Poison ivy, or Toxicodendron radicans, is a trailing vine with short tributary stems that sport triple-leaves. It contains urushial in the sap, a compound that causes an intensely itchy and blistering skin rash on approximately three out of four people who come into contact with it. It is shade tolerant and grows thickly on the trunks of many trees in Mississippi. The old saw that says "leaves of three, let it be" also applies to its non-climbing cousin, poison oak.
Eastern poison oak, or Toxicodendron pubescens, has a triple leaf that very closely resembles poison ivy, but poison oak is a bush, not a trailing vine. It propagates locally and intensively, and you can run into vast swathes of the plant on forest floors throughout Mississippi. Poison oak sap contains the same compound as poison ivy, urushial, and the symptoms of contact are the same as for poison ivy. The third Mississippi plant that contains urushial in the sap is poison sumac.
Poison sumac, or Toxicodendron vernix, cannot be avoided using the three-leaf rule. It is a small tree, which has seven to nine leaves per stem, arranged in a herringbone pattern. It looks remarkably similar to a plain sumac tree, the latter of which is harmless. The trick to avoiding poison sumac without living in fear of sumacs is to remember that poison sumac likes soaking wet soil and grows only along watercourses, lakes, ponds and marshes.
The other class of "itchy" plants that grows around Mississippi is the nettle. The most common is the stinging nettle, or Urtica dioca. Stinging nettle is a perennial wild flower, that can grow as high as two meters. The identification tip-off is the leaf, which is soft with a very serrated border. The leaf appears "hairy." Those hairs are like miniature hypodermic needles that inject histamine into the skin of the unfortunate soul who rubs against them, provoking a sensation of both itching and stinging, but without the blistering of poison ivy and its cousins. Baking soda mixed with a little water is often used to relieve the sting and itch until the sensation recedes (in a few hours). Stinging nettle is very common throughout Mississippi, especially in sunny, overgrown fields. In recent years, a Northern cousin has migrated into Mississippi that has the same capacity to create discomfort--the wood nettle.
The wood nettle, or Laportea canadensis, is a shade-loving member of the same family as the stinging nettle. This wildflower has similar "hairs" with similar properties to the stinging nettle and tiny clusters of white flowers. It seldom exceeds four feet in height. Again, the tip-off is the hairy appearance of the leaf. If the leaf is fuzzy-looking, and you don't know what it is, don't touch it.