WESTERN POISON-OAK
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.
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.
| 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. |
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 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 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.