Slippery Elm in the Herbal Marketplace - Past, Present & Future
by Eric P. Burkhart, PhD
Slippery Elm (Ulmus rubra Muhl.) is one of the most well-known, and widely used, medicinal tree barks native to the United States. It is currently included on the United Plant Savers “At-Risk” List due to concerns over the continued industry dependence on wild harvested material to fill market demand.
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. It 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) (Braun and Cohen, 2010).
Native Americans utilized a variety of tree barks in their pharmacopoeia (Moerman, 1998). Slippery elm was one of a few (including sassafras and black cherry) to be accepted by European settlers. It has subsequently become a commonly traded bark in the US herbal marketplace. 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 (AHPA, 2012).
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 (Cooley and Van Sambeek, 2016). 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 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).
In the nineteenth century, the de-barking of slippery elm in the eastern United States for medicine was apparently commonplace. In his published profiles of American Forest Trees (1913), for example, 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 stripping of live trees for bark continues today in parts of Appalachia. Disappointingly, there have also been a number of visible “poaching” incidents reported during the past decade (e.g., Associated Press, 2006, Crawford, 2007, Toncray, 2012, USFWS, 2016).
Attempts to cultivate trees for bark have been sporadic and inadequate. The amount of cultivated material reported in AHPA surveys is a small percentage of the overall trade volume (e.g., generally less than 10,000 pounds) (AHPA 2012). Moreover, there have been no published scientific growing trials to guide slippery elm plantation establishment and management nor to provide insights into any differences in bark yield, quality, and chemistry as a result of tree diameter. Any transition to cultivated sources has undoubtedly been hindered by the long time horizon involved until crop ‘maturity’ coupled with a relatively low price offered for raw product. 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 (Associated Press, 2006, Toncray, 2012); however, tree diameter and age are not provided in these reports, and both would clearly influence yields.
Despite the long history of exploitation, slippery elm is considered ‘secure’ or ‘un-ranked’ within its native range of the central and eastern United States (NatureServe, 2016). However, there are additional threats beyond bark harvesting which are rapidly inflicting havoc on wild populations and ultimately threaten wild abundance and supply to the herbal trade. It is well known that slippery elm succumbs to many of the same diseases as American elm–most popularly Dutch elm disease (Ceratocystis ulmi). Increasingly, however, slippery elm is also being killed by ‘elm yellows’ or ‘elm phloem necrosis’ (Candidatus Phytoplasma ulmia, a wall-less bacteria called a phytoplasma). These two diseases are so virulent and widespread that slippery elm seldom reaches commercial size and volume today.
As the ‘newcomer’ of these two elm diseases, the origin of elm yellows remains a mystery. It was first described in the Midwestern United States in the 1930s, but it may have been present much earlier (Sinclair, 2000). The only known host of elm yellows is elm (Ulmus spp.). There is no known resistance in North American native elms, but some Eurasian species appear to be tolerant or resistant. In the United States the disease is transmitted by the white-banded elm leafhopper (Scaphoideus luteolus) (and possibly other insects) that feed on the sap. Slippery elm usually dies within a year or two after symptoms appear (U.S. Department of Agriculture, 2012).
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 odor 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 odor similar to maple syrup (Gibson et al.,1981).
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 lowered quality, especially if the smell if “off” (i.e., smells like wintergreen or maple syrup) and/or is discolored. 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.
In coming years, there will be an urgent need for herbalists, bark harvesters, herb traders, and academia to work together to observe, gather and share information about elm yellows throughout the eastern United States. There is little awareness or effort being directed around this developing problem, 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. Grass roots cooperative efforts might include proactive monitoring networks, seed distribution from possible resistant trees (if/when they are found), and cooperation between buyers and harvesters to more closely examine questions regarding differences in bark quality as a function of degree of disease infection symptoms. The United Plant Savers member and business network could potentially play a bridging role in these efforts.
