Bark medicine, or Todd's Bark Thesis.


On two mailing lists in Apr96:

Todd Caldecott had a neat project on bark medicines. He's asked about them on two mailing lists (that I've seen), and got rather good answers, so I'm putting the whole string here.

The focus of my thesis isn't the harvesting of bark medicines per se, but rather the energetic assessment of them (these are plants from the New World, and aren't listed in any materia medica of the most sophisticated energetic systems eg. Ayurveda, TCM). Harvesting obviously plays a key part and perhaps the time of year in which they are harvested will yield different medicines for different indications. Applying an energetic model may help in discovering the bark's usage. Any and all information is of benefit to me.


On Herbal Hall, in April '96
From Todd Caldecott:

I'm wanting to gather some opinion as to the appropriate time to collect bark medicines. Most conifers, I've heard, can be harvested at almost any time. For deciduous trees, some say in the spring, just before the blossoms open, others say the late spring/summer during the major part of photosynthesization, and still others prefer the fall, as the leaves drop. What's the consensus?

From Howie Brounstein, to above:


I don't know what "the consensus" is, but for me, I like to harvest barks in the spring before the flowers open, when the sap is running from the roots upward, or in the fall as the sap flows down.

from me: Barks: Harvest barks in the spring and fall, when the sap is running. In the spring, the sap runs from the roots up to the aboveground parts of the plant, as it "wakes up" for the year. The reverse is true as it goes dormant for the winter. During these times, the bark should easily peel from the wood. If you have to whittle one little piece of the bark at a time, the sap isn't flowing. Many barks are damp inside with sap and fluids at the proper time. There's the story of the person who moved to Alaska to find freedom, and build his own log house in the process. He felled some appropriate trees and set to the task of stripping the bark off. Being new to the woods, he didn't take anyone's advice to wait for the right time. He immediately set to the grueling task of stripping the bark, chip by chip. After a few months of solid work, fall came about, and the bark on the rest of the trees practically fell off. Hopefully he learned an important principle about plant harvesting practices and accepting advice.

Of course, there are always exceptions. The conifers don't follow this completely, but I bet the bark is still easier to peel in the spring/fall.

And also, there's probably that one tree that someone will bring up that is best harvested at a different time. There always is.

Howie Brounstein

From Christopher Hedley, to above:

I don't remember ever having been taught how to harvest barks- the consequence, no doubt, of the growing trend towards academic education in UK herbal medicine. Our main source of practical herbal knowledge is Mrs Grieve's modern herbal which was written in the 1930s and, of course our own direct experience. We learn that rule no. 1 for harvesting herbs is that there are no rules. Each plant has to be treated differently, according to what the plant wishes and to what we wish the plant to do. My own experience agrees with Howie's ie. I harvest barks when it peels off most easily, usually early summer.

Sap is taken in early spring, when it is most abundant, by boring about 2 inches into the wood. Sap is taken for making wine and syrup, when all that sugar is needed.

Barks are harvested for their inate properties; healing, laxative, astringent etc, and constituents; resins, tannins, mucilage etc. Mrs Grieve gives late spring and early summer except for Elder bark, which is used for its laxative resins and is harvested in the autumn. This practice must come from direct observation. Willow bark is a bit of a problem as it can vary a lot, but more between spp and strains than over the year. I go by taste and tend to cut the older suckers from white willow. I am reluctant to cut from main trunks- except when the tree has been cut down for other reasons.

I also have a tendency to harvest herbs when the spirit gives me a clear indication. A patient of mine showed me a photograph of a fever bark tree in the grounds of Lagos university which had blown down in a gale. By the next day all the bark had been stripped by local people.

And then, on the phytopharmacognosy list in April '96:
From Todd Caldecott:

I am wondering if anyone can say with a certain degree of accuracy when the best time harvest bark is. Traditional practice in herbal medicine has meant harvesting the bark in the spring and fall, when the "energy" of the plant ascends and descends. This, to my understanding of botany, makes some sense, as the nutrients stored in the tissues of the roots rises through the xylem to nourish to the opening buds in spring. However, this would mean that the nutrients are not in the inner bark at all, but within the cambium, and harvesting the bark at this time would be pointless. It rather makes sense to harvest the inner bark during the better part of photosynthesis, when the nutrients are being transported from the leaves to non-photosynthetic cells and roots. This generally means summer and fall. Am I correct in my understanding?

