Home News and Features Gardeners: Beware Invasive Species 

Gardeners: Beware Invasive Species 

Japanese knotweed. Photo courtesy of Wikipedia.
As Vermonters venture out to yards and gardens, many will decide which species to plant or remove. While aesthetic considerations can be important, another crucial question is what role a plant might play — not only in a lawn or garden, but in the larger Vermont environment. Invasive species can easily hop a garden wall or fence, becoming a nuisance or even a danger to other plants and animals that call Vermont home

“Invasive plants, they’re not inherently malicious; they’re just doing their thing,” said Sean Beckett, program director of the North Branch Nature Center. “But in the process of them just living out their life, they elbow out and eradicate a tremendous amount of biodiversity … and by biodiversity I mean numbers of species of other plants — that were there before.”

Despite the way we often talk about invasives “they are not “evil,” said Montpelier Tree Board chair John Snell. “They are simply a plant that is in an out-of-balance place, usually resulting from human action.”

Invasives to Watch Out For 

Garlic mustard — a tasty wild edible —is a common invasive plant that appears along roadsides and in yards (among other places), according to the Vermont Invasives’ website (vtinvasives.org). Garlic mustard has the typical invasive trait of spreading easily and outcompeting native plants for space, sunlight, and other resources; it tolerates shade and spreads readily in forests, crowding out other groundcovers. But it has another detrimental quality: it alters nearby soil chemistry, making it more difficult for seeds of other plants to reestablish once garlic mustard has claimed its spot.

Goutweed (or bishop’s weed), too, is happy on a shady forest floor, where it overtakes native groundcover plants while preventing young trees from getting established. Autumn olive, on the other hand, is an invasive shrub that fills in open areas, overtaking habitat from native species, according to Vermont Invasives. 

“It is tough or impossible to get totally clean of some invasives,” said John Snell, chair of the Montpelier Tree Board. “Goutweed will grow on our graves. Knotweed and Phragmities are also VERY difficult to eliminate. Garlic mustard can be eliminated but only if you persist for 5–10 years, best in spring and fall.”

Multiflora rose often appears along trails and woodland edges, where it can form dense stands leaving no room for native species. Common and glossy buckthorn are two more to look out for, per the Vermont Agency of Natural Resources. Glossy buckthorn, like most invasives, takes over habitat and shades out native plants. Common buckthorn does, too, but also produces berries that act as a laxative, so mammals eat them but can’t digest them.

Honeysuckle is harder to avoid because in addition to four invasive types, there’s one native variety, Vermont Invasives notes. Like common buckthorn, honeysuckles produce berries. The invasive honeysuckle species are attractive to birds, but not very nutritious. These plants take up space and resources  that could have gone to plants that would feed other native species via relationships that have evolved over millions of years. 

Honeysuckle. Photo by John Snell.
“Because many of these invasive species are not adapted to this local area — they’re not historically part of the flora of the Northeast — it means none of the other coadapted insects and birds and whatever have evolved to use that plant,” Beckett said. “Invasive species, if they’re not part of the flora, they’re essentially ecologically irrelevant. They’re almost like ghosts when it comes to their impact on the landscape.” 

To see the difference between a plant that plays a needed ecological role and one that’s just taking up space, Beckett suggested residents “go and try to find a leaf of Japanese knotweed that has bites taken out of it. And then go to native shrubs and try to find leaves that don’t.”

The Big One: Japanese Knotweed

No discussion of invasive plants would be complete without a huge, exasperated shoutout to Japanese knotweed. Knotweed is particularly difficult to remove because it expands via a vast network of underground rhizomes. Even a small bit of its root can grow a new plant. 

“It only takes a few grams of knotweed root biomass in order to establish a new plant downstream,” Beckett noted. “That is exactly what it’s evolved to do, is to have a weak root system so it can get washed away really easily.” Knotweed often appears along riverbanks, bad news for basically every other multicellular organism nearby. First, knotweed spreads so quickly that it outcompetes native flora. And, noted Beckett, unlike the roots of native plants, knotweed roots don’t stabilize the soil, which means riverbanks, once overtaken by knotweed, easily erode. 

“Disturbance is a normal and natural thing that happens all the time in rivers, but the scope and scale to it has changed a lot in that if all you have is knotweed, then there’s nothing holding that bank in place,” Beckett said. “So it doesn’t take nearly as much of a change in flow direction or storm event to be able to incise and cut out a huge area of land where otherwise a healthy root system of shrubs would’ve kept that in place under those circumstances.” Climate change is a compounding factor, Beckett added, with more intense storms occurring more frequently, creating extreme shifts in water levels when they hit. 

An example of knotweed’s rapid spread can be seen along the North Branch River Park’s Invasives Trail, Beckett noted, which hosts a series of interpretive panels identifying invasive species. Except none of the species pictured on the panels is still there, said Beckett — because they’ve all been replaced by knotweed. It happened so quickly, Beckett said, that “I don’t even think knotweed is one of the plaques.”

How to Keep Invasives At Bay

So what if you’ve (gasp) found one of these invasive plants in your yard? Different methods of eradication are recommended depending on the species you’ve found. At least one Facebook group is solely dedicated to its members’ ongoing battles against Japanese knotweed. Beckett suggests two general approaches.

First, don’t wait. “Get on top of it sooner rather than later because it will spread exponentially … but it’s much easier to keep after a patch that’s a couple square feet than a patch that’s 20 meters across.” And, Beckett said, keep any invasive species you remove in a separate location in your yard — NOT your regular compost pile. This will make it easy to keep an eye on the location to ensure nothing has escaped and begun to take off across your yard or into the woods. 

Finally, Beckett said, it’s on individuals to ask questions about the plants they purchase. He suggested shopping at nurseries that sell native plants and confirming that plants you select are native to Vermont, not just the Northeast. 

Seas of green are not all the same, said Beckett, even if to the untrained eye the riverbank looks the same as it did a few years ago. “[If] there’s now a patch of knotweed instead of a patch of dogwood, to the average person walking by, they might just think oh, well, it’s just as leafy … what’s the big deal?” It is a big deal, though, Beckett explained: “One plant harbors hundreds and hundreds of different specialized interactions with bugs and insects and everything, and the other harbors none of it, so even though they might look the same at a glance, invasive species are really one of the biggest threats to biodiversity in the country.”

Dealing with Invasives

Controlling invasives is a 3–10 year task, with treatments such as cutting often repeated every month or more. 

  • Start at the EDGES of the area and work toward the middle. Too often people start at the middle and never reach the edges, so the invasive just keeps growing. 
  • Norway maple shades out other native plants; it is easy to distinguish it by pulling a leaf off and looking for the white sap that quickly exudes. Best to girdle these trees to the wood at two feet and four feet off the ground; they will typically die in 2–3 years and then you can cut them. If you cut them down rather than girdle them, they will just sprout back up.
  • Barberry, unless huge, is easy to pull after digging around the roots and then pulling the whole plant; hang it somewhere until roots dry and the plant is dead. Small roots left in the ground will sprout back, so keep an eye on it for a couple years and dig them out early.
  • Buckthorn is best cut a few inches above ground level and then a THICK piece of plastic wired around it. After a couple years most will be dead. If you dig them up, you simply create a bed for seeds to sprout.
  • Herbicides should be used only by a professional as part of an overall management plan.
  • Many invasive plants are still being sold by nurseries! Do not buy or plant them. 
—John Snell