In a garden in tune with nature, the majority of woody plants should be native species of trees and shrubs that have co-evolved with the insects, birds, and other animals of the region. Such a garden grows as part of the local ecosystem and helps sustain local biodiversity.
But this idea of a garden should not preclude planting of non-native trees and shrubs. It is a matter of balance.
Have you noticed that one of the strongest selling points used to market a non-native tree or shrub are the words “pest free”? And it’s a fact. Non-native plants typically are not the target of native herbivores because they have no evolutionary connection with them. By the end of summer, most of the leaves on red maples and sugar maples are tattered and torn by the munching of native insects while the leaves of nearby Norway maples remain intact.
I’ve noticed the same lack of herbivory on the redvein enkianthus (Enkianthus campanulatus, native to China) and katsura (Cercidiphylum japonicum, native to Japan) in our garden. Their autumn leaves are as clean and fresh as if new.
I have no misgivings about planting the few non-native trees and shrubs that grace our garden. None are invasive and each has special significance. The redbud (Cercis canadensis) reminds me of my Georgia roots. The enkianthus, one of Marjorie’s favorite small trees, enriches our lives in every season (the bumblebees treat it as a native plant when it is in bloom) and our Octobers would be poorer without the katsura.
If I had to guess, I would say that non-native woody plants make up less than one percent of the woody plants in our garden. Three red oaks and several yellow birches, favorites of insect herbivores and predators, more than tip the scale, their canopies alive with birds throughout the year.
So, as a gardener in tune with nature, you can feel good about growing your favorite non-native trees and shrubs. Just do your homework to make sure none of your favorite non-natives is considered invasive (and thus a threat to local biodiversity), and keep the balance tipped toward native plant species.
Screening non-native plant species for invasive potential is the work of gardeners; no one else is going to do it for us. Growers and garden center managers have demonstrated, for the most part, a “buyers beware” operational philosophy. Many will eagerly sell you a Norway maple, Japanese barberry, burning bush, or any of several other known invasive species; all you have to do is ask.
Screening for invasiveness requires the gardener to do a little research, a process greatly facilitated by the Internet. Simply coupling the scientific name of a plant with the phrase “invasive species” in a search engine will bring up helpful links. For example, if you search “Acer platanoides invasive species”, you will be inundated with links to sites that leave no question about the invasive potential of Norway maple.
Entering “Enkianthus campanulatus invasive species”, you find sites that recommend the use of redvein enkianthus as a substitute for the non-native invasive burning bush (Euonymus alatus). You will find no evidence that enkianthus is an invasive threat anywhere in the United States.
There are more elaborate screening tools, including a decision-making model developed by Dr. Sarah Reichard of the University of Washington’s School of Forest Resources. Dr. Reichard specializes in the biology of invasive species.
Her model is a set of questions, each answered “yes” or “no”, and starts with the question, “Does the species invade elsewhere, outside of North America?” For Norway maple, the answer would be an easily documented “yes”.
The second question, “Is it in a family or genus with species that are already strongly invasive in North America”, would also be answered “yes” for Norway maple, since box elder (Acer negundo) has proven to be invasive outside of its native region. This leads quickly to the decision to reject Norway maple as a candidate for North American landscapes. Unfortunately, Dr. Reichard’s decision-making scheme was created long after the Norway maple was unleashed upon American landscapes.
Invasive species are available for sale in local garden centers not from lack of evidence regarding their invasive potential, but because there is a lack of concern by growers and retailers for the destruction they cause to ecosystem integrity and biodiversity. This means that it is up to gardeners, you and me, to stop the trade in invasive species with informed, intelligent choices. We have to screen non-native plants for invasive potential before buying.
A redvein enkianthus grows in Marjorie’s garden and it is the highlight of our late autumn garden. But you can bet that every year, about this time, I check to make sure Enkianthus campanulatus has not become an invasive weed somewhere in the world. It is a gardener’s duty.