Where did that plant go? (or, when good plants go bad.)
Observant visitors to the Botanical Garden will have noticed that a very large broadleaf evergreen plant missing from the landscape south of the viewing platform (the “Ting”) at the end of the entrance boardwalk. The plant, an Asian avocado (Machilus viridis), came to us as a small seedling from the Washington Park Arboretum in Seattle in 1987. In the mountains of Southwest China where it is native,the species is a dense, suckering, domed bush that can eventually become a 25-m-tall tree. Machilus viridis has both long-lived foliage and branches that usually persist to the ground for a number of years. The copiously produced fruits of the Asian avocado are, like the more familiar American avocado, rich in oils, and they are much sought after by birds. Unfortunately, bird-transported seeds of this plant germinate readily in shade, in both irrigated and unirrigated soil, and seedlings have the ability to shade out other plants. This combination of traits—wide ecological adaptability, success in competing for resources (light, water and nutrients) and easily dispersed seeds—usually spells trouble. Other plants with similar reproductive characteristics, such as English ivy (Hedera helix), holly (Ilex aquifolium), European mountain ash (Sorbus aucuparia), cherry laurel (Prunus laurocerasus), Portuguese laurel (Prunus lusitanica), Japanese knotweed (Fallopia japonica), purple loosestrife (Lythrum salicaria), Scots broom (Cytisus scoparius), Himalayan blackberry (Rubus armeniacus) and the annual Himalayan jewelweed (Impatiens glandulifera) are now so common around the campus and the region that each might reasonably be taken for native plants. Each was originally introduced for its significant ornamental value, but all have now spread out of control. Rather than waiting to see whether seedlings of the Asian avocado started showing up in Pacific Spirit Park, we made the decision to remove the plant entirely.
As in the case of the Asian avocado, and despite significant ornamental appeal, we have been compelled to remove a number of species entirely, and to strongly recommended against their introduction as ornamentals. Development of an assessment protocol for invasive potential in the Botanical Garden is ongoing, as UBC continues to acquire exotic plants for its collections. Some of the plants we are closely monitoring include a variety of mountain ash (Sorbus) species and a small number of climbing roses and magnolia vine (Schisandra) species. Most of the plants on the watch-list are adapted to cool, shaded forest conditions and that produce fruits attractive to birds; however, wind-distributed seed producers, such as the western American seaside daisy (Erigeron glaucus) and the magnificent Kamchatka meadowsweet (Filipendula kamtschatica)have also become locally weedy in the Garden and also demand our scrutiny. Some rhizomatous plants, including the normally rather frost tender mallow bindweed (Convolvulus althaeoides) from southern Europe have become aggressive among the boulders in the E.H. Lohbrunner Alpine Garden. This pretty little ornamental species could be considered a threat elsewhere on similarly well-drained, exposed sites. In ponds in the same area, Eurasian water fringe (Nymphoides peltata) is now an increasingly visible pest.
As a botanical garden, we take seriously the threat of exotic plants becoming pests. One of the Garden’s roles is to introduce new plants to cultivation, but we believe that an even more important role is to critically assess cultivated plants for signs of invasiveness. If readers have questions about invasive plants, or what the Botanical Garden is doing about these plants, please don’t hesitate to contact us.
Submitted by Douglas Justice, Associate Director, Horticulture & Collections, February 28, 2014.