From Robin J. Marles to above:

Dear Todd and others interested in bark medicines:

I would hesitate to generalize about gathering bark, since it isn't necessarily the nutrients that we are interested in. The first question is where in the plant we are taking bark. In terms of tissues, the "inner bark" is a rather loose term presumably including active phloem which transports photosynthate and various other nutrients throughout the plant, the cambium layer which is really a very thin layer of actively dividing cells producing secondary phloem to the outside and secondary xylem to the inside, and perhaps also this active xylem, whose main function is to bring water and dissolved minerals up from the roots. Biologically active compounds would be transported partly by phloem and partly by active transport across other cell membranes, and may also be biosynthesized in situ, so it would depend on the particular species and particular metabolites of interest as to where the highest concentration would be. The outer bark, composed of old phloem and cork cambium (phellogen) that produces cork tissues (phellem to the outside and phelloderm to the inside, collectively called periderm or cork), has lots of bioactive compounds such as tannins or even acetylenes in one bark I studied (Minquartia guianensis Aubl., Olacaceae) stored in a complex mixture that is more stable than the purified constituents. These make effective barriers to penetration by insects or fungi. There are also lots of active compounds in resin ducts present in many species, especially gymnosperms. The wood (old secondary xylem) also has many compounds stored in it, including condensed tannins, and has resin ducts too. Thus we must ask a traditional healer to show us what he collects so we can see what he means by bark or inner bark, etc.

The next question is time of collection, because of course various biochemical processes are seasonal, and this too cannot be generalized. Gymnosperms maintain some physiological activity all year round whenever the temperature gets above the minimum for activity of its enzymes for metabolism. Annual flowering plants devote the first part of their season to rapid growth, the latter to reproduction, and metabolic resources devoted to defense compounds will be primarily for small, potent molecules that take few materials and energy away from primary metabolism. Perennial angiosperms invest more of their metabolic resources in maintenance so we tend to find defense compounds that are larger, more complex molecules. Some are made constantly and stored until needed, others (phytoalexins) such as coumarins and flavonoids which are fairly small, can be made upon demand, i.e. their biosynthesis is induced by chemical signals released when a bug chews on a leaf or fruit, or when a fungus starts to invade tissues. These chemical signals can also pass from one individual plant of a species to its neighbours either as volative chemicals through the air or as soluble chemicals (salicylic acid?) through the roots (the "Talking Trees" hypothesis). If you are interested in these constituents it is better to collect them from an infected plant (accidentally or deliberately, e.g. search the term "hairy root culture" for deliberate bacterially induced production of natural medicines), so you have to know when infect is common.

During active transport of bioactive metabolites they are often in the form of glycosides which makes them more water soluble, and they may be stored as glycosides or as the aglycones. If the compound is not water-soluble it is probably from a storage vesicle somewhere in the plant. Often in a related group of metabolites we can find an increasing degree of oxidation of the compounds the later in the season they are harvested, and that may mean more activity, less activity, or different activity, depending on where in the molecule they are oxidized. I found this to be a critical point in studying the antimigraine constituents of feverfew, Tanacetum parthenium (L.) Schultz-Bip., Asteraceae). Sesquiterpene lactones that were more oxidized in particular positions on the molecule showed more specific antiplatelet aggregation activity and less broad-spectrum toxicity than the primary active ingredient, parthenolide. There have also been a few papers published on diurnal variation in bioactive phytochemicals, and that certainly fits in with traditional medicine which in some cases recommends harvest at night, under a full moon, early morning, etc.

Sorry I can't give you a short sweet answer to your question about when and what part of the bark to collect, but unfortunately nature resists being pigeon-holed, and every generalization has its significant exceptions. Barring scientific evidence, go with the empirical evidence of the traditional healer, who has often been proven correct although perhaps his reasoning is different from ours. His knowledge is really scientific too, because the essence of the scientific method is reproducibility, and all that differs is the interpretation of the evidence.

P.S. Although I am an Associate Professor of Botany, my Ph.D. is in pharmacognosy and I am actively involved in ethnopharmacological and pharmacognostic research in a variety of geographical and subject areas.

Best wishes,


Robin J. Marles,
Ph.D. Associate Professor,
Botany Department Brandon University,
Brandon, MB R7A 6A9 CANADA