The western poison-oak (_Rhus diversiloba_) of the Pacific Coast States
is usually known as poison-oak, but is occasionally referred to as
poison-ivy or yeara. It is likely to be found in the region designated
by shading in figure 12, and from central Oregon northward through
Washington may be growing intermixed with common poison-ivy. The term
"poison-oak" is a misnomer, as this species is in no way related to the
oak but is related to poison-ivy. Anyone familiar with either of these
plants in their several forms will likely recognize the other, both as
plants to be avoided.
_Figure 12._--The shaded area is the region where the western poison-oak
is likely to occur. Other forms may also be in the same region.
The most common growth habit of the western poison-oak is in the form of
a rank upright shrub with many small woody stems rising from the ground.
It frequently grows in great abundance along roadsides (see fig. 16) and
in uncultivated fields or on abandoned land.
Sometimes western poison-oak attaches itself to upright objects for
support and takes more or less the form of a vine. A plant of this type
is shown in figure 13, where it is growing on a telephone pole by the
roadside. It may be noted even in this case that the tendency is for
individual branches to continue an upright growth rather than become
entirely dependent upon other objects for support. In some areas,
especially in woodland, poison-oak may grow principally as a vine. Areas
have been noted where 70 to 80 percent of the trees leaves and flowers.
in a wooded area support vines running more than 25 or 30 feet in height,
although the most common form of the plant is the shrub.
_Figure 13._--Western poison-oak on a telephone pole. This vine form is
not uncommon, especially on trees in woods.
_Figure 14._--Western poison-oak: Upright shrub, with fruit, growing in
open pasture field.
_Figure 15._--Western poison-oak, showing common leaf types: _A_, The
more common leaves with irregular margins; _B_, less typical, although
not uncommon, leaves with even margins.
_Figure 16._--Western poison-oak growing alongside roadside. This rank
shrubby growth is typical for most localities.
_Figure 17._--Western poison-oak, showing common leaf types: _A_, Leaves
with thick leathery appearance; _B_, leaves irregular in shape with
much-curled or distorted surfaces; _C_, branch, with leaves and flowers.
In open pasture fields western poison-oak usually grows in spreading
clumps from a few feet to several feet tall (figs. 14 and 16). Where it
develops extensively it greatly reduces the area for grazing and becomes
a serious menace to most people who frequent the area or attend cattle
that come in contact with the plants while grazing.
_Figure 18._--Western poison-oak: _A_, Top of shrub, with leaves and
fruit; _B_, clusters of fruit after leaves have fallen.
Low-growing plants, especially those exposed to full sunlight, are often
quite woody and show no tendency for vining. Low-growing plants with
woody stems are common in pasture areas or along roadsides. Livestock in
grazing do not invade the poison-ivy shrub, and as a rule these plants
continue to spread both by root-stock and seed.
As in other ivy the leaves consist of three leaflets with much
irregularity in the manner of lobing, especially of the two lateral
leaflets. The Latin name _dirersiloba_ refers to the irregularity
occurring in the shapes of the leaves on different plants and on the same
plant or even irregularity in the lobing of leaflets of the same leaf.
Sometimes lobes occur on both sides of a leaflet, thus giving it somewhat
the semblance of an oak leaf. The middle or terminal leaflet is more
likely to be lobed on both sides and resembles an oak leaf more than the
other two. A study of the leaves shown in figures 15 and 17 shows the
variation in lobing. Some plants may have leaflets with an even margin
and no lobing whatsoever, as those in figure 15, _B_. The surface of the
leaves is usually glossy and uneven, so that they have a thick leathery
appearance.
_Figure 19._--A large poison sumac shrub about 15 feet tall, growing on
the edge of a swamp.
The flowers are borne in clusters on slender stems diverging from the
axis of the leaf, as may be noted in figure 17, _C_. The individual
flowers are greenish white and about one-fourth inch across. The cluster
of flowers matures into greenish or creamy-white berrylike fruits about
mid-October. These are about the size of small currants and are much like
other poison-ivy fruits, having a smooth greenish-white glossy surface
striped somewhat like the segments of a peeled orange. Many plants bear
no fruit, although others produce it in abundance, as shown in figure 18,
_A_. The fruits are not always spherical, but sometimes have a somewhat
flattened appearance, as is shown in figure 18, _B_. They remain on the
plants throughout fall and winter and are quite helpful in identifying
poison-oak in seasons after the leaves have fallen.
_Figure 20._--The shaded area shows the region where poison sumac is
likely to occur in bogs or swamps, although isolated plants are sometimes
found in dry soil.