Common Hop Tree

Issue: 
May-June 2002

THE COMMON HOP TREE is a rare plant in Ontario. It is the most northerly representative of the citrus family (Rutaceae) and ranges all the way from Mexico to southern Ontario where it is restricted naturally to growing in a few places along the shore of Lake Erie. Nevertheless, it has been planted elsewhere, presumably as an ornamental, and it thrives in Montreal at the Botanical Garden and in and around Guelph. It is winter hardy, thus, it is surprising that it has not been recommended for horticultural use. Let me explain.

Apart from being winter hardy, hop trees are really treelets. They grow to a height of 5 metres, rarely more, and the trunks rarely attain girth of more than a 10 cm. They do not spread as shrubs with multiple stems. The foliage is bright green, resembling that of poison ivy in hue and trifoliate shape. Crushed leaves have a pungent citrus odour that some people find unpleasant and presumably gave rise to another common name, stinking-ash.

It is winter hardy, thus, it is surprising that it has not been recommended for horticultural use.

The plants are unisexual with pale green flowers in clusters that attract many kinds of pollinating insects. The flowers of the male plants are somewhat larger than those of the female plants. Both secrete nectar, and the males also provide pollen for some pollinators. After pollination, the fruits on the female trees start to grow and eventually become two-seeded fruits about 2.5 cm in diameter. These comprise a flat, disc-like circular “wing” or samara that surrounds the seeds – probably they are adapted for dispersal on wind. The fruits presumably gave the tree yet another common name, the wafer-ash. The name hoptree originates from the use of the fruits as a substitute for hops in brewing. Perhaps an enthusiastic home-brewer would like to try this and report back!

Apart from their attractive stature, foliage and flowers, hop trees display winter beauty. After the leaves have yellowed and fallen, the treelets’ reddish bark becomes striking, especially in contrast with the white backdrop of winter snow. The browned and ripe fruits, or keys, are retained as clusters on the female plants almost all winter. In the wind, they wave and audibly rattle.

The hop tree has colour all year round, sings in winter, and fits well into home gardens and public spaces. It is used by nesting birds and supports a variety of attractive pollinating insects from flower flies (Syrphidae) to various solitary bees. We urge that anyone interested in growing this tree obtain stock that hails from Ontario. Germination of the seeds requires care and stratification.

— Peter Kevan, University of Guelph. Photos from Trees of Ontario, Lone Pine Publishing, 2001.

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