Toxicodendron radicans rydbergii By Matt Young November 23, 2015 12:00 MST Toxicodendron radicans rydbergii – western poison ivy, South Boulder Creek Trail, Boulder, Colorado, July, 2015. As I was taking the picture, a little voice approached me and asked, “Do you know —? Is that —? Could that be —? Poison ivy?” Yes, and it was one of the lushest fields of poison ivy I have seen this side of New Jersey, growing right along the trail. You can identify it because it has 3 leaflets, and often the outer ones are shaped like mittens, though not as distinctively as these. Poison ivy is red only in the fall; we will see that in 2 weeks.