The Hop and Prickly Ash

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For several years now, we have been harvesting seeds and growing trees and shrubs that have been forgotten by the public and the nurseries. Such 2 plants are the Hop tree and the Prickly Ash. We purposely grow these since they are the caterpillar plants for the giant swallowtail butterfly. Since both these plants are scarce in the wild, the giant swallowtail has now become a member on the endangered list.


The Hop tree is listed as rare for occurrence in Canada. In the botanical world, it is ‘the tree that got away’. This is the only tropical tree that has survived and genetically adapted to our colder climate. It derives its name because the pioneers of this area would harvest the fruit from this tree, the hops, and use it as a hop substitute for making beer.

The other interesting name for this tree is, stinking ash. Most botanical books indicate that the flowers are foul scented and are only pollinated by flies. In reality, the University of Guelph conducted a survey where they found that the Hop scent was highly variable and over 102 insects were found feeding on its flower – this is a huge number of pollinators. So don’t let the name mislead you – the name does not do it justice.

To the settlers of this area so long ago, Hop looked like poison ivy on a stick. They could see no wood value or wildlife value and so it was ruthlessly ripped out and eradicated from their fields. Even today, they are ripped out or sprayed mistakenly identified as poison ivy. Even landscapers, city planners and conservation authorities ignore this plant not seeing it true value.

This tree totally lends itself to restorative plantings. It likes full sun and favours disturbed ground. Considering a vast majority of southern Ontario is disturbed soil sites or abandoned fields this tree would be perfect for restoration projects. Even in raw sites where wind is a factor, the tree will adjust its size from small tree to large shrub.

There are really no diseases or pests to bother the Hop tree. My only recommendation is to wrap them every winter since they seem to be ‘vole candy’.

So why are people hesitant to grow the Hop tree with all the hardiness they possess? Add the bonus you have the largest butterfly in North America visiting them – it would seem obvious to give the Hop tree a try!


A fascinating bush that everybody has ignored and has faded away from the natural landscape. Here again, the common names are misleading for Prickly Ash is not a true ash but, rather, the most northern member of the citrus family. It is very fragrant when the leaves are crushed, like tangerines. The other common name is ‘toothache tree’. In colonial times the volatile, aromatic oil was used as a numbing emergency treatment for toothaches.

Like the Hop, it has been overlooked as a naturalization choice though it is an obvious choice. It is truly an adaptive plant where the nature and actual appearance of the Prickly Ash colony will conform to the planting location. If the colony is growing in moist, wooded conditions, the appearance of the colony is open with the Prickly Ash plants being well spaced at 10-13 feet apart. If the colony is growing in the wide open, dry soils, embankments or ditches, the plants form a thicket.

To avoid disappointment, plant both the Hop tree and Prickly Ash on the same site. My favourite arrangement is to plant a group of Hop trees surrounded by Prickly Ash. Though giant swallowtail butterflies can use both these plants as a larval/caterpillar source, they are odd as butterflies go in that they tend to reproduce on what they are accustomed to. For example, if a giant swallowtail lays her eggs on a Hop host, the caterpillars will be accustomed to Hop, not Prickly Ash, and tend to gravitate towards Hop when they become breeders. You maximize your chances of attracting giant swallowtail butterflies when you have both host plants.

So when you see your Hop trees and Prickly Ash having eaten leaves, REJOICE, don’t spray, for the giant swallowtail has made your place home. A truly satisfying experience.