Stinging nettles Each serrated dark green leaf and stem (pictured) harbors thousands of hollow spines ready to deliver a painful injection of histamine and formic acid. Distinguish nettles from lookalike mint by crushing the leaves with your boot and sniffing the air. Nettles grow in moist, shady areas in every state except Hawaii.
Poison oak Rounded, urushiol-laced leaves change color from bright green in spring to yellow-orange in summer to pinkish red by fall. Along the West Coast, shrubs grow in shady spots, while vines climb redwoods and Douglas firs. In the East, knee-high bushes lurk from Texas to New Jersey in dry, open woodlands and pine forests with sandy soil.
Poison ivy The leaves are bad, but the vine is just as toxic. The vine version carpets rock ledges and tree trunks with dense clusters of reddish-brown rootlets. All parts of shrub and vine contain urushiol, which spawns blistering rashes in 85 percent of people. Leaves that are dark green in summer turn bright red by the fall.