The Friends of the Wildflower Garden, Inc.

Trees & Shrubs of the Eloise Butler Wildflower Garden

The oldest public wildflower garden in the United States

Hop Tree

Common Name
Hop Tree (Wafer Ash, Stinking Ash, Common Hop-tree, Swamp Dogwood)


Scientific Name
Ptelea trifoliata L.


Plant Family
Rue (Rutaceae)

Garden Location


Prime Season
Spring Flowering



Hop Tree is a North American native aromatic shrub that can grow into a small tree of up to 20 feet in height, with a rounded crown. It can also be shrubby with multiple stems. Crushed twigs and foliage produce an unpleasant odor, hence the alternate common name of 'stinking-ash'.

Bark is a thin brownish-gray to reddish-brown in color, smooth to slightly scaly, with horizontal lenticels and with warty corky ridges on larger trunks.

Twigs are a lighter brown, covered with fine hair, with short fuzzy buds and U-shaped leaf scars.

Leaves are alternate and palmately compound with 3 leaflets on a long leaf stalk. The individual leaflets are ovate to elliptical in shape, 2 to 4 inches long, the lateral leaflets without stalks, the terminal leaflet on a short stalk, shiny blue-green above, paler below, either with smooth margins or slightly toothed. The undersides are marked with little gland dots which produce the chemical giving off the aromatics. Fall color is yellow.

Flowers are male, female, and sometimes perfect, and appear in the spring in terminal compound clusters (cymes), each stalked flower 1/4 to 1/2 inch wide with 4 or 5 narrow greenish-white hairy petals, 4 to 5 stamens on the male flowers. The sepals are much shorter than the petals. Female flowers have a single flattened pistil. Flowers also have an unpleasant odor.

Fruit: Flowers mature in summer into drooping clusters of one inch wide, light brown, round samaras that are wafer thin, hence the old common name of 'wafer-ash'. The samara contains 2 to 3 brown seeds which are surrounded by the thin rounded wing that shows many reticulated membranes. These remain on the tree into winter which helps with identification of this tree. The samaras have the odor of hops.


Habitat: Hop-tree will grow in rich woodland soils with adequate moisture and partial sun and can be found as an under-story tree to the larger woodland trees, on sandy stream edges and ledges. The root system is woody but does not reproduce from the underground parts.

Names: The species name, trifoliata, is a reference to the 3-leaflet leaf. The genus, Ptelea, is from the Greek for an elm, which also has similar, but smaller, winged seeds. Despite the alternate common name of Wafer-ash, it is not a member of the ash family. That name comes from the thin samara that is round like a wafer, not elongated like most ash seeds. but like the ashes, the thin samara surrounds the seed. The author name for the plant classification - 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.

See bottom of page for notes on the Garden's planting history, distribution in Minnesota and North America, lore and other references.

Hop Tree Hop Tree twig Hop tree bark

Above: 1st photo - Hop Tree can frequently form a multi-stemmed shrub. 2nd photo - twigs are light brown with U-shape leaf scars. 3rd photo - On smaller stems the bark is thin, brownish-gray to reddish-brown in color, smooth to slightly scaly, with horizontal lenticels.

Below: Flowers appear in cymes, male and female usually separate.

Hop Tree flower cluster

Below: 1st photo - Female flowers have 4 or 5 narrow greenish-white hairy petals, and a flattened pistil. 2nd photo - Male flowers have 4 or 5 narrow greenish-white hairy petals and 4 to 5 stamens.

Hop Tree flower Male Flower

Below: 1st photo - The large compound leaf. 2nd -photo - Seeds are housed in round samaras, in drooping clusters.

leaf Hop Tree Green seed

Below: Each samara contains 2 to 3 brown seeds which are surrounded by the thin rounded wing that shows many reticulated membranes.

Hop Tree green seedsHop Tree brown seed


Notes: Hop Tree is not indigenous to the Garden. It was originally introduced to the Garden by Eloise Butler in 1908 with plants sourced from Kelsey's Nursery in North Carolina. Then in Oct. 1917 she planted 4, obtained from the Park Board Nursery which had begun growing the tree. Martha Crone noted it in bloom in 1938. Garden Curator Susan Wilkins added additional plants in 2008. Hop Tree is found in Canada in Ontario, Quebec and P. E. Island. In the U.S. its range is the eastern half of the country with Minnesota being the NW corner (although tenuous) of the range and with Texas the SW corner, then eastward. It is considered endangered or threatened in PA, NY, NJ and of special concern in Wisconsin. Within Minnesota, the only known specimen collected was in Hennepin County and that is thought to have been planted, as the location is far from the nearest known native populations in southern Wisconsin. It does exist in other landscape plantings besides the Garden, such as the Minnesota Landscape Arboretum.

Uses: In years past the samaras were used as a substitute for hops (and hence the common name) in brewing beer. However, the bitter taste did not prolong the use. But the chemical make-up of the samaras also resulted in an additive to yeast for bread-baking. A decoction of the samaras added to the yeast produced a rapid increase. (Ref. #6). Likewise, the bitter bark and root found some use in folk medicine as a tonic, stomatic and stimulant. The active ingredient is known as Petelein. (Ref. #12). Moore (Ref.#30) reports on the use of the leaves, which contain a volatile oil, for making a tea to treat poor digestion, heartburn or dyspepsia.

References and site links

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.

graphicIdentification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.