The saying "Leaves of three, let it be" is often used to describe the poison plants. This is a handy way to recognize poison ivy and poison oak, but it's better to memorize what these plants look like. Not all three-leaved plants are toxic. And some that are toxic have more than three leaves.

Poison Ivy. The Latin name for poison ivy is T. radicans. This hardy plant may grow as a vine that climbs on trees and other surfaces up to about 50 feet. It may also grow as a shrub about 4-5 feet high.

Poison ivy can grow in a variety of conditions. It especially likes sandy, stony, or rocky shores of streams, rivers, and lakes. It also sprouts in thickets, along the borders of woods, and in forest openings. Birds feed on the plant's fruit and spread its seeds in their droppings.

The leaves of poison ivy are usually arranged in groups of three, but they may also appear in fives or sevens. In each cluster, the middle leaf grows on a stalk that is much longer than those on the sides. The leaves in a cluster are usually fairly equal in size, from one-half inch to two inches long. Their edges may be slightly notched or smooth. They are shiny when young and turn a brilliant red in fall. The flowers of poison ivy are yellowish to greenish white, about one-quarter inch in diameter, and grow in clusters on a slender stem. Small, berry-like, whitish or greenish fruit, about one-sixth of an inch across, appears after the flowers have faded and are glossy and dry when ripe.

Poison Oak. As its name suggests, the leaves of T. diversilobum, or poison oak, are lobed like those of an oak tree. Poison oak leaves are a little larger than poison ivy's and grow in groups of three, five, or seven. The plant's flowers and fruit are similar to those of poison ivy. Also like poison ivy, poison oak may grow as a vine or a shrub up to 3 feet high. The leaves have short, smooth hair on the undersides, and berries are fuzzy and white. Poison oak loses its leaves in the winter, but the plant remains toxic in all seasons.

Poison Sumac. T. vernix, or poison sumac, is less widespread than poison ivy and poison oak. More common in the Midwest, poison sumac grows as tall as a tree or shrub with clusters of 7 to 13 leaves arranged in pairs with one leaf at the end. Its small, yellowish flowers grow in clusters and mature into whitish-green berries that hang in loose clusters up to 12 inches long. There is also a species of sumac that is not poisonous and has red berries. To be on the safe side, though, always assume that any plant resembling poison sumac is probably toxic.

Poison ivy, oak, and sumac all contain an oil called urushiol. This oil is not really a poison, but it is made up of chemicals that are powerful allergens (substances that cause an allergic reaction). In half to two-thirds of people, urushiol causes an allergic reaction known as contact dermatitis. The resulting rash consists of swollen, itchy, red bumps and blisters that appear wherever the oil has touched the skin.

Urushiol is present in the stems, leaves, and roots of the poison plants. The only parts of these plants that do not contain the oil are certain parts of the flower (the anther), the pollen, the outermost membrane of the stem and leaves (the epidermis), and a particular kind of tissue found inside the stem (xylem). For all practical purposes, though, it's best to consider all parts of these plants as able to cause the rash.

Urushiol is released when the epidermis of the plant is broken. Even the tiniest scrape or break in this membrane can release the oil. These plants are poisonous at all times of the year but are even more so in spring and summer, when the leaves are tender and bruise easily.

You don't have to touch these plants directly to get the rash. Urushiol is a sticky, long-lasting substance that can easily remain on your clothing and shoes. Dogs, cats, and horses can carry the oil on their coats and transfer it to your skin long after you've left the woods. Especially under dry conditions, the oil can retain its effects for a very long time. In one case, museum workers handling a 100-year-old specimen developed a rash from the oil! Smoke from the burning of these plants can also cause the rash and can affect the nose, throat, eyes, and lungs.

The rash from poison ivy, oak, and sumac usually appears within 24 to 48 hours after contact with the plant. In some cases, though, the rash may not appear for a few days afterward. The worst stage of the rash usually occurs within a week of exposure. Depending on how severe it is, the rash may take 2 or 3 weeks to heal.

The rash from these plants occurs most often on parts of the body where the skin is thinnest, like the wrists, ankles, neck, and face. At first the area may appear reddish, and you may feel a mild stinging or itching. Red bumps then appear, often in streaks or patches where you rubbed against the plant. Itching can become severe before the bumps turn into blisters. These may ooze a clear, yellowish fluid. The blisters begin to crust over and dry up as the rash subsides.

The rash from poison plants is not "contagious." Only the oil itself can be spread to other parts of the body or to another person. Sometimes, after the rash has developed in one place on your body, it may seem to suddenly appear elsewhere. This fact leads many people to think that the rash can be spread by scratching or bathing, or by touching another person's rash. This is not true, however. The rash may take days to appear after contact with the plant, and your skin varies in thickness over different parts of your body. This is why all of the affected areas may not show the rash at the same time.

In fact, keeping the area of the rash clean with soap and water is a good way to help it heal faster. Neither the blisters themselves nor the fluid they secrete contain urushiol. Touching them will not spread the rash to a new location on either your own body or someone else's, unless urushiol is present on the skin. Still, it is best to avoid touching or scratching the rash.

The best way to treat the rash is to catch it early. At the very first sign of a rash, or if you think that you have come in contact with one of these plants, wash the affected area with plain soap and cool water as soon as possible. Keep the area clean, cool, and dry as much as possible. And above all, don't scratch!

Most cases of poison ivy, oak, and sumac may be extremely uncomfortable but do not pose a serious health threat. Several types of over-the-counter medications can provide relief. Creams and ointments, such as Benadryl or hydrocortisone cream or Caladryl lotion, can soothe the itching. If you do use a cream or ointment, wash it off and dry the area before reapplying it. An oral antihistamine, such as Benadryl, can lessen the allergic symptoms. You can also soothe the itching by applying cool compresses or soaking the area in cool water with baking soda.

If the rash covers a large area of your body or is near your eyes, call your doctor. He or she may prescribe an oral corticosteroid to lessen the swelling and itching. There are also prescription creams and ointments that may provide relief.

Some persons may have a severe reaction to poison ivy, oak, or sumac, to the point where hospitalization is needed. If the rash covers more than one-quarter of your body, the allergens in the plant oil may cause a systemic reaction. Go to the emergency room of a hospital if you have any of the following symptoms along with a severe rash:

• Fever, headache, or nausea
• Trouble breathing or shortness of breath
• Extremely sore or painful rash that interferes with normal activity
• Swollen lymph nodes in your neck, under your arms, or in the groin
• Blisters that continue to ooze after a few weeks

Prevention is the best way to avoid the effects of the poison plants. The first step is to learn what they look like at all seasons of the year in the area where you live. Pay attention to the plants around you when you are outside, especially in wooded or overgrown areas.

When you come in from your walk in the woods, wash your clothing in warm water. If you encountered the poison plants while outside, scrub off your boots or shoes before bringing them into the house. Remember, the oil from these plants can remain on surfaces for days or weeks.

If you find one of these plants in your yard, remove it with care. Wear long pants and sleeves, heavy work gloves, and closed shoes or boots. Pull or dig out the entire plant, making sure to get as much of the root as possible. Avoid touching any part of the plant, and keep children and pets out of the area until you are finished. Never burn the plants. Instead, place them in a heavy, tightly sealed plastic lawn bag and dispose of them.

If you think a pet has been near an area with poison ivy, oak, or sumac, bathe the animal in warm, soapy water. Wear long sleeves and rubber gloves to avoid getting the oil on your skin.

If you live in a rural area, you may be able to control the growth of poison ivy, oak, and sumac with livestock grazing. Goats and cows will eat these plants with no ill effects, but grazing needs to continue for several years to be effective.