Western Poison Ivy is and erect native perennial shrub that produces a woody stem, usually unbranched. Unlike the Eastern Poison Ivy it does not have aerial roots and does not climb. However, a robust plant can produce a stout woody stem reaching several feet in the air.
Leaves are alternate and divided into 3 widely oval leaflets that have a pointed tip. Side leaflets are usually shorter than the terminal leaflet. Leaves are on long hairless leafstalks that are usually attached at the top of the stem and the base of the stalk is enlarged leaving a U-shaped scar after the leaf drops away. The upper surface of the leaf is a smooth shiny dark green (bronze-green when young). Margins may be smooth or have shallow teeth. Size varies from 1 to 6 inches long and 1 to 4 inch wide. Fall color is yellow to orange-red.
The inflorescence is an elongated slightly branched panicle, with each panicle cluster having 25 or fewer stalked flowers. Flower clusters occur at the axils of the leaves at the top of the old growth stem.
The Flowers are very small, 5-part, with a cup shaped hypanthium with a short green calyx that has 5 triangular lobes. The corolla has 5 white to greenish-white triangular petals that spread and reflex slightly when the flower is open. There are five stamens that have white filaments and yellow anthers that surround a central pistil. The globose ovary and pistil are green with a 3 lobed stigma. Flowering time varies with the latitude.
Fruit: Flowers mature to round dry droop, green initially, turning to a dull ivory at maturity and may remain on the stem all winter. Each contains a single seed. The entire plant is toxic - see notes at bottom of the page.
Habitat: Western Poison Ivy grows from a much branched rhizomatous base that will form colonies of plants. It grows best on wood edges and openings and in open spaces with full to at least partial sun. Soil conditions can range from poor to rich but there must be adequate moisture.
Names: The genus name, Toxicodendron, means 'poison tree' and is used for the various poison ivies. The species name rydbergii, is named after Per Axel Rydberg (1860-1931), Swedish-American Botanist. The authorship of the plant classification is complex. The first to describe the plant - ‘Small’ - refers to John Kunkel Small (1869-1938), American Botanist, first curator of the New York Botanical Garden, best known for Flora of the Southeastern United States. His work was amended by Dr. Rydberg ('Rydb.') who was the first curator of the New York Botanical Garden Herbarium. He received a commission from the US Dept. of Agriculture for a botanical exploration of several places in the western states and his expertise was in the flora of the Great Plains and Rocky Mountains. He described approximately 1,700 new species. Finally, additional work was done by ‘Greene’ who was Edward Lee Greene (1843-1915), American botanist who wrote Landmarks of Botanical History, named or re-described over 4,400 species of plants in the American west and was the first Professor of Botany at the University of California.
Comparison: The close relative of Western Poison Ivy is Eastern Poison Ivy, Toxicodendron radicans, which has leaves with lobes, and has aerial roots for climbing.
Above: 1st photo - The branched flower cluster occurs in the leaf axils where new growth springs from the old stem. 2nd photo - The large three-part leaf from which comes the warning "leaves of three, let them be." 3rd photo - The old twiggy stem of prior years growth has characteristic U-shaped leaf scars.
Below: Individual flowers are stalked and have 5-part white to greenish-white spreading petals. 1st photo - After fertilization, the anthers have turned dark and the green ovary is exposed. 2nd photo - The yellow anthers are erect around the style until fertilization. The last photo on this page shows a rather large inflorescence on a stout plant.
Below: The flowers mature to a round ivory color berry, bearing a single seed. They persist all winter.
Below: 1st photo - Young leaves of springtime begin with a greenish-bronze color. 2nd photo - Fall color can be a nice red. Note the mature berries. Young or old, the leaves, berries and stems are toxic.
Below: An example in Summer Green.
Notes: Western Poison Ivy is indigenous to the Garden. Eloise Butler catalogued it on May 25, 1907. She also brought into the Garden the Eastern Poison Ivy in 1912 from an East Coast source. Both are still extant. While called 'western' this species is found coast to coast in North America with the exceptions being the states of the SE part of the U.S. and California; and in Canada it is found in all the lower provinces except Labrador and Newfoundland. Within Minnesota, it is widespread throughout the state with most exceptions being in the western half of the state whereas the Eastern Poison Ivy is only known in 7 widely separated counties.
Toxicity: Fear of poison ivy is well-known. All parts of the plant contain the clear colorless oil called urushiol which causes dermatitis outbreaks. Most humans are susceptible to the itchy blistering rash. There are modern over-the-counter remedies for it but prior to their development a treatment was to to wash with alcohol (avoiding eyes, mouth and open sores) as alcohol dissolves the oil. Water does not and simply spreads the oil around unless combined with a powerful bar soap such as Fels-naptha which works well against the oil if used soon after exposure - otherwise Fels-naptha is a laundry soap, not to be used for washing skin normally. The oils remain potent for months after leaving the plant. Thus tools and clothing that has touched the plant will still be toxic. Burning anything that contains the oil spreads it in the smoke. Inhaling the smoke is the worst possible case as it infects areas you cannot treat - the inner linings of the mouth and throat. One must be careful in the seasons when leaves are not present as the woody twigs sticking up from the ground still are dangerous and may be confused with other woody plants unless you see those U-shaped leaf scars. Poison Ivy can be eliminated with glyphosate based herbicides and most products labeled Brush or Stump Killers.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"