2. Toxicodendron radicans (L.) Kuntze (poison ivy)

Rhus radicans L.

Pl. 201 f–h; Map 836

Plants shrubs 0.5–3.0 m tall or more often lianas to 30 m or more, spreading by rhizomes, often with abundant aerial rootlets. Leaves scattered along the branches, the petiole 6–15 cm long. Leaflet stalks 2–5 mm long in lateral leaflets, 2–5 cm long in terminal leaflets. Leaflets 5–18 cm long, 4–16 cm wide, ovate, angled to truncate, often unequally so on lateral leaflets, usually angled or tapered to a sharply pointed tip, the margins entire or coarsely and bluntly saw-toothed, sometimes irregularly lobed, those of the lateral leaflets usually somewhat asymmetrical (deeper on the lower side than on the upper side), the upper surface glabrous to sparsely hairy, the undersurface glabrous or sparsely to moderately hairy, especially along veins. Inflorescences 4–12 cm long. Sepals 1.2–1.4 mm long, narrowly ovate-triangular. Petals 2.4–2.6 mm long, narrowly elliptic, yellowish white with dark veins. Fruits 3–4 mm long, 4–5 mm wide, subglobose, the outer layer shiny, green at first (July–August) and turning creamy yellow or tan at maturity (September), glabrous, rarely with sparse, minute papillae or hairs, the outer layer becoming papery at maturity and separating, revealing a powdery white middle layer spotted with small, black lines (resin ducts). 2n=30. May–July.

Common throughout the state (eastern U.S. west to South Dakota and Texas; Canada, Mexico, Central America, Asia). Bottomland forests, mesic upland forests, sand savannas, thickets in upland prairies and loess hill prairies, banks of streams, rivers, and spring branches, margins of ponds, lakes, and sinkhole ponds, and ledges of bluffs; also fencerows, railroads, roadsides, and shaded, disturbed areas.

This species is extremely variable in growth form. To the north of Missouri, in sand dunes along the Great Lakes, it takes on a low, nearly herbaceous habit, but in Missouri it produces woody stems. In open areas the plants tend to remain shrubby, but when growing under a tree canopy, the plants usually become stout lianas that climb to the tops of trees and have thick-barked stems that can eventually reach 10 cm in diameter. Steyermark (1963) noted that a plant in Indiana measured more than 50 m in length. Thick masses of adventitious roots are produced relatively evenly (vs. usually in discreet clumps or patches in Parthenocissus) wherever a climbing stem makes contact with a substrate and these act to anchor the plant securely to rocks, tree trunks, etc. Although poison ivy is not recommended for cultivation in gardens, it has a long history of being grown as an ornamental in parts of Europe, presumably for its bright fall foliage.

Toxicodendron radicans exhibits a wide range of leaf forms and pubescence types that are difficult to categorize into discrete subspecies. Although Barkley (1937) had lumped all of the temperate North American plants into the single var. radicans, Gillis (1971) recognized a number of subspecies that are roughly correlated with broad geographical regions. There exist many intermediate, transitional, or possibly hybrid plants. The three subspecies said to occur in Missouri are separated in the following key. Although these subspecies may be distinct in some parts of their range in other states, they are not clearly distinguishable throughout Missouri. Their inclusion as distinct taxa in Missouri can only be considered tentative at present.