1. Toxicodendron pubescens Mill. (eastern poison oak)

Rhus toxicodendron L.

T. toxicarium (Salisb.) Gillis

R. toxicodendron var. quercifolium Michx.

T. quercifolium (Michx.) Greene

Pl. 201 e; Map 835

Plants low shrubs 0.3–0.5 m tall, spreading by rhizomes, not climbing, not producing aerial roots. Leaves often clustered near the tip of the stem, the petiole 5–15 cm long. Leaflet stalks 1–3 mm long in lateral leaflets, 0.5–2.0 cm long in the terminal leaflet. Leaflets 3–10 cm long, 3.0–7.5 cm wide, ovate to obovate, angled at the base, rounded to broadly and bluntly pointed at the tip, the margins with 3–7 coarse, deep, rounded lobes, those of the lateral leaflets usually somewhat asymmetrical (deeper on the lower side than on the upper side) and sometimes with a few blunt teeth, the upper surface sparsely hairy, the undersurface sparsely to moderately hairy, especially along the veins. Inflorescences 1–4 cm long. Sepals 1.0–1.2 mm long, narrowly ovate, green. Petals 2.2–2.4 mm long, oblanceolate, cream-colored with dark veins. Ovary densely hairy. Fruits 4.0–4.5 mm long, 4.0–4.5 mm wide, globose, greenish white to tan, the outer layer separating at maturity to reveal a powdery white middle layer, the stone kidney-shaped with a smooth surface. 2n=30. May–June.

Uncommon in the southern portion of the Ozark Division and in the northern part of the Mississippi Lowlands (southeastern U.S. west to Kansas and Texas). Glades, openings of dry upland forests, sand prairies, and sand savannas; also roadsides.

Toxicodendron pubescens is recognized by a combination of its shrubby, nonclimbing habit, deeply lobed leaves, hairy fruits, and occurrence in low-nutrient, sandy or rocky soils. It produces aerial stems from rhizomes, but it never climbs or produces aerial roots as in T. radicans. In its typical form, the leaflets of T. pubescens are deeply lobed and reminiscent of the leaves of white oak or blackjack oak, hence the common name poison oak. Unfortunately, this common name is frequently misapplied to lobed forms of T. radicans as well. According to Gillis (1971), the lobes are deeper on staminate plants than on pistillate plants. Although the fruits are usually sparsely hairy, occasionally they are nearly glabrous. A few putative hybrid specimens between T. pubescens and T. radicans have been identified, but the relative rarity of hybrids was a factor in the recognition by Gillis (1971) of T. pubescens as a distinct species.

The nomenclatural history of T. pubescens is quite complex (Fernald, 1941a; Gillis, 1971; Kartesz and Gandhi, 1991; Reveal, 1991a). The lobate-leaved, pubescent forms of poison ivy originally were described as Rhus toxicodendron L. However, when classified into the segregate genus Toxicodendron, the name Toxicodendron toxicodendron (L.) Britton is not allowed under the International Code of Botanical Nomenclature because it would form a tautonym (a binomial in which the generic and specific epithets are identical). Gillis (1971) recognized poison oak under the name T. toxicarium, which was in turn an illegitimate combination, leaving the next available legitimate name within Toxicodendron as T. pubescens.