Poison-ivy and poison-oak are known by a number of local names, and
several different kinds of plants are called by these names. The plants
exhibit a good deal of variation throughout the United States, so much
so that even technical botanists cannot agree on the number of species
and names that should be applied to them. They grow in the form of (1)
woody vines attached to trees or objects for support, (2) trailing shrubs
mostly on the ground, or (3) erect woody shrubs entirely without support.
They may flourish in the deep woods, where soil moisture is plentiful,
or they may be found in very dry soil on the most exposed hillsides. The
plants are most frequently abundant along old fence rows and edges of
paths and roadways, and they ramble over rock walls and climb posts or
trees to considerable heights. They often grow with other shrubs or vines
in such ways as to escape notice.
The leaf forms among plants or even on the same plant are as variable
as the habit of growth; however, the leaves always consist of three
leaflets. The old saying, "Leaflets three, let it be," is a reminder of
this consistent leaf character but may lead to undue suspicion of some
harmless plant. Only one three-parted leaf leads off from each node on
the stem. The leaves never occur in pairs along the stem. The wide range
in habit of growth of plants and leaf types may be noted from a glance at
the several pictures of plants and leaves.
The flowers and fruit are always in clusters on slender stems that
originate in the axis of the leaves along the side of the smaller
branches. The fruits usually have a white, waxy appearance and ordinarily
are not hairy, but may be so in some forms. The plants do not always
flower and bear fruit. The white or cream-colored clusters of fruit, when
they occur, are significant identifying characters, especially at the
season after the leaves have fallen.
For convenience, these plants are discussed in this bulletin under three
divisions: (1) Common poison-ivy, (2) oakleaf poison-ivy, and (3) Western
poison-oak. A technical botanist would recognize many species of plants
in what are included as the common poison-ivy, and anyone will recognize
that various forms occur within this grouping.