Home - View all weeds - Photo Key - Credits

POISON-IVY
Toxicodendron radicans

Other Names | Origin & Distribution | Plant Description | Similar Species | Biology | Toxicity | Facts & Folklore

Poison-ivy seedling with cotyledons and 2 true leaves (composed of 3 leaflets). Poison-ivy seedling with cotyledons and the first leaflets of 2 true leaves. Poison-ivy seeds. Poison-ivy leaves; note the various shades of green and red. Poison-ivy leaves. Mature poison-ivy plant growing on a fence. Poison-ivy flowers. Poison-ivy growing in no-till corn. Poison-ivy berries.

Family: Cashew Family (Anacardiaceae)

Other Names: Rhus radicans, markweed, mercury, picry, poison creeper, poison vine, three-leaved ivy.

Origin and Distribution: Poison-ivy is an American native that has a range extending from Canada to South America. It is distributed throughout every county in Ohio. Poison-ivy grows in many habitats including disturbed sites, woodlands, and wetlands. Because birds and animals often disperse the seeds, it is common to find poison-ivy growing in fence rows, on roadsides, at the base of trees, or along the edges of woods. Also, it has been observed in no tillage fields. Although poison-ivy grows in many soil types, it prefers soils with high calcium content.

 

Plant Description: This is a deciduous woody perennial distinguished by its leaves that have three leaflets. The stalk attached to the middle leaflet is considerably longer than that attached to either of the two outer leaflets. It grows in a variety of forms including trailing, shrubby, or as a vine. Reproduction is primarily by seeds that are dispersed by birds and animals. Also, it may spread by rhizomes (horizontal underground stems). Stems are capable of forming roots and sending out new shoots when in contact with soil.

  • Root system - Poison-ivy produces aerial roots that attach to plants and other things when it grows as a vine. These aerial roots give stems of older plants a hairy appearance.
  • Seedlings & Shoots - First to emerge from a seed are 2 leaves (cotyledons) that are narrow and oblong. The characteristic 3-parted leaves appear next.
  • Stems - Woody stems have gray bark and grow either horizontally along the soil surface with upright leafy stalks or as a climbing vine.
  • Leaves - Leaves are alternate (1 leaf per node) and compound consisting of 3 leaflets. Leaflets are 2 to 4 inches long, glossy, and have a pointed tip. Their shape varies from elliptic to egg-shaped. Their edges also vary from smooth, to toothed or lobed. They appear droopy and reddish green in spring, become level and change to dark green when mature, and turn yellow, orange, or bright red before falling off in the fall.
  • Flowers - The small, greenish flowers have 5 petals and form in cluster that are 1 to 3 inches long and often hidden in the leaf axils. Male and female flowers occur on separate plants.
  • Fruits & Seeds - The small white berries are round, hard, and about 1/8 inch in diameter. Their surface has ridges that resemble segments of a peeled orange. Each berry contains a single seed.
 
Similar Species: Seedlings of box-elder (Acer negundo) have the same alternate, 3-parted leaves that distinguish poison-ivy, but its leaflets are less shiny. Poison oak (Toxicodendron toxicarium) grows more erect than poison-ivy and its leaves have blunt tips with hairs on the upper and lower surfaces. In the U.S., poison oak is usually found growing from New Jersey southward. Leaves of poison-ivy are pointed and smooth on the upper surface, although they may be hairy on the underside. Virginia creeper (Parthenocissus quinquefolia) is similar in appearance to poison-ivy but its leaves have 5 leaflets and it climbs by way of tendrils. Also, its fruit is a blue berry. Hog-peanut (Amphicarpa bracteata) has 3-parted leaves similar to poison-ivy, but it is a twining vine lacking woody stems and its purplish, pea-like flowers are larger than those of poison-ivy. Also, its leaflets have 3 strong veins apparent on either side while poison-ivy leaflets have only one centrally located midrib.
 

Biology: Flowers appear from May to July. Seeds usually form after September and may remain on the plant throughout winter. Over 50 species of birds are known to eat poison-ivy seeds. Seeds are often dispersed far from the parent plant by animals and birds. Poison-ivy generally establishes on sites that have been repeatedly disturbed but not recently cultivated. It grows low to the ground and spreading, upright and bushy as a shrub, or vine-like and spreading. Slow vegetative spread by rhizomes can result in formation of large patches. The weed is easy to control by repeatedly cultivating, cutting, or mowing. Its shallow rhizomes are easy to dig up and remove. However, care should be taken to wear heavy protective clothing and repeatedly wash clothing and tools after use. Several herbicides are available that selectively control poison-ivy if applied to growing plants such that all foliage is completely covered.
 

Toxicity: All parts of poison-ivy release an oil upon bruising that causes severe dermatitis with swelling and blistering. Sensitivity to the toxin varies among individuals, plants, and circumstances under which the person was exposed. If contacted, affected areas should be washed immediately with soap and water as well as any clothing or objects that may have come in contact with the oil. This activity will not decrease the severity of the reaction, but it will lessen the chance of spread. Unless removed by washing, the oil, which is similar to lacquer, can remain on plant parts, skin, clothing, and tools for an indefinite period of time without loosing potency. Fluid contained in blisters is not allergenic. Objects and animals can pick up the oil and transfer it to humans. Smoke of burning poison-ivy plants can cause allergic reactions inside the lungs of susceptible people. If affected, consult a pharmacist for ointment to treat the affected area and a doctor if the case is severe.
 

Facts and Folklore:
  • 'Toxicodendron' is Greek meaning 'poison tree'.

  • 'Leaflets three, let it be -- berries white, poisonous site.'

  • Each year, reactions to poison-ivy are one of the most often cited causes of workers' compensation claims.

  • Application of crushed leaves of jewelweed (Impatiens capensis) relieved the effects of recent exposure to poison-ivy in 108 out of 114 people tested.

  • Contrary to a widely-held belief, eating a poison-ivy leaf will not result in immunity to its toxin.

  • Botanists have contracted dermatitis from handling 100-year-old dried plants.

  • Poison-ivy has been cultivated in gardens and sold as an ornamental in Europe and Australia.

  • In the Netherlands, where its attractive fall foliage is prized, it is planted along dikes.