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A couple of weeks ago, Sharon Barnes asked a question on this listserv about the fate of poison ivy during composting. I found the question intriquing so I decided to use it for the BioCycle Q&A column. Below is the “awaiting-final-edits” version of the Q&A column. It will be in the May issue. Thanks to eveyone who replied to Sharon for the information and leads.

In the end I did not find much definitive information. One of the more interesting references was a paper from the Journal of Investigative Dermatology Volume 4 (1941) by B. Shelmire. Shelmire ponded, dried, crushed, washed, drowned and “roofed” pieces of the plant and still found it a potent source of urushiol even after a year and a half (to sensitive individuals). Unfortunately more recent data seems scarce. I would appreciate learning of other studies if anyone knows of them. By the way, the USDA Poisonous Plants lab in Logan Utah does not have much information on poison ivy (they are mainly concerned with plants poisonous to grazing animals) but they were a good place to start. —
Bob Rynk

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Q. What is the fate of poison ivy in a composting system? The owners of a large pile of brush have asked to burn the pile. They say composting will not destroy the oils in the plant. Will burning pose even a greater threat through airborne contamination? (This question was originally submitted to the U.S. Composting Council’s Internet Listserv in April)

A. While there is a great deal of information available about the poison ivy plant, and the rash that it causes, there is little guidance available about how to dispose of it. Fact sheets, books, and web pages caution us not to touch poison ivy, and not to burn it, but they stop short of recommending how to dispose of poison ivy vegetation. Burning poison ivy is a bad idea. People have developed severe reactions from breathing in smoke particles from burning vines. Burying poison ivy in the ground and putting it in the trash have been suggested as disposal methods but these alternatives have obvious disadvantages, especially if poison ivy is mixed with large quantities of yard trimmings. Apparently, there isn’t a good method, or at least a certain method of safely disposing of poison ivy.

Intuitively, some poison ivy experts and composting practitioners guess that the poison ivy toxin decomposes during composting but there are plenty of “ifs” and “maybes” in their statements. Outside of the compost pile, there is ample evidence that the poison ivy toxin can persist and remain potent for years. Unfortunately, there isn’t a clear cut answer about the fate of poison ivy during composting. Here is what we do know.

The rash that occurs from poison ivy, poison oak and poison sumac is caused by a family of organic compounds called urushiol (pronounced oo roo-shee-ohl , or you-ROO-she-ol, depending whether you read it in the north or south). Urushiol includes several organic molecules with an aromatic structure, categorized as catechols. The literature on poison ivy also refers to the toxin as an oil or oleoresin but urushiol is the more specific term used to identify the oily toxin in the sap.

The urushiol-carrying sap is present in the roots, stems, leaves and fruit (but not in the pollen). The roots, in particular, contain high concentrations of urushiol. The sap is released when the plant is damaged by bruising, cutting, mowing, scraping, wind, or insects. One also can contact urushiol indirectly after the sap gets on clothing, shoes, tools, pets, and bark and firewood previously covered with poison ivy. Urushiol is not volatile but shredding, mowing, or burning the plants can create airborne particles that increases the chance of exposure. The plant is most potent in the spring and early summer when the sap is rising and the urushiol content is high. The dreaded sap is less abundant in the winter but still present and potent in the roots, stems, vines and twigs of the plant. Therefore, if yard trimmings contain poison ivy (or poison oak or sumac) residue, there are urushiols about.

Urushiol is a stubborn substance. It has been shown to remain potent on dead plants, clothing and contaminated objects for years (bad news for everyone). In one study, plants stored in dry conditions and in submerged water for 17 months were less potent but stilled caused a rash in sensitive individuals. However, these are not the biologically-rich conditions of a compost pile. Because urushiol is organic, it is subject to biological degradation (good news for composters). One reference even suggests that under hot humid conditions, urushiol becomes inert in about a week (more good news for composters). Unfortunately, the research basis for this statement is not identified.

Still, there are other indications that urushiol decomposes naturally. First, we are not overrun with cases of poison ivy despite the persistence of the toxin and ubiquity of the plants. Secondly, according to Susan Carol Hauser, author of Nature’s Revenge(1), leaves that naturally fall off the plant do not contain urushiol. Furthermore, leaves do gradually lose potency over time. Finally, there is anecdotal testimony. For instance, commenting to the U.S. Composting Council Internet listserv, the manager of a large yard trimmings facility mentioned that users of raw shredded yard trimmings mulch have reported developing the poison ivy rash while no users of the composted product have (users were cautioned about the poison ivy risk with accompanying written instructions).

Large-scale composting could be a rational approach for disposing of poison ivy-laden yard trimmings, given the lack of good alternatives and the fact that urushiol is thought to decompose under conditions typical to composting. However, without more scientific evidence, it remains a risky venture. In any case, it deserves due caution and common sense in handling the raw feedstocks and the products. To adequately decompose, the urushiol must be exposed. Because poison oak, sumac, and even poison ivy are woody plants with fairly thick stems, the plant material needs to be shredded prior to composting. However, shredding also releases the sap, spreading it among the shredded material and possibly into the air near the shredder. This raises concerns about using uncomposted mulch made from the shredded yard trimmings containing poison ivy. As a side note, given the uncertainties, it is wise to keep poison ivy vegetation out of the backyard compost pile.