Poison Ivy is a plant that is mentioned in Diary of a Wimpy Kid: Old School.
When Mr. Jefferson sets up a fire and tells the rest of Greg's cabinmates to get some kindling for a fire but nobody knows what kindling is, they bring back everything they think might burn. Then his son, Rowley, comes back with bunch of weeds and tosses them onto the fire, which totally smothers it. It turns out Rowley had collected a bunch of poison ivy leaves which was placed on the fire, he got covered with poison ivy spots on Saturday morning after he woke up. His father Mr. Jefferson woke up too and he discovered that he had trouble breathing after he inhaled the fumes and got it into his lungs. The Nurse called Mrs. Jefferson to come pick them both up. This means Greg's group is the only group without a chaperone because Mr. Jefferson] has left due to breathing difficulty. Greg mentioned that The Essential Book For Young Lads had a chapter on how to identify poison ivy, which would have come in handy yesterday for Rowley and Mr. Jefferson. This is also the second time when Rowley had to leave the camp after the first incident involving him and Gareth Grimes during the trust fall with a tooth stuck on his forehead, resulting Rowley had to leave the camp first to get checked out to see a doctor for Gareth's tooth stuck on his forehead.
- Diary of a Wimpy Kid: Old School (First and only appearance)