While backpacking in Yellowstone or the Grand Tetons, you can pretty much forget worrying about poison ivy. It is extremely rare in both of these national parks.
Yet “rare” is not the same as “absent”! Poison ivy is a deciduous shrub with a skin-irritating oil with which most folks are familiar. It grows in temperate climates with adequate moisture, which excludes much of the arid and high altitude West. Since most of the Yellowstone and Tetons region is above 7,000 feet, and those areas that are lower are mostly semi-arid, this region is not conducive to robust populations of poison ivy (“poison oak” is a variety that grows mostly along the Pacific coast).
Over the years, I have hiked much of the Teton Range and have never seen poison ivy there. I have, however, read that it does occur at the foot of the mountains along the western shore of Jackson lake, a relatively low elevation area with plenty of moisture. By contrast, in Yellowstone I have seen poison ivy — but only in one location: a low-elevation south facing slope with spring-fed moisture near the confluence of Hellroaring Creek and the Yellowstone River. And that’s it. Nowhere else. Yet I have read that poison ivy is absent from Yellowstone, which obviously isn’t true. But that small patch of Toxicodendron radicans may be the only population in Yellowstone, though it probably is not.
Anyway, again, don’t worry about it while backpacking in Yellowstone. Keep your eyes open, sure, but be much more vigilant about drowning, falling, lightening, bison, bears, falling trees, and most important, the always dangerous drive to the trail-head!
