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Daily Plant Masthead

Volume XVII, Number 3790
Friday, Jan 31, 2003

IN PRAISE OF POISON IVY

No one seems to like Toxicodendron radicans but poison ivy is an important plant in our urban natural areas. Poison ivy (Anacardiaceae, the cashew family) is a common woody vine native to the United States and Canada from Nova Scotia to Florida and west to Michigan and Texas. It is also found in Central America as far south as Guatemala. In New York City it is all but ubiquitous in natural areas. It has been recorded in over 70 wooded parks and other natural areas in the City.

Poison ivy does have certain drawbacks for many people who are allergic to its oily sap. The toxins in poison ivy sap are called urushiols, chemicals containing a benzene ring. These chemicals can cause itching and blistering of skin but they are made by the plant to protect it from being eaten by insects and vertebrate herbivores such as rabbits and deer.

Poison ivy is recognized in summer by its alternate leaves with three, shiny leaflets and by the hairy-looking aerial roots growing along its stems. In autumn, the leaves rival those of sugar maple for red and orange colors. Winter leaf buds are narrow and pointed, and "naked" (without scales). It forms extensive colonies from underground stems and can cover large areas of the forest floor with an understory of vertical stems, especially in disturbed woodlands and edges. However, it generally only blooms and sets fruit when it finds a tree to climb. When a poison ivy stem encounters a tree trunk, or other vertical surface, it clings tightly to the bark with its aerial roots and climbs upward, reaching for the light (unlike several notorious exotic vines, it does not twine around or strangle trees). Once it has found enough light, it sends out long, horizontal branches that produce flowers and fruit.

Flowers of poison ivy are small and greenish-white and not often noticed, except by the honeybees and native bees which visit them for nectar and to exchange pollen among the flowers. Honey made from poison ivy nectar is not toxic. Fruits of poison Ivy are small and waxy-coated berries that remain on the vine into winter. They are eaten by woodpeckers, yellow-rumped warblers and other birds. Crows use poison ivy berries as crop grist (instead of, or along with, small stones) and are major dispersers of the seeds.

It is as ground cover that poison ivy performs its most vital functions in urban woodlands. It can grow in almost any soil and it enjoys full sun, but can grow in closed canopy woodlands. It is an ideal ground cover, holding soil in place on the steepest slopes, while collecting and holding leaf litter and sticks that decay to form rich humus. It captures rain, causing the water to sink into the ground, slowing runoff, renewing groundwater, filtering out pollutants and helping to prevent flooding.

Poison ivy is usually found with many other plants growing up through it that also live in the forest understory. It seems to "get along" with other plants, unlike Japanese honeysuckle or Asian bittersweet, which crowd out or smother other plants. Poison ivy is also important as shelter for birds, and many invertebrates.

While those who are severely allergic to poison ivy have reason to dislike and avoid it, Toxicodendron radicans has an important place in our urban natural areas. No one would advocate letting it grow in playgrounds, picnic areas, or along heavily used trail margins, but it belongs in our woods and fields and should be treated with respect, not hatred.

Uva, R. H., J.C. Neal and J. M. DiTomaso. 1997. Weeds of the Northeast. Comstock Publishing. Ithaca, NY.

Written by Margaret B. Gargiullo

QUOTATION FOR THE DAY

"Every moment of one’s existence one is growing into more

or retreating into less. One is always living a little more or dying a little bit."

Norman Mailer

(b. January 31, 1923)

 

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