A plant in sympathy with Greta Garbo's social sentiments, the poisonwood tree (Metopium toxiferum) just wants to be left alone. Like many of its Cashew (Anacardiaceae) family cousins, poisonwood protects itself from intrusive humans with skin-blistering sap. Avoiding a painful encounter with the poisonwood tree isn't difficult. Its natural U.S. habitats are limited to the Florida Keys and a handful of southern Florida counties, where it's also known as hog gum or the Florida poison-tree.
Size and Form
Poisonwoods typically reach 10 to 40 feet tall, with short, slender trunks, rounded, spreading canopies and drooping branches. They’re most common as trees, and one particularly robust South-Florida poisonwood stands 63 feet high, according to the Institute for Regional Conservation, but it's an exception.
Poisonwood in the Wild
Although poisonwoods grow in U.S. Department of Agriculture plant hardiness zones 9b though 11, the trees are most common among the closed-canopy hardwoods of southern Florida's limestone-rich rockland hammocks. In the open-canopy pine rocklands, they're understory shrubs. Poisonwoods do best in moist, well-drained soils but tolerate poor ones, and are drought-resistant once established. The hammock trees need a barrier of other plants to protect them from salt-spray damage, and they won't survive saltwater flooding.
The No-Trespassing Signs
You'll have no trouble identifying a wild poisonwood tree. Its trunk produces enough sap to soak through the reddish-brown bark in dark, oily patches. It also leaks through wounded branches and clumps on the surfaces of the glossy-green, curled-under compound leaves. Water dripping from the leaves during a rainstorm may contain enough sap to trigger an allergic reaction. Even their appealing yellow berries are laced with the toxic fluid. Your wisest course when encountering poisonwood is to steer clear.
Poisonwood sap contains urushiol, the same toxin found in poison ivy (Toxicodendron rydbergii, USDA zones 3 through 10). The University of Iowa Hospitals and Clinics estimates that urushiol causes an allergic reaction in 75 percent of people exposed to it. It begins with itchy skin and progresses to blistered, burning red rashes that often weep. They take between 24 hours and three weeks to surface, and one to four weeks to subside. Poisonwood tree lives up to its name.
It's Not All Bad
Although poisonwood has no place in a landscape design, the wild trees benefit several butterfly and one threatened bird species. Bahamian swallowtail, large orange sulphur, Florida white and mangrove skipper butterflies all harvest nectar from poisonwood's modest, pale-green flowers. The threatened white-crowned pigeon's only U.S. nesting grounds are in southernmost Florida's mangrove forests. The adult pigeons fly several miles each day to collect poisonwood berries for their nestlings.
- University of Florida IFAS Extension: Identification of Poison Ivy, Poison Oak, Poison Sumac, and Poisonwood
- UF School of Forest Resources & Conservation: 4-H Forest Resources: Poisonwood
- The Institute for Regional Conservation: Natives for Your Neighborhood -- Poisonwood, Florida Poisontree
- Keysnews.com: Human-Harming Poisonwood Trees Provide Food, Habitat for Animals
- Zipcodezoo.com: Metopium Toxiferum
- LandScope America: LandScope Florida -- Pine Rocklands and Rockland Hammocks in Florida
- University of Iowa Hospitals and Clinics: Poison Ivy --The Most Common of Allergens
- Zipcodezoo.com: Toxicodendron Rydbergii
- National Park Service: Everglades -- White-crowned Pigeon Species Profile
- Photo Credit cuatrok77photograph/iStock/Getty Images