Unknowingly tromping through a patch of poison ivy can turn a pleasurable hike or run in the woods into a nightmare. If you touch the plant or something that has touched it (like your dog), then the oils can get on your skin and cause a nasty, itchy, blistery rash that lasts for up to three weeks.

If you know you've gotten into some poison ivy, you want to rinse with cold water within an hour of exposure. A hot shower is the worst idea because it will open your pores and let the oils in. If you wash the oils off immediately, hopefully you'll avoid a rash altogether. If you do develop the rash, don't pop the blisters. The liquid inside won't cause the rash to spread, but you could get an infection. Use a topical anti-itch cream to relieve your symptoms and speed up the healing process.

If you learned the rule, "Leaves of three, let it be," you can throw that out the window. While poison ivy does grow in clusters of three leaves, it can also grow up to nine leaves in a group. To hear what else you should you watch out for and see some photos of this treacherous plant read more.

• This plant needs light to grow, so often you'll find it lurking on the edge of woods, roads, parking lots, and fields.
• Poison ivy generally grows in a cluster of low weed-like plants, but it can also be a vine that climbs up trees.
• Many plants look like poison ivy, but you'll know it's something to avoid if the base of the leaves are slightly wide. Also, the edges of the leaves have tiny "teeth."
• The leaves aren't always green. They are reddish in the spring, green in the summer, and yellow or orange in the fall.
• The berries are typically white.

Check out these photos from the poison-ivy.org website, so you know exactly which plants to avoid and which are harmless.

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