Ptelea is a small genus of deciduous trees and shrubs in the citrus family, Rutaceae, placed in the order Sapindales. Native to North America, the roughly three to nine recognized species range from the eastern United States and Canada westward to California and south into northern Mexico. Members of the genus are commonly called hoptrees, a name that reflects the traditional use of the fruit as a substitute for hops in brewing.
The plants are typically multi-stemmed shrubs or small trees, seldom exceeding six metres in height. Leaves are trifoliate and aromatic, and all parts of the plant emit a somewhat disagreeable odour. The flowers are small, greenish-white, and notably pungent, attracting carrion flies as pollinators. The fruit is a distinctive two-seeded samara with a broad papery wing, closely resembling an elm fruit — the morphological similarity that led Carl Linnaeus to name the genus after the Greek word for elm.
The best-known species, Ptelea trifoliata (common hoptree), is widespread across eastern North America. Ptelea crenulata (California hoptree) is endemic to California, where it grows in canyons and chaparral. Ptelea aptera from Baja California lacks the conspicuous fruit wing that characterises most of the genus.
Ptelea trifoliata has a long history of ethnobotanical use. The root-bark has been used medicinally as a tonic, stomachic, and antiparasitic agent, and was employed by various Native American peoples to treat fevers, poor digestion, and intestinal worms. The bitter, hop-like fruit was used as a brewing adjunct and as a leavening aid for bread.
Etymology
The genus name Ptelea is the classical Greek word for the elm tree. Carl Linnaeus applied it to this genus in reference to the close resemblance of its flat, winged samara fruits to those of elms, even though the two groups are unrelated.
Distribution
Ptelea is native to North America, ranging from eastern Canada and the eastern United States west to California and south into Baja California and northern Mexico. Ptelea trifoliata occupies the broadest range across the east, while Ptelea crenulata is endemic to California (canyon habitats) and Ptelea aptera is restricted to Baja California.
Ecology
Hoptrees grow in moist places, rocky slopes, woodland edges, and alluvial thickets. They tolerate a wide range of soil textures (sandy to clay) and pH (mildly acid to mildly alkaline), and can grow in semi-shade or full sun, though they prefer well-drained, moist conditions. The strongly scented flowers are pollinated by carrion flies rather than bees. Plants are notably resistant to honey fungus.
Cultivation
Ptelea trifoliata succeeds in any fertile, well-drained, moisture-retentive soil in full sun or light partial shade. It is slow-growing and generally short-lived in the wild. Young spring growth is susceptible to late frosts. Ptelea crenulata is grown by California native plant nurseries as an ornamental shrub or small tree, valued for water-conserving and habitat-restoration plantings.
Cultural Uses
The hop-like fruits of Ptelea trifoliata have historically been used as a substitute for hops in beer-making and as an additive to yeast for leavening bread, giving the genus its common name. The root-bark has wide ethnobotanical use: it has been employed as a tonic, stomachic, anthelmintic, and antibacterial agent, and was used by Native American peoples to treat intermittent fevers (including malaria), heartburn, roundworms, pinworms, and poor digestion. The bark is also applied externally to wounds.
Propagation
Seed is best sown fresh as soon as it is ripe in a cold frame. Stored seed requires approximately three months of cold stratification at 5°C before sowing. Seedlings should be grown on in light shade for at least their first winter before planting out after the last frosts. Vegetative propagation is possible via half-ripe cuttings taken in July or August in a frame, or by layering.
History
The genus was described by Carl Linnaeus, who adopted the Greek elm name ptelea for the group due to the elm-like appearance of the samaras. Ptelea trifoliata was the earliest species described and remains the type species. The genus has long been recognized within Rutaceae; modern molecular phylogenies confirm its placement in the order Sapindales alongside the citrus relatives.
Taxonomy Notes
Ptelea belongs to the family Rutaceae, order Sapindales. Depending on the taxonomic treatment, the genus is considered to contain between 3 and approximately 11 species, with GBIF recognizing around 20 infrageneric taxa. Some older treatments recognized far more species, but many have been reduced to synonymy or subspecific rank under Ptelea trifoliata.
