How Poison Ivy Works

Poison Plants
The leaves of the poison ivy plant turn red and yellow for fall.
The leaves of the poison ivy plant turn red and yellow for fall.
Photo courtesy Jon Sachs,

Because urushiol is found in all parts of the poison ivy plant -- the leaves, stems, and roots -- it's best to avoid the plant entirely to prevent a rash. The trouble is, poison ivy grows almost everywhere in the United States (with the exception of the Southwest, Alaska, and Hawaii), so geography won't help you. The general rule to identify poison ivy, "leaflets three, let it be," doesn't always apply. Poison ivy usually does grow in groups of three leaves, with a longer middle leaf -- but it can also grow with up to nine leaves in a group.

Here are some other ways to identify the poison ivy plant:

  • It generally grows in a cluster of low, weed-like plants or a woody vine which can climb trees or fences.
  • It is most often found in moist areas, such as riverbanks, woods, and pastures.
  • The edges of the leaves are generally smooth or have tiny "teeth." Their color changes based on the season -- reddish in the spring; green in the summer; and yellow, orange, or red in the fall.
  • Its berries are typically white.
Poison sumac
Photo courtesy Jon Sachs,

Poison ivy's cousins, poison oak and poison sumac, each have their own unique appearance.

Poison oak grows as a shrub (one to six feet tall). It is typically found along the West Coast and in the South, in dry areas such fields, woodlands, and thickets. Like poison ivy, the leaves of poison oak are usually clustered in groups of three. They tend to be thick, green, and hairy on both sides.

Poison sumac mainly grows in moist, swampy areas in the Northeast, Midwest, and along the Mississippi River. It is a woody shrub made up of stems with rows of seven to thirteen smooth-edged leaflets.

­We'll look at how poison ivy affects the immune system in the next section.

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