(Editor's Note: This article was originally published on April 14, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)

We'll start with the most common three dangerous plants found in the landscape, but not necessarily in order of toxicity: Poison Ivy, Poison Oak, and Poison Sumac. These plants are contact-poisonous, meaning they will cause an external reaction if touched. These plants are so widespread that they account for an estimated 10% loss of work time for the U.S. Forest Service.1 Firefighters battling the California fires are often disabled by the airborne toxins.

Poison Ivy (Toxicodendron radicans). "Leaves of three, let it be" is good advice! But it's interesting to learn that many people don't recognize this fast-growing intruder. Many homeowners mistake it for a close look-alike: Virginia Creeper (Parthenocissus quinquefolia), of which only the berries are toxic, and only if eaten.Image

Both plants are climbers, and both plants have brilliant red foliage in the fall. However, though the tiny, newly emerging leaves of Virginia Creeper are indeed three-lobed, if you follow the stem back toward older growth, you'll see five distinct leaves with toothed edges. In contrast, Poison Ivy has shiny leaves with smooth edges.

All parts of Poison Ivy are toxic (leaves, stems, roots, blooms, berries); the culprit is urushiol, a substance that seems to have an unlimited shelf-life. Individuals who are highly allergic to Poison Ivy can react to even the handling of dead leaves and stems, or petting a dog or cat that has brushed against the plant. Even people who are not normally sensitive can have severe reactions by breathing the smoke of burning Poison Ivy. Eradicating this weed from your landscape should be done thoroughly and with care. NEVER burn Poison Ivy. When working around it, cover up, wear gloves, and dispose of the plants in plastic bags. Some commercial products can be applied before contact to prevent or at least lessen exposure.
Poison Oak (Toxicodendron diversilobum) is the western counterpoint of Poison Ivy. Poison Oak grows solely in the western United States, from southern British Columbia to northern Mexico; references also cite subspecies identified in Malaysia, China, and Japan. Poison oak can be difficult to identify because it displays in several forms: low ground cover, vining, or very mature specimens in shrub form. Leaves with three lobes are oak-shaped and shiny; the leaves turn brilliant red in fall. Poison Oak forms small white berries that remain on the stem after the leaves drop.

All parts of the plant are toxic and should be avoided. When taking Poison Oak from your property, exercise care, always cover up and wear gloves. NEVER burn either living plants or those that have been pulled or cut. Dispose of the materials in heavy plastic bags.

Special care should be taken in early spring when these plants emerge as new growth. They often blend in with the surrounding foliage and it only takes one brief contact for them to do their damage.
Poison Sumac (Toxicodendron vernix) is listed as one of the "U.S. Invasive Weeds"2 and is considered more virulent and dangerous than either Poison Oak or Poison Ivy. Poison Sumac can be small and shrubby, or grow up to 20 feet tall. The frond-like pinnate leaves are attractive, especially in fall when they turn scarlet. In June and July, the plant blooms, forming long panicles of yellow-green flowers. Poison Sumac grows as far west as Idaho (though only in the southern part); the plant prefers swamps and bogs, or other very wet soil.

The same precautions should be taken when working around Poison Sumac as with all other contact-poisonous plants. Use good sense and, when necessary, call in a professional to deal with large areas of poisonous materials.

1 http://waynesword.palomar.edu/ww0802.htm
2 http://plants.nrcs.usda.gov/cgi_bin/invasive/all.cgi