They both grow along fence rows, waste areas, open and cut over forest lands, stream banks and rocky canyons. In the fall, both species' leaves turn brilliant red. Unwary people who gather these colorful leaves sometimes suffer acute allergic reactions, sometimes requiring hospitalization. Both poison oak and poison ivy are readily identified by their leaves, which are divided into three leaflets. Shiny when young, the leaflets usually range from half an inch to two inches long, although poison oak leaves may be larger.
Flowers are greenish white, about a quarter-inch across and are borne in clusters in the leaf axils.
The fruits are white, berrylike, glossy and dry when ripe, about a sixth of an inch in diameter in poison ivy and a little larger in poison oak. All parts of poison oak and ivy except the pollen are poisonous, year-round. Human reactions vary from extreme susceptibility to near immunity. And people may become allergic later in life after repeated exposure. Pets, especially dogs, are another source of the irritating oil, said Ball.