5. Poison Oak (Toxicodendron Diversilobum)
While technically a shrub or small tree, Poison Oak earns its place on this list with its potent urushiol oil. Just brushing against its leaves can cause a severe, itchy rash that lasts for weeks.
Found across North America, especially in forests and along hiking trails, it’s a stealthy threat to outdoor adventurers. Recognize the “leaves of three” and steer clear.