and B.H. Honkala). U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2. Available at: http://na.fs.fed.us/spfo/pubs/silvics_manual/volume_2/ulmus/rubra.htm
- AHPA – American Herbal Products Association. 2012. Tonnage Surveys of Select North American Wild-Harvested Plants, 2006–2010. Silver Spring (MD): American Herbal Products Association.
- AHPA – American Herbal Products Association. 2020. Tonnage Surveys of Select North American Wild-Harvested Plants, 2011–2017. Silver Spring (MD): American Herbal Products Association.
- Associated Press. 2006. Slippery elm trees fall victim to poachers. Available at: http://www.nbcnews.com/id/14272015/ns/us_news-environment/t/slippery-elm-trees-fall-victim-poachers/
- Toncray, M. 2012. Five arrested for stripping bark from trees. Available at: http://www.maysville-online.com/news/five-arrested-for-stripping-bark-from-trees/article_efaca1b0-ed1d-54a5-a5db-c2228744b3c6.html
- Crawford, B. 2007. Bark thieves taking bite out of slippery elms. Available at: http://archive.courier-journal.com/article/20070530/COLUMNISTS04/705301300/Bark-thieves-taking-bite-out-slippery-elms
- U.S. Fish and Wildlife Service. 2016. Forest Service increases watch for illegal taking of slippery elm bark. Available at: http://www.fs.usda.gov/detail/dbnf/news-events/?cid=FSEPRD495630
- Sinclair, W.A. 2000. Elm Yellows in North America. In: The Elms: Breeding, Conservation and Disease Management (Ed. C.P. Dunn). Kluwer Academic Publishers, Boston.
- Baker, W.L. 1948. Transmission by leaf hoppers of the virus containing phloem necrosis of American elm. Science. 108: 307-308.
- Marcone, C. 2016. Elm yellows: A phytoplasma disease of concern in forest and landscape ecosystems. Forest Pathology. Available at: http://dx.doi.org/10.1111/efp.12324.
- Swingle, R.W. 1938. A phloem necrosis of elm. Phytopathology 28: 757-759.
- Rosa, C., Mc Carthy, E., Duong, K., Hoover, G., Moorman, G. 2014. First report of the spittlebug Lepyronia quadrangularis and the leafhopper Latalus sp. as vectors of the Elm yellows associated phytoplasma, Candidatus Phytoplasma ulmi in North America. Plant Disease 98(1): 154-155.
- U.S. Department of Agriculture Forest Service. 2012. Pest Alert: Elm Yellows. Northeastern Area NA–PR–04–12. Available at: http://na.fs.fed.us/pubs/palerts/elm-yellows/elm_yellows-pest-alert-120709_high-res.pdf
- Gibson, L.P. et al. 1981. How to differentiate Dutch Elm Disease from Elm Phloem Necrosis. United States Department of Agriculture Forest Service NA-FB/P-11. Available at: http://www.na.fs.fed.us/spfo/pubs/howtos/ht_dednecrosis/ht_dednecrosis.htm
- Gibson, H.H. 1913. American Forest Trees. Hardwood Record. Chicago, IL.
