Given the commercial importance of slippery elm within the herbal industry, it is perhaps surprising that there are no published studies to help guide ‘sustainable’ harvests. The following guidance is based on available publications (mostly forestry-related), harvester experiences, and common sense:
There is no accepted season for harvesting slippery elm bark
Some bark harvesters will peel bark any time of the year. However, bark tends to peel easiest from late March to early June while the sap is flowing since spring bark should be especially full of mucilage, sugars and nutrients as the tree prepares to break bud. Some claim that the spring bark has the highest concentration of phytochemicals and nutrients but there is no scientific information to back this claim.
Diameter is a better indicator of bark yield than age
Some bark buyers claim that ten-year-old bark is the best. While this may be true, keep in mind that the diameter of a tree is not always correlated with age. Trees that are ‘open grown’ often increase in diameter more rapidly than trees under competition or on poor sites. Thus, a smaller-diameter tree is not always younger than a large-diameter tree, and so age can be very difficult to predict based on tree size.
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 while the bark is still fresh. High quality, rossed bark will be smooth, leathery and creamy white in colour. 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.
Limit bark removal to the branches of live trees
Avoid harvesting bark from the main stem(s) and trunks of trees. Instead, limit the harvest to branches of slippery elm where the diameter is large enough to be practical, as part of regular maintenance activities such as pruning, training, and thinning.
Choose dying trees for harvest
Unfortunately, an increasingly common sight in the landscape is 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 moulding. 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.