Slippery elm is a tree with a long tradition of use and exploitation in the United States. Despite historical harvest pressures, recent poaching incidents, and a lack of widespread cultivation, the species is presently considered “secure” within much of its range. This conservation 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 thwart any possible supply crisis. The future of slippery elm may be slipping away if we as a community of plant stewards don’t begin to pay attention to what is happening with slippery elm in the wild and work together to conserve, and make wise use of, this important North American medicinal tree.
Slippery Elm Harvest: Stewardship Guidelines
Remarkably given its commercial importance to the herbal industry, there are no published studies to help guide ‘sustainable’ slippery elm harvests. The following suggestions are therefore based on available publications, information, and common sense:
There is no accepted season for harvesting slippery elm bark. Some bark harvesters will peel bark anytime of the year. It tends to peel easiest from late March to early June while the sap is flowing, however. This makes sense because sap and bark harvested in the spring should be especially full of mucilage, sugars, and nutrients as the tree prepares to break bud.
Diameter is a better indicator of bark yield than age. Some bark buyers claim that ten-year-old bark is the best. Keep in mind that the diameter of a tree is not always strongly correlated with age. Trees that are “open grown” often increase in diameter more rapidly than trees under competition or on poor sites.
Slippery elm bark is used most often in rossed form. Rossed means that the rough outer bark is removed by scraping or peeling it from the inner bark while the bark is still fresh. High quality, rossed bark will be smooth, leathery, and creamy white in color. The following video of elm bark stripping shows how skilled and labor intensive this process of rossing elm bark can be: https://www.youtube.com/watch?v=N-GxB34BjlA
Only remove thin vertical strips from live trees. It is possible to harvest the bark of slippery elm by removing only segments of bark at any given time. However, when one girdles the tree, it is likely to die. The inner layers of the bark provide for the flow of water and nutrients throughout the tree, and this process is cut off when the bark is completely or mostly removed. From a commercial perspective, it will take more time and effort to harvest only thin sections of live standing trees.
Choose dying trees for harvest. Unfortunately, an increasingly common site in the landscape are losses due to Dutch-elm’s and Elm Yellows. Keep an eye on local slippery elm trees and watch for signs of disease. The bark should be harvested before or soon after symptoms are first observed. Infected trees will decline rapidly, and the inner bark quality will as well.
Choose ‘over stocked’ locations to harvest from. Thinning a forest stand by removing trees that appear to be less vigorous or closely spaced together can help the residual trees by reducing stress due to competition. This will not only help improve the future elm stand, but a product can still be harvested on an annual basis.
Bark drying requires heat and airflow. If you choose to dry the bark, lay strips of bark flat and separated in a dry area and turn daily to prevent molding. Hanging strips of bark from building rafters also works well. Depending on the drying location and weather conditions, slippery elm bark should be dry in less than a week. Once the bark has dried, fold it into strips for storage or sale.
Eric Burkhart, PhD, is an ethnobotanist interested in the husbandry, conservation and supply chain aspects of native Appalachian medicinal forest plants. His work and research program (at Penn State University) is focused on developing sustainable medicinal crop management and production systems in Pennsylvania and the mid-Atlantic region through agroforestry and plant husbandry practices.
American Herbal Products Association. 2012. Tonnage Surveys of Select North American Wild-Harvested Plants, 2006–2010. Silver Spring (MD): American Herbal Products Association.
Associated Press. 2006. Slippery Elm Trees Fall Victim to Poachers. Available at: www.nbcnews.com/id/14272015/ns/us_news-environment/t/slippery-elm-trees-fall-victim-poachers/
Braun, L. and M. Cohen. 2010. Herbs and Natural Supplements: An Evidence-based Guide, Third Edition. Elsevier, Australia.
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
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
Gibson, H.H. 1913. American Forest Trees. Hardwood Record. Chicago, IL.
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
Moerman, D.E. 1998. Native American Ethnobotany. Timber Press, Inc. Portland, OR.
NatureServe. 2016. Comprehensive Report
Species - Ulmus rubra. Available at: http://explorer.natureserve.org
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.
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
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
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