Leaves of Three
Remember the old adage, leaves of three, let it be OR leaflets three, quickly flee? These proverbs were meant to help identify poison ivy (above pic) but they contribute to confusion since many plants we encounter are trifoliate (i.e., a compound leaf with three parts or leaflets). The confusion frequently involves another common woody native, boxelder. Why? Frankly it is a novice mistake since the two species are quite different. Let's distinguish and discuss them.
Boxelder (Acer negundo -- a type of maple) is a fast-growing, weedy species characteristic of low-lying areas. It is a potentially medium-sized tree that rarely produces an attractive ornamental specimen. The wood is comparatively lightweight (not dense), the branches are straight, relatively thick and brittle/weak, the bark of new stems is green, and the heart wood is a distinctive red -- some woodturners utilize it. The box portion of the name refers to its use as a crate wood. Boxelder's leaves and leaf scars are oppositely arranged on the stems whereas poison ivy is alternate. A boxelder was my favorite climbing tree at my paternal grandparent's property in Flora IL, but it is a trash species. I always recommend it be removed. Speaking of removal, getting rid of poison ivy is a challenge. Attempting to do so will involve applying an herbicide after the leaves have emerged in spring -- in my Rantings book I show and describe a way to spray without harming surrounding vegetation. But, rather than eradication, even with spraying, about the best one can expect is suppression.
Poison ivy (frequently referred to as PI) is reviled for good reason. All parts of the plant except the pollen contain an oily mixture (urushiol) that causes skin rash (contact dermatitis) in most people, Curiously, about 15% of the population never have the allergic reaction and the severity of those that do can vary over time. It is an acquired reaction since no one gets it initially. Know that the urushiol remains active for several years after the plant is dead. As the adage warns, let it be. Washing the contact area with warm soapy water within 20 minutes usually will remove the oil -- take care to get between the fingers. The scientific name for PI is Toxicodendron radicans which translates to the toxic wood, with stems that take root. Interestingly, PI belongs to the same plant family (Anacardiaceae) as cashew and pistachio and is more closely related to maples than ivy -- true ivy is Hedera, an invasive you should avoid. Poison ivy is found in practically every woodland and fence row in the Midwest. In northern Indiana the plants are much shorter; often appearing as a short single stemmed shrub. As one proceeds south the plants can get bigger and climb much higher. I have seen the vine clinging to tree bark about 100 feet high. The branches are distinctively short and ascending with the tips turned up -- somewhat gray, they standout from the rough brown main stem. As I noted in a previous rant, the state record PI is almost 15-inches in diameter -- reported from Posey Co. (IN) in the 1880's by the famous Smithsonian ornithologist, and Mt. Carmel (IL) native, Robert Ridgway. Evidently, the size of the specimen led him to initially presume it was a boxelder tree. A 4-foot-long section of a 5-inch diameter PI stem I collected from a wooded area at Park Tudor School (Marion Co, IN) was mounted, trophy like, on my classroom wall when I taught there. Poison oak has hairy leaves and, contrary to the common belief, is NOT found in Indiana. However, there is a form of PI that forms only a short shrub (i.e., not a climbing vine) and is considered by some a variety or subspecies of T. radicans. Some authorities consider it a distinct species (T. rydbergii).
While I obviously do not promote boxelder, most people would be shocked to hear that I find poison ivy interesting and beautiful -- from the glistening tan-red to green new emerging leaves, to the fabulous orange-red fall coloration, to the clustered white fruit that persist into winter. To complete the aforementioned adage, berries white, poisonous sight. However, since the berries are not present for much of the year, they often cannot serve to help identify. PI is a wonderful plant for wildlife. I also fondly remember collecting an unusual host specific fungal pathogen from PI leaves for an assignment in my fall semester (1979) plant pathology class at EIU.
LESSON & ADVICE: consider taking a general botany, natural history or local flora class as well as participating in field trips -- manmade gardens and natural areas. None available in your area. Make it happen. Strive to learn as much as possible about the vegetation and associated organisms you encounter. Doing so will make the plants more interesting, you a better gardener, and a better human.