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The saying, "leaflets three, leave them be" refers to the trifoliate
threat of poison ivy (Toxicodendron radicans) and poison oak (T.
diversiloba). Nearly 75 percent of people are allergic to these noxious
plants. In fact, severe allergic reactions account for a substantial number
of the workers compensation claims in the United States. Poison ivy and
poison oak shrubs and vines are a nuisance to arborists and to those who
enjoy outdoor recreation. To avoid missing work and play this summer,
it is essential to understand the identification, reaction, treatment,
and control specifics of these so-called "evil weeds."
Identification
Poison ivy and poison oak are two related plant species with a long
history of irritating (both figuratively and literally) arborists and
others who spend much time outdoors. Poison ivy is common in most of the
United States, although diverse varieties inhabit different regions. Poison
oak is concentrated throughout western North America. The plants usually
grow as shrubs from 3 to 10 feet tall, but they also can grow as vines
with hairy, aerial roots that wind around the bases of trees and other
shrubs. Both poison ivy and poison oak have three leaves, but the edges
of the poison ivy leaf are notched and each has a stalk, while the leaves
of a poison oak are lobed and feature a stalk only on the terminal leaflets.
The cream-colored fruit grows in clusters on the plants and is 1/6 inch
in diameter on the poison ivy and only slightly larger on the poison oak.
The signature three-leaf clusters of the poison ivy and poison oak turn
from red to shiny green as they mature. In autumn, these colors change
to various shades of yellow, orange, and red, attracting more than a few
unsuspecting victims.
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International Society of Arboriculture
