Part 3

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.

POISON SUMAC

Poison sumac grows as a coarse woody shrub or small tree (fig. 19) and never in the vinelike form of its poison-ivy relatives. This plant is known also as swamp sumac, poison elder, poison ash, poison dogwood, and thunderwood. It does not have variable forms, such as occur in poison-oak or poison-ivy, and botanists agree to call it Rhus vernix. The area in the Eastern States where it is likely to be found is chiefly eastward from eastern Minnesota, northeastern Illinois, Indiana, central Kentucky and Tennessee, and southeastern Texas, as shown on the shaded area of the map, figure 20. This shrub is usually associated with swamps and bogs, and the most typical growth occurs along the margin of an area of wet acid soil.

Figure 21.--Small branch of poison sumac with six compound leaves.

Figure 22.--Fruit of poison sumac, one-third natural size. These fruits somewhat resemble those of poison-ivy. They are always on drooping slender stems attached at the side of the small branches and are never terminal as in the species of sumac that are not poisonous.

The plants range in height from 5 or 6 feet to small trees that may attain a height of 25 feet. The poison sumac shrub shown in figure 19 is growing on the edge of a swamp and is quite typical of the general appearance of the larger shrubs, which approach a treelike form. As a rule the shrubs do not have a symmetrical upright treelike appearance. They are more or less inclined to lean and have branched stems with about the same diameter from the ground level to the middle height of the shrub.

Occasional isolated plants are found outside of swampy regions. Apparently these plants are started from seed distributed by birds. The plants in dry soil usually do not become more than a few feet tall. They are likely to cause poisoning to unsuspecting individuals, as single isolated plants are not readily recognized, especially out of their usual region.

The leaves of the poison sumac are divided into 7 to 13 leaflets, arranged in pairs with a single leaflet at the end of the midrib (fig. 21).

Figure 23.--A, Smooth sumac and dwarf sumac growing in a mixed stand. These plants are not poisonous. B, Terminal fruiting spike of smooth sumac, about one-seventh natural size. This form of terminal fruiting spike is typical of all the species of sumac that are not poisonous.

The leaflets are an elongated oval shape without teeth or serrations on the margins. They are 3 to 4 inches long and 1 to 2 inches wide, with a smooth velvetlike texture and bright orange color when they first appear in spring. Later they become dark green and glossy on the upper surface and pale green on the lower and have scarlet midribs. Early in fall they turn to a brilliant red-orange or russet shade.

The small yellowish-green flowers are borne in clusters in slender stems arising from the axis of leaves along the smaller branches. The slender clusters of flowers have much the general appearance of the poison-ivy flower, but hang in much longer clusters. The flowers mature into ivory-white or green-colored fruits resembling those of poison-oak or poison-ivy, except that they are usually less compact and hang in loose clusters that may be 10 to 12 inches in length (fig. 22).

CONFUSION OF POISONOUS AND HARMLESS SUMACS

Because of the same general appearance of several common species of sumac and the poison sumac, considerable confusion has occurred as to which one is poisonous. Throughout most of the range where poison sumac grows, three other species are the only ones that are likely to be confused. These are the smooth sumac (Rhus glabra), staghorn sumac (R. typhina), and dwarf sumac (R. copallina). All the nonpoisonous species have red fruits that together form a distinctive terminal seed head, as shown in figure 23. These are easily distinguished from the slender hanging clusters of white fruit of the poison sumac, as shown in figure 22. Sometimes more than one species of the harmless sumac grow together, as shown in figure 23, A.