Screen plant choices for invasive potential

Posted Oct. 30, 2009, at 7:55 p.m.

My favorite plant in Marjorie’s garden at the moment is a redvein enkianthus, Enkianthus campanulatus, planted 10 years ago and pruned to form a multitrunk small tree. This past week, as the last yellow leaves of overhanging birch and maple branches floated down to the ground, this enkianthus peaked in fall color, a vibrant reddish yellow with individual leaves at the stem tips blushing red. Marjorie places the foliage color close to that of ripe mangoes; for me it brings back memories of ripe persimmons.

Low-angled shafts of early sunlight pierce dark green conifers to light up the foliage of this small tree. Clusters of dry seed capsules glow like polished teakwood beneath each whorl of leaves. In a garden largely devoted to native plant species, this Japanese plant takes center stage.

When it comes to the garden worth of plants, I am not a native plant purist. I want a garden that is ecologically functional, providing food for its human and nonhuman occupants, offering shelter for birds, squirrels, mice and other animals, and attracting pollinators with nectar and pollen sources. These goals may dictate a mainly native flora, but they do not rule out the occasional non-native plant — as long as it is also not invasive.

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 proved 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.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@shead.org. Include name, address and telephone number.

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