Invasive Plants
In the Garden with Andrew
Andrew Bunting. Photo courtesy of the Pennsylvania Horticultural Society.
A couple weeks ago, several of us toured Little Crum Creek Park. A large portion of the tour focused on invasive plants and how they are impacting the natural areas of the park. As a gardener, I think it is valuable to have a working knowledge of invasive plants. With knowledge, our actions can help remedy their damage; conversely, in ignorance, we can inadvertently contribute to the problem.
I often hear people say, “If it is a living tree or shrub, it must be good.” It’s true that all plants produce oxygen and sequester carbon. However, an invasive species in a natural ecosystem can be very destructive. In a healthy ecosystem, many species of plants, mammals, birds, amphibians, reptiles, insects, etc., live in balance with each other. An invasive organism disrupts that balance. When an invasive plant moves in, it outcompetes other trees and shrubs, which diminishes the food sources for mammals, birds, reptiles, and bugs adapted to feed on flowers and fruits that are no longer there. This can create a cascading effect that impacts every living component of the ecosystem.
Most invasive plants were once popular ornamentals that have now invaded natural areas, causing habitat destruction by disrupting fragile ecosystems. Most of the plants we saw invading Little Crum Creek Park are the same shrubs and trees that have injured natural ecosystems in the Crum Woods and surrounding areas.
One of the most destructive invasives in our area is the Norway maple, Acer platanoides. After most of the American elms, Ulmus americana, were wiped out by Dutch elm disease, this tree gained popularity across the United States as a shade and street tree. The Norway maple is indeed durable and fast growing, but it also produces thousands of very viable seeds. These readily germinate and take hold in a variety of conditions. As the trees grow up, they cast a lot of shade, which makes it harder for desirable native species to germinate. Additionally, the roots are allelopathic, which means they produce a chemical that prevents other species from germinating and/or growing. Over time, as the Norway maple thrives and other native species struggle, our native woods slowly turn into a monoculture of Norway maples. In Swarthmore, many properties still have Norway maples as street or shade trees. Even if you don’t have an intentionally cultivated specimen, some are likely growing in unkept areas or along your fence or property lines.
The foliage and fruit of the invasive shrub burning bush, Euonymus alatus. Photo: Andrew Bunting
It would be impractical for everyone to remove all their Norway maples. However, once you are aware that you have one, you may find opportunities to remove it and replant with more desirable species.
While the Norway maple is the most prominent invasive species in this area, there are many more that you should be aware of, so you can either take appropriate actions to eradicate them or avoid adding them to your garden. Invasive plants are region-specific. A good resource for Pennsylvania can be found at: swat.ink/PA-invasives. This list continually evolves, as plants once thought not to be problematic come to be classified as invasives.
Like the Norway maple, the Bradford pear, Pyrus calleryana, was a popular medium-sized street tree and spring flowering ornamental. However, it too is a prolific spreader. It primarily invades the edges of woodlands where there is more sun. If you drive north or south on I-95 in the spring, you will see spire-like trees covering the verges of the highway. In most municipalities, the Bradford pear has been phased out and replaced with more regionally appropriate trees.
Many shrubs that were once popular ornamentals are now among the most invasive plants. We saw lots of common privet, Ligustrum sinense, on our walk in Little Crum Creek Park. Many viburnums that were once coveted both for their ornamental fruits and flowers, like Siebold’s viburnum, Viburnum sieboldii; Linden viburnum, Viburnum dilatatum; and one of my favorites, the double-file viburnum, Viburnum plicatum f. tomentosum, are now considered highly invasive. Several honeysuckles, including Lonicera maackii and four other species, are also problems. The entire understory of some Connecticut forests is covered with the Japanese barberry, Berberis thunbergii. Burning bush, Euonymus alatus, once coveted for its fire-engine fall color, is also highly invasive.
In the 1980s and 1990s, ornamental grasses started to become popular. Some, like most cultivars of Miscanthus sinensis as well as the fountain grass Pennisetum alopecuroides, have invaded natural meadows and open areas.
An entire article could be written on invasive vines, but the ones in this area to be most concerned with are bittersweet, Celastrus orbiculatus; English ivy, Hedera helix; Japanese honeysuckle, Lonicera japonica; Wisteria sinensis and Wisteria floribunda; and the five-leaf Akebia, Akebia quinata.
Many botanic gardens and arboreta are much more proactive in combating invasive plants than they used to be. Many have policies for removing invasives, along with protocols to determine the potential invasiveness of plants before adding them to their collections.
As gardeners, we need to be responsible stewards of both our gardens and the surrounding natural areas. We should strive to have a working knowledge of invasive plants so we can avoid adding them to our gardens — or, if we have them, plan for their removal.
Reader Questions
Q: I have Elaeagnus umbellata (reader, I planted it. 23 years ago), which I believe is now considered a bad actor. The bees do really seem to like it in the spring, though. I don’t suppose that makes it ok?
A: Elaeagnus umbellata, also known as Japanese silverberry or spreading oleaster, is highly invasive. Like many invasive plants, it sets an abundance of fruits that are attractive to birds which helps exacerbate the spread. Probably best to take it out, if you can.
Happy gardening!
Andrew Bunting
Send your gardening questions to editor@swarthmorean.com.
Andrew Bunting is vice president of public horticulture at the Pennsylvania Horticultural Society and vice president of the Swarthmore Horticultural Society.
