"Leaves of three, let it be; berries white, take flight." (Photos by Mary Holland)

About the only characteristic that holds true for any poison ivy (Toxicodendron radicans) plant is that its leaves are divided into three leaflets. Other than this, poison ivy is a master of disguise.

Botanically speaking, it is a deciduous woody vine, but it can also take the form of a shrub, or thick ground cover. The vine itself can be thin and sprawling, or rope-like and up to four inches thick. While the compound leaves always have three leaflets, they can be rounded or pointed, shiny or dull, and their margins toothed or smooth. It is understandable why this plant is frequently not recognized.

North American poison ivy was described in 1612 by Captain John Smith in Jamestown, Virginia as resembling English ivy, but "causeth redness, itchynge, and finally blysters." While poison ivy was relatively common in southern United States at this time, it has only become well-established in New England during the past 100 years or so.

Much of the Northeast was cleared in the 1800s, and poison ivy quickly moved into the open, sunny pastures, where it thrived. However, grazing held it in check. As farming declined in the 20th century, there was no holding back the natural spread of this member of the cashew/sumac family, and poison ivy spread rampantly through both fields and woodlands.

Although toxic to humans, poison ivy berries provide mcuh needed sustenance for birds in the winter.

At this time of year, poison ivy is at its most beautiful, for its green leaves turn a brilliant red in the fall. At the same time, the three-inch clusters of flowers that it produced in the spring have developed into white berries that persist well into winter. Botanists feel that the color change of poison ivy leaves at the very time that its fruit is ripening is no coincidence. In fact, they've even given this process a name – foliar fruit flagging.

The theory is that the red color of the leaves evolved as an attractant to birds (including many fall migrants) which consume the fruits and disperse the seeds in their droppings, often many miles from the original vine. Whether or not the red color of its leaves serves as an avian lure, poison ivy does provide food for over 60 species of birds. Those for whom poison ivy berries are a preferred food include northern mockingbirds, black-capped chickadees, ruffed grouse, hermit thrushes, ruby-crowned kinglets, starlings, yellow-rumped warblers, white-throated sparrows, wild turkeys, northern flickers and both hairy and downy woodpeckers.

While poison ivy provides much needed sustenance for birds, especially during the most trying season of the year, its negative effect on approximately 40 to 50 percent of the humans that come in contact with it can't be ignored. All parts of the plant (except the pollen) contain resinous compounds called urushiols that, as John Smith noted, cause inflammation of the skin, blistering and itching. The compounds can be transferred to people by direct contact with the plant or with objects or animals that have been exposed to the plant, including tools, pets and clothing. The smoke produced when burning poison ivy also contains droplets of these compounds, which are easily breathed in and carried to the lungs. You can be affected year round by this plant, and the compounds remain active on objects and in dead plants for years; century-old herbarium specimens of poison ivy have produced rashes on botanists.

Should you not be part of the population that is immune to these compounds, soap and hot water applied immediately after contact is said to be an effective deterrent. It only takes 10 minutes for the compounds to be absorbed by the skin, so time is of the essence. The sap of crushed jewelweed, or touch-me-not, is a remedy that some swear by, as is plantain and sweetfern sap. No one can be complacent, however, as immunity can be ephemeral, present one year and gone the next.

Virginia creeper, another woody vine whose compound leaves turn red in the fall, is often mistaken for poison ivy. A close look reveals that Virginia creeper leaves have five, not three, leaflets, however. In addition, if you look closely at a Virginia creeper vine, you will see that the tendrils it sends out have an adhesive disk at their tip with which the vine clings to the bark of the tree or stonewall it is climbing. Poison ivy has hairy rootlets that extend outward from the vine, but they lack terminal disks.

Mary Holland is the author of "Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England." She has a natural history blog which can be found at www.naturallycuriouswithmaryholland.wordpress.com.