May 31, 1903: 116 years ago

Watch out for poison ivy!  Both because it is a fascinatingly cool plant but also a potentially dangerous one. 

This flowering specimen of poison ivy was collected on May 31, 1903 by John Shafer (first curator of botany at Carnegie Museum) and O.P. Medsger in the Laurel Highlands of southwestern PA.
 
Poison ivy (Toxicodendron radicans) is a woody vine found in forest understories, but does especially well on forest edges. It is very common in urban and suburban environments. It takes many forms, climbing high up a tree, along the ground, or even a short shrubby thing. The leaves take a range of shapes, but as the adage goes (“leaves of 3, let it be”), it has three leaflets.
 
Poison ivy is known by many for its chemistry, producing a class of organic compounds call urushiol found in the plant’s sap and on leaves/stems/roots that cause a nasty rash to those who touch it. Not all people have a reaction (but most are sensitive), but don’t be too confident - it can develop with repeated exposure.  Incredibly itchy, the rash can become a serious health issue if infected, and especially if it oils get in contact with your eyes, face, or throat.  The rash can appear days after exposure and can last for weeks.
 
Research indicates poison ivy will get more poisonous with climate change, both in terms of growth and producing higher quantities of forms of urushiol particularly toxic to humans.
 
It is often considered a “weed” because of its fear-causing rash, but it is native to eastern North America.  The species has a wide distribution across our region, and across the world (native subspecies in China). Although native, the species seems to be getting more common as we create more forest edge habitat.   The colorful foliage can be quite beautiful in the fall, and the berries are a food source for birds and other animals. Humans seem to be the only ones allergic to it.
 
Poison ivy is often confused with other non-poisonous and poisonous plants. Some common confusions are 1) Virginia creeper (Parthenocissus quinquefolia), a vine in the grape but different leaves, and not with leaves of three and 2) Box elder maple (Acer negundo), a tree that has compound leaves unlike most stereotypical maples,  but with samaras (aka “helicopter” seeds).  
 
Poison ivy and some other rash-causing plants are in the cashew family (Anacardiaceae) – which also includes poison oak (not found in Pennsylvania, but in western USA), staghorn sumac (not rash causing), poison sumac (rash causing, but you likely won’t come across it unless in very wet habitats)....and even a few familiar species like  pistachio,  cashew, and mango (some people are allergic).  
 
What an amazing plant. 

This specimen image is now publicly available online: http://midatlanticherbaria.org/portal/collections/individual/index.php?occid=12232529&clid=0

In July 2017, I collected poison ivy, and was not careful.  Poison ivy rash on  your finger  tips is an awful experience.  This picture was from weeks after.