POISON IVY RULE:
"Leaves three, let it be"
Its upward-growth as a vine often
makes it look like a shrub or tree.
of Pennsylvania" is a 1002-page treatise on all species of
plants found in Pennsylvania, native and introduced. It lists three
related species known to cause contact dermatitis: poison ivy (Toxicodendron
radicans), giant poison ivy (Toxicodendron rydbergii) and
poison sumac (Toxicodendron vernix) as growing in
Pennsylvania. Poison oak is not listed as a native or introduced
species. Poison oak is also known as Pacific poison oak or western
poison oak (Toxicodendron diversilobum). Although it is as
common in the Pacific Ocean states as poison ivy is here, it is not
known to occur in Pennsylvania. All of these plants can cause an
itchy, blistering dermatitis in susceptible individuals because
their tissues are suffused with a compound called urushiol.
LET IT BE
The old saying, "leaves three, let it be" is mostly correct.
Most poison ivy has leaves composed of three leaflets; mature plants
can have anywhere from five to seven leaflets. It typically grows as
a vine, but can grow as weak-stemmed shrub if there is nothing handy
for it to climb. The stems-especially very mature ones-are covered
with aerial rootlets that give them a very hairy appearance. Poison
ivy has beautiful red fall color and produces white fruits in late
summer and early fall when mature.
Poison ivy has beautiful fall color
ivy is considered a threatened or rare species limited to dry, rocky
woods. Its appearance is very similar to common poison ivy.
Poison sumac is a shrub or small tree with an upright growth habit
and produces a grayish-white fruit in fall. It has alternate,
pinnately compound leaves comprised of seven to thirteen leaflets.
Poison sumac is an obligate wetland species, and is “occasionally
found in swamps, bogs, fens and marshes” throughout Pennsylvania.
Your friend’s “poison oak” may be a native vine known as Virginia
creeper (Parthenocissus quinquefolia), based on the
description she gave you. It has palmately compound leaves comprised
of five leaflets that turn scarlet in fall. Virginia creeper
produces dark purple fruits, also in fall. Although it is often
mistaken for poison ivy, it generally does not cause dermatitis.
Most plants-including poison ivy-are likely to grow bigger and
faster due to increased CO2 in the atmosphere. A study from Duke
University found that exposing poison ivy to higher levels of CO2
resulted in faster, lusher growth due to the fertilization effect of
the higher CO2 levels. It also found that higher CO2 levels made
urushiol more potent. These findings could have a significant future
impact on human health, since more potent urushiol will likely
increase the number of people requiring medical treatment for the
resulting contact dermatitis. As of now, roughly 80 percent of the
population is susceptible to urushiol. An interesting sidebar from
the Duke study: “Other species in the Anacardiaceae family,
including mango, cashew, and pistachio, also can be allergenic. It
is possible that these plants, too, may become more problematic in
Increased vine growth could have severe consequences for our
increasingly fragmented and degraded woodlands as well. Tree
regeneration is reduced when rampantly growing vines of any kind
block sunlight and reduce the saplings’ ability to manufacture
carbohydrates via photosynthesis. If they cannot produce enough
carbohydrates to sustain themselves, the saplings die.