The Latin epithet radicans means “with aerial roots.” Poison ivy climbs high up on trees with the help of its clinging roots. Ascending a red maple in Purgatory Swamp. Norwood, June 15

As any woody vine, poison ivy is always in search of a lighted position. It climbs tall trees in order to reach full-sun situations. Tidmarsh Sanctuary, Plymouth, June 30

Ripe fruits. Agawam River, Wareham, October 4

When there is enough sunlight at the ground level, poison ivy may grow prostrate, appearing as a low, inconspicuous shrub. Tidmarsh Sanctuary, Plymouth, March 10

Fruits may remain on branches until the spring. Tidmarsh Sanctuary, Plymouth, March 10

New leaves in spring. Each leaf consists of three leaflets, the upper one on a stalk longer than those of the two side leaflets. A warning rhyme “Leaflets three—quickly flee!” is familiar to everyone. Blue Hills Reservation, Canton, May 6