Some form of the common poison-ivy may be found in almost every part of
the country but the extreme West. The shaded area on the map in figure 1
shows the extensive range where some form of poison-ivy is likely to be
found. The plant is known by various local names--poison-ivy, threeleaf
ivy, poison creeper, climbing sumac, poison-oak, markweed, piery, and
mercury. The most widespread form is classified by botanists under the
botanical name of _Rhus radicans_ or varieties of it; however, many of
the variations are sometimes given other botanical names. The common
poison-ivy may be considered as a vine in its most typical growth habit.
_Figure 1._--The shaded part shows the extensive area where some form of
the common poison-ivy is likely to be found. Other forms also may occur
in parts of the same region.
_Figure 2._--Large poison-ivy vine growing on a tree for support. This is
the usual form of the common poison-ivy in wooded areas.
_Figure 3._--_A_, Common poison-ivy growing in a hedge and on a shade
tree on the edge of a lawn. _B_, Typical vine, showing leaves and the
roots that attach it to the tree.
The vining type most often occurs in wooded areas (fig. 2), where it
depends upon trees for support. Vines often grow for many years, becoming
several inches in diameter and quite woody. Slender vines may run along
the ground, mixed with shrubbery, or take support from a large or
small tree. That a plant growing along the edge of a lawn and into the
shrubbery may be quite inconspicuous as compared with a vine climbing on
a lawn tree is shown in figure 3. The vine readily develops roots when in
contact with the ground or with any object that will support it. When the
vines grow on trees these aerial roots attach the vine securely ( fig. 3.
B). A rank growth of these roots often causes the vines on trees to have
the general appearance of a "fuzzy" rope, which sometimes serves a good
purpose for identification. The vines and roots apparently do not cause
injury to the tree except where growth may be sufficient to cover the
supporting plant and exclude sunlight or break the plant from excessive
weight. The vining nature of the plant makes it well adapted to climbing
over stone walls or on brick and stone houses.
The fact that poison-ivy often becomes mixed in with ornamental shrubbery
and vines often results in its cultivation as an ornamental vine by
people who do not recognize the plant. An ivy plant growing on a house,
as is shown in figure 4, is often prized by an unsuspecting owner. The
vine is attractive and quite effective as an ornamental, but such use
should not be tolerated, as it most certainly will result in cases of
accidental poisoning and serve as propagating stock for more poison-ivy
in the vicinity.
_Figure 4._--Common poison-ivy growing on side of house with ornamental
shrubs. This accidental or intentional use of the attractive vine may
lead to many cases of accidental poisoning.
_Figure 5._--Common poison-ivy growing along fence row. In open sunlight
and over some large areas it is more of a shrub than a vine.
_Figure 4._--Common poison-ivy growing on side of house with ornamental
shrubs. This accidental or intentional use of the attractive vine may
lead to many cases of accidental poisoning.
When poison-ivy becomes mixed in with other vines it is quite difficult
to detect except by one who has carefully trained himself in observing
and recognizing the plant. Also, some other vines and young plants
resemble poison-ivy in many respects. The Virginia creeper and some forms
of the Boston ivy are often confused with it. The Virginia creeper can
always be recognized by its five leaflets radiating from one point of
attachment, as compared with the three leaflets of poison-ivy arranged
in the same manner. The Boston ivy with three leaflets is sometimes
difficult to detect; however, as a rule not all the leaves on the plant
will be divided into three leaflets. By studying a large number of the
leaves it is usually possible to detect on every Boston ivy plant some
leaves that have only one deeply lobed blade or leaflet, while the
poison-ivy always has the three leaflets. A number of other plants are
easily confused with poison-ivy. No specific rule can be observed that
will easily rule out all possibility of confusion. The best policy is to
learn through practiced observation to know poison-ivy on sight and then
to make the determination from observation of all parts of the plant, and
never to judge from a single leaf or part.
_Figure 6._--Leaves of common poison-ivy, about half natural size. These
may occur on the same or different plants: _A_, _B_, _C_ and _F_, less
common; _D_, with even margins, the most common and generally recognized
form; _E_, with lobed margins, nearly nearly as often seen as the shape
in _D_.
_Figure 7._--Common poison-ivy vine with clusters of flowers in the axis
of each leaf.
Common poison-ivy in full sunlight along fence rows or in open fields
grows more in the form of a shrub than a vine. In some localities the
common form is a low-growing shrub from 6 to 30 inches in height.
This form, as well as the vining type, usually has a rather extensive
horizontal system of rootstocks or stems at or just below the ground
level. The vining form under some conditions later becomes a shrub.
Plants of this type are shown in figure 5, where the vine, supported
on the fence, has extended upright stems that are shrublike. In some
localities the growth form over a wide range is consistently either a
vine or a shrub type, while in others what seems to be the same species
may produce either vines or shrubs.
The leaves of the common poison-ivy are extremely variable, the three
leaflets being the only constant character. It is impossible to describe
the great range of variation in the shape or lobing of the leaflets. A
study of the leaf patterns, especially the six shown in figure 6, will
give a fair range of types, although other forms may be found. One plant
may have a large variety of forms of leaves or it may have all leaves of
about the same general character. The most common type of leaf having
leaflets with even margins is shown in figure 6, _D_, and one with
leaflets having slightly lobed margins is shown in figure 6, _E_. The
other forms shown in figure 6 are not quite so widespread, but may be the
usual type throughout some areas.
_Figure 8._--Common poison-ivy, about natural size: _A_, Flowers; _B_,
mature fruit.
Most of the vines or shrubs of poison-ivy produce some flowers that are
always in clusters arising on the side of the stem immediately above a
leaf. The flowers are rather inconspicuous among the leaves, as may be
noted in figure 7, but they are in quite distinct clusters, as shown in
figure 8, _A_. Frequently the flowers do not develop or are abortive and
no fruit is produced. When fruits do develop they serve as a positive way
of identifying the plant. The berries are not easily confused with the
fruits of other plants. They are white and waxy in appearance and have
rather distinct lines marking the outer surface, as the segments appear
in a peeled orange. These lines may be noted in figure 8, _B_, which
shows dense clusters of mature fruit about natural size. Some forms of
the poison-ivy have the fruit covered with fine hair, giving it a kind
of downy appearance; however, in the more common form the fruits are
entirely smooth. The fruit is especially helpful in identifying plants
late in fall, in winter, and early in spring, when the leaves are not
present.
_Figure 9._--The shaded area is the region where the eastern oakleaf form
of poison-ivy is likely to occur. Other forms also may be in the same
region.