Ptelea trifoliata L.

Common Hop Tree

© KBildner

Family - Rutaceae

Habit - Shrub or small tree, usually 1-2 m, incompletely dioecious, usually producing at least a few perfect flowers.

Stems - Branches ascending, the bark smooth or somewhat roughened, brown with prominent lenticels, occasionally developing a network of shallow, longitudinal ridges on large old trunks, unarmed, the twigs green, becoming brown, glabrous or sparsely pubescent with short, inconspicuous hairs, rarely densely hairy.

	Stem, older and younger portions.
© SRTurner

Leaves - Alternate, trifoliate, long-petiolate. Leaflets 2-15 cm long, the terminal leaflet longer than the lateral pair, ovate-elliptic, elliptic, rhombic, or oblong-ovate, the lateral pair somewhat oblique, angled or short-tapered to a sessile base, angled or tapered to a bluntly or sharply pointed tip, the margins minutely scalloped or toothed, the upper surface green, glabrous, the undersurface pale green, glabrous or sparsely hairy, mostly along the main veins, rarely densely hairy.

	Leaf adaxial.
© SRTurner

	Leaf abaxial.
© SRTurner

	Leaf abaxial.
© DETenaglia

Inflorescences - Terminal, sometimes on short branches and thus appearing lateral, rounded panicles appearing as loose, branched clusters, the individual flowers long-stalked.

	Inflorescence.
© SRTurner

Flowers - Unisexual or less commonly perfect. Sepals 4 or 5, 1-2 mm long, ovate, minutely hairy, at least along the margins, not persistent at fruiting. Petals 4 or 5, 4-6 mm long, narrowly elliptic-ovate to narrowly oblong, usually minutely hairy, creamy white. Stamens 4 or 5 (reduced to 4 or 5 small staminodes in pistillate flowers), alternating with the petals, the filaments free, flattened, and densely hairy toward the base, attached to a lobed, glabrous nectar disc below the slightly elevated ovary base. Ovary 2-locular, flattened (reduced to a more or less conic rudiment in staminate flowers), glabrous, each locule with 2 ovules, rounded or short-tapered to a short, slender style, the stigma more or less 2-lobed.

	Calyces.
© SRTurner

	Flowers (staminate)
© SRTurner

	Flowers (pistillate)
© SRTurner

	Pistillate flower.
© SRTurner

	Pistillate flower, rear view. Staminodes are clearly visible.
© SRTurner

Fruits - Samaras, 2.0-3.0 cm long, with a broad lateral wing, this circular to obovate-circular, papery, with a network of strong veins, short-tapered to cordate at the base, shallowly to deeply notched at the tip, the body 5-7 mm long, obovate to nearly circular in outline, flattened, with 2 locules, the outer layer hard, glabrous, strongly veined, pale green to straw-colored at maturity.

	Fruits.
© DETenaglia

	Fruit.
© SRTurner

Flowering - April - June.

Habitat - Limestone glades, prairies, forest openings, streambanks, pond margins, pastures. Nearly always on limestone soils.

Origin - Native to the U.S.

Lookalikes - Staphylea trifolia.

Other info. - This shrubby species is common in much of Missouri, except for the northwestern quadrant of the state and the Mississippi Lowlands Division. It also occurs throughout most of the eastern half of the continental U.S., most commonly in a southwest-to-northeast band stretching from Oklahoma to Michigan. It is easily identified from its long-petiolate trifoliolate leaves and small flowers, which are commonly found with both four and five petals within the same inflorescence. The plant is most commonly found on calcareous glades.

The species has been subdivided into numerous subspecies and varieties. Of these, apparently only ssp. trifoliata occurs naturally in Missouri.

Traditionally the plant has been used for a variety of purposes. The fruits were used by German immigrants as a substitute for hops in brewing beer, leading to the common name of "hop tree." Another common name, "stinking ash," refers to the unpleasant odor emitted by the flowers and bruised foliage, and also illustrates the fallacy of common names, as the plant is unrelated to ashes. Native Americans used parts of the plant as seasoning and for medicinal purposes, and the roots and bark were used as a quinine substitute. Today it is sometimes cultivated for its attractiveness to the giant swallowtail butterfly.

Photographs taken at Alley Spring, MO., 5-22-03 (DETenaglia); also at Weldon Spring Conservation Area, St. Charles County, MO, 5-11-2010, Shaw Nature Reserve, Franklin County, MO, 5-21-2015 and 5-20-2022, Danville Conservation Area, Montgomery County, MO, 5-15-2018, and Klondike County Park, St. Charles County, MO, 6-11-2022 (SRTurner), and in Lemay, St. Louis County, MO, 5-19-2020 (K. Bildner).