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03/15/2024 12:22 PMEvery landscape task has its best moment. Many would nominate March and April for seed starting, pruning, and cleanups. I’d like to plant a different idea: These months are a great time to cut invasive shrubs and vines. This is particularly true of Japanese barberry, which is among the first shrubs to produce leaves in spring.
Repetitive cutting before June can seriously set back and eventually end woody plant invasions. Why? Cutting deprives them of the one thing all plants need: sunshine. Cut them at ground level, and you deprive them of branches that sprout fresh leaves.
After a few years of cutting in late winter and early spring, the plants lose vigor, fail to generate seeds, and eventually die. These cutting techniques are called “root depletion” or “carbohydrate starvation.”
I learned about barberry and other woody shrubs the hard way, firsthand, as a volunteer “weed warrior” at The Preserve State Forest in Old Saybrook since 2016. Along with many other volunteers, I have spent countless hours cutting the woody stems of barberry, burning bush, privet, autumn olive, multiflora rose, oriental wisteria, and oriental bittersweet.
Though cutting is labor intensive, we’ve successfully removed thousands of woody shrubs and vines from over ten woodland acres.
You might wonder, why is cutting better than pulling? It’s because cutting allows soil to remain undisturbed. Pulling, on the other hand, can raise the buried seeds of unwanted plants, and it disturbs communities of beneficial soil microbes.
If you’re looking for more detailed instructions for specific plants, visit the Connecticut Invasive Plant Working Group website for links to fact sheets and advice on timing. (See CIPWG.uconn.edu and the links at the end.)
Woodlands and Roadsides
It’s reasonable to ask: Why are invasives so vigorous in woodlands and along roadsides?
Partly, it’s because these spaces are unmanaged by people with mowers and other cutting tools. The plants may be present, but not very noticeable in a backyard if they are repeatedly mowed. When they spread to unmanaged spaces, however, there are no blades to thwart their progress.
The seeds of invasive plants can be transported by wind, in the digestive tracts of birds and animals, and on the fur of deer and other woodland mammals. The seeds also arrive in woodlands with human visitors--on hiking boots, dog paws and fur, horse hooves, bike treads, and the tires of illegal ATVs.
Furthermore, seeds can be spread with “fill,” a loose term that describes the soil and compost deliveries that arrive during the last stages of construction.
How Did These Invaders Arrive?
Invasive plants are nothing new. According to a timeline supplied by an organization called Eat the Invaders, many plant invasions started with European arrival. (See link at the end.)
European barberry is a local example. According to Steven Johnson, Milford’s assistant director of public works, “The plant was imported by early colonists to mark field boundaries. Later, it was discovered that the plant carried wheat rust that affected crops.” The problem led to a 1797 ordinance that allowed anyone to enter anyone else’s land to remove the plant, pictured here.
Today, European barberry is banned from sale in Connecticut. But its cousin, Japanese barberry, is not banned, though it is listed as invasive on the state’s invasive plant list. This may change soon.
Research shows that the nursery industry has been a primary source of invasive ornamental plants since the 1800s.
Two recent studies from UMass-Amherst underscore this unfortunate fact. In one study, researchers observed that 55 percent of invasive species at invaded sites were also for sale at garden centers within 13 miles. See resource links below.
A recent study of seven midwestern states reported that 80 percent of invasive plants there originated in the nursery trade. See resource links below.
In other words, everyday gardeners who buy non-native plants at their local nurseries could unwittingly start or perpetuate an invasion from their backyards.
This is not to say that growers intend to unleash problem plants on their customers. Nurseries—like all businesses--respond to purchasing patterns. Many of these plants were imported for their decorative qualities. Some non-native plants deliver tempting benefits. Japanese barberry and burning bush, for instance, are very deer-resistant, require little maintenance, and offer bright autumn colors. Other invasives, such as oriental wisteria, offer exotic flowers. Given those and other benefits, the public has been purchasing these plants for a long time.
We have met the enemy, and it is us.
Should We Buy “Sterile” Invasive Plants?
Some growers offer “cultivars” of non-native invasive plants with the claim the plants are unable to produce fertile seeds. A cultivar is a plant variety altered by humans through plant breeding.
Charlotte Pyle, Ph.D., a retired ecologist from Tolland County, says we should be cautious with these so-called "sterile” cultivars.
“When talking about plants, the word "sterile" doesn't always mean what you might think,” says Pyle. “The word can mean reduced fertility, which includes plants with very few viable seeds or plants with no viable seeds at all.”
She notes that when a plant with reduced fertility produces just two percent of the species’ usual seed, it can still make a lot of seedlings. For instance, a single fertile crabgrass plant can produce 10,000 or more seeds. If there were such a thing as “sterile” crabgrass (which there is not), 200 little crabgrass babies could sprout in next summer’s lawn.
“A cultivar may be declared sterile after multi-year studies,” says Pyle. “While the plant may be sterile during the study period, it may become capable of reproduction when plants grow larger, in the case of a shrub, or when it is grown under different environmental conditions.”
Pyle points out a more vexing problem that results when "sterile" cultivars cross with wild plants: the case of the non-native European wand loosestrife. Cultivars of this plant were touted as the solution to the problem of the pretty but invasive purple loosestrife. Although the European wand cultivars were sterile in certain limited situations, they readily cross-pollinated with invasive purple loosestrife. They produced viable seeds and thus spread invasive offspring.
Pyle concludes, “The genetic manipulation behind 'sterile' cultivars is complicated, and there are scientific studies indicating that 'sterile' plants can revert to a fertile state.”
Even if plants such as specific cultivars of Japanese barberry are sterile, the plant offers nothing for native insects, birds, or wildlife. They may be decorative landscape placeholders but contribute little to local ecosystems.
Is Removal Worth The Effort?
Many people ask two questions, “Is invasive plant removal worth the effort? Won’t invasives just come back?” There are two parts to the answer.
First, if we don’t remove invasive plants at the backyard or community level, we can only expect this problem and its side effects to cover more and more land.
Second, it is true that if you remove plants and then leave the area untended and unplanted, the invasives may come back. But if you replant wisely and monitor the area for remaining invasive plant seedlings, you will likely succeed at growing a new land cover.
If you replant with as many native trees, shrubs, perennials, and grasses as possible, the new plants will provide more than beauty. They will benefit native creatures and contribute to local ecosystems. That last piece is the biggest benefit of all.
Links For Further Research
Kathy Connolly writes about plants, landscape design, and ecology from Old Saybrook. Kathy@SpeakingofLandscapes.com.