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 two 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.
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.