Poison ivy and Poison oak are the most common causes of allergic contact dermatitis
in North America. Poison ivy grows east of the Mississippi River and Poison
Oak, west of the Mississippi River (ie: this means Poison Oak does not grow
in Pennsylvania). Donald Wyman, author of Wyman's Gardening Encyclopedia explains
the challenge this way "Poison ivy - poisonous and native to the eastern US
- is found everywhere, along roads, scrambling over walls or up trees, at the
seashore within reach of salt spray and in the best of gardens." Also, poison
ivy seeds are spread by more than 50 varieties of birds.
