Gardeners tend to appreciate the diversity of the plant world, but there are a few plants we could live without. Poison ivy must top most gardeners’ list of undesirables in the eastern half of the country, where it occurs with particular abundance. Neither strictly poisonous nor truly and ivy, Toxicodendron radicans is a member of the Anacardiaceae, or sumac family, along with poison oak (T. diversilobum), which is prevalent in the West, and poison sumac (T. vernix), which also grows in the East.
In 1908, Japanese researchers isolated the chemical that many members of this plant family secrete. They named it urushiol, after urushi, the Japanese word for lacquer. (In Asia, the rash has long been accepted as an occupational hazard for those that harvest and handle the “lacquer tree.” T. vernicifluum, and the varnish produced from it.) Urushiol is a particularly fierce allergen; as little as one one-thousandth of a milligram can cause a rash in the susceptible. Few people react allergically to the substance upon first exposure. Generally, repeated exposure is necessary, and it will cause progressively more serious rashes. Sensitivity sometimes subsides with advanced age. Gardeners and hikers are not the only ones affected; according to the Journal of the American Medical Association, poison ivy and oak are the greatest cause for workers’ compensation payments related to outdoor work injuries.
Urushiol is insoluble in water and retains its potency for remarkably long periods of time. For example, 10 months after the fact, Walter C. Muenscher and John M. Kingsbury of the Cornell University botany department washed a canvas glove worn to collect poison ivy in hot water and copious soap for 10 minutes. They dried and ironed it, then presented it to a volunteer, who developed a rash. Even fire doesn’t degrade urushiol, which merely regroups into droplets that can cause lung irritation, entire body dermititis, fever and temporary blindness. Urushiol-filled smoke is one of the greatest hazards to firefighters in the western forests, and the leading cause of injury to them.
In the cases of poison ivy, oak and sumac, urushiol remains in roots, leaves, flowers and fruit throughout the year. Contact with the plant’s sap can result in rashes for sensitive humans, even when the plant is dormant or dead. Contact with the outer surface of the plant will not—but these are fragile plants that are easily torn or broken by people and animals brushing past them. Even the wind can break their stems and leaves.
Urusiol must penetrate the skin to reach the dermis, which why thin skin will show symptoms before thick skin. It is far more common for the rash to occur in the spaces between the fingers than on the palm of the hand. The rash rarely appears sooner than 12 hours after contact, and sometimes can take as long as 3 days to surface. It can occur without direct contact with the plant. Touching shoes or clothes that have rubbed against the plant can cause a response, and pet owners can get a rash after touching dogs or cats that have passed through thickets of poison ivy, oak or sumac.
Ridding the Yard of Poison Ivy, Oak and Sumac
The only certain way to avoid an unpleasant experience is to learn to recognize and avoid the plants. There are not many good options for eradicating plants if they grow in your yard. The two herbicides used for poison ivy, Roundup (glyphosate) and Ortho Poison Ivy Killer (triclopyr), will kill many other plants as well. When used sparingly, they can eradicate poison ivy and leave the trees upon which it grows undisturbed, but these products cannot be applied near shrubs, broadleaf groundcovers or herbaceous plants. The poison ivy vines can be pulled away from desirable plants and then wiped with the herbicide, or a shield can be used to direct the spray toward the poison ivy alone.
Joseph Neal, Ph.D., an associate professor of weed science at Cornell University, says manual removal can be effective is you are diligent, but every bit of the roots must be thoroughly removed. He suggests wearing plastic gloves over cotton gloves when pulling the plants. I have found that dealing with poison-ivy seedlings while they are small is by far the best way to eradicate them and keep their spread contained. Once the first true leaves emerge—distinctively lobed and glossy—they are easy to spot. Use a stick that can be thrown far away or the tine of a weed scratcher to dig under seedlings’ shallow roots. The plants should be placed in plastic bags and disposed of. Do not compost them, which would result in future exposure. Do not burn the plants. Breathing their smoke can cause serious illness.
When Contact Occurs
The application of barrier creams may be effective when contact with poison ivy, oak or sumac is anticipated and unavoidable. The American Academy of Dermatology recommends wearing long pants, long sleeves, gloves and boots whenever contact is expected. However, urushiol is soluble in rubber, and tight-fitting garments can drive the chemical farther into the wearer’s skin.
Some gardeners swear that the juice of jewelweed (Impatiens biflora) and plantain (Plantago major and P. lanceolata) can help to soothe the rash once it occurs, but they do not claim that these plants can prevent the rash from starting.
One time-honored method for removing urushiol from the skin after contact, in the hope of sidestepping the rash, involves hot water and “yellow laundry soap,” or borax. But recent research indicates that strong soaps may make the skin more vulnerable to urushiol, because they irritate it. Milder soap, as long as it does not contain moisturizing agents, may be more effective. And cold water, not hot, is now considered the best medium for quickly removing the oil from exposed skin. In fact, cold running water alone may be the very best treatment. Its temperature encourages the pores of the skin to close, while at the same time inhibiting urushiol’s virulence. There is just one hitch: for plain cold water to be truly effective, the effected area must be washed within three minutes of contact.
Most recently, dermatologists recommend alcohol as the best option for combating the plant’s oily residue. Alcohol not only removes oil upon the surface of the skin, but with repeated swabbing it may even flush out oil that has already soaked into the sebaceous glands. I’ve found that packaged towelettes that come presoaked in an alcohol-based liquid can be very helpful. Keep them handy for use immediately following contact. Then take a break from gardening until tomorrow—the alcohol will remove your skin’s natural protection too.
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