A year ago I wrote a blog post arguing that poison ivy should be declared the state plant of Massachusetts. I still feel that way. Poison ivy owes a lot to the big biped invasives. It doesn’t grow very well in shade, though it’s pretty happy with any type of soil and very drought tolerant. When humans arrived the forests covered more ground. Contrary to popular opinion, Native Americans cleared a lot of forests using fire (see Cronon’s Changes in the Land -- just do it, you won’t regret it). When the nefarious Europeans showed up to ruin the continent they cleared even more land. This continuing process creates lots of disturbed edge habitat between fields and forests, roads and forests and the like. We create the perfect environments for poison ivy.
If you plan a tryst in the deep forest, you’re not likely to get a rash from poison ivy on your tuckus. If instead your tryst occurs beside a road in the suburbs, you'll likely get an itchy rash (from the poison ivy). We are poison ivy’s best friend, though it does have a strange way of returning the favor. The active ingredient, urushiol, is a mixture of compounds that irritates human skin. Most other animals are unaffected, though apparently guinea pigs can be sensitized. It is as if it were sitting here in North America waiting for us to show up.
The rash referred to above is likely caused by one of two species. They’re commonly called poison oak and poison ivy. Once you learn to recognize one you’ll recognize them both; they’re in the same genus and look pretty similar. The leaves vary quite a bit, but I think they often look like mittens, with a thumb sticking out one side. Usually, as in the picture, the leaves aren't very symmetrical: one side is thinner than the other.
So let’s say you spent a part of your weekend relaxing in a pile of poison-ivy. You will almost no doubt have a friend, perhaps one who showers even less than I do, who will recommend Impatiens pallida ( jewelweed or touch-me-not) as a remedy. In “preparing” for this column, I read lots of bloggers who claim that they don’t have to worry about poison ivy now that they know about jewelweed. The trouble is that the effect cannot be replicated in the lab. In double blind studies, volunteers treated with jewelweed get just as itchy as others (Zink et al 2010 Wilderness Medicine). You’re best off using cortisone or an oatmeal bath.
How do you get rid of the plants then? The old mater recently asked me what to do about some poison ivy tangled in a blueberry bush. This bloody blueberry bush has been growing in my grandfather’s yard since long before I was born. There isn’t another blueberry bush in shouting distance so it’s never set a berry. I’d say get rid of the whole thing, put in two or three good young bushes and move on. This suggestion didn’t fly. She bought something that claimed to be an herbicide specific to poison ivy.
Sounds like a load.
You can use an herbicide, rip it out (repeatedly), or put heavy mulch on top of it. That’s how you get rid of poison ivy. There are “natural” herbicides like clove oil and strong vinegar. These do provide some control, but they’re still herbicides. It is unclear how long it will take for the clove oil to break down in the environment. Natural does not mean safe. Witness, say poison ivy.