Invasive plants on the rise in Vermont

Japanese Knotweed.  Photo courtesy KENPEI, Wikimedia Commons
Japanese Knotweed. Photo courtesy KENPEI, Wikimedia Commons

Ever had phytophotodermatitis? If you had, you’d probably know it: blisters, reddened skin like second-degree burns, terrible discomfort.

The kicker is, it’s caused by a relative of the carrot. And it’s a newcomer to Vermont.

Poison parsnip is a plant with broad, low leaves and yellow flowers on a tall stalk. Touch the sap, expose it to sunlight, and — ouch — the plant sap reacts with sunlight and burns your skin.

There are a lot of plant newcomers in Vermont these days, some of which, like poison parsnip, are considered noxious weeds. Some scientists say climate change will help noxious weeds spread and thrive, while making it harder for many native species to make a living. Others aren’t ready to take a stand.

Tim Schmalz, a plant pathologist with the Vermont Agency of Agriculture, Food & Markets, says he’s skeptical of studies that say particular weeds will spread as the climate changes.

“There are a lot of things that may happen or may not happen,” said Schmalz. “It’s very hard to predict environmental change on that scale.”

Lewis Ziska, a plant physiologist with the U.S. Department of Agriculture, says otherwise. According to Ziska, weeds in general will do better in a warmer world — there will be more weeds, bigger weeds, different kinds of weeds.

“The factors in climate change that are relevant here are really related to carbon dioxide and temperature. In that context, and I’m speaking in very broad terms, what we see is that weeds tend to respond more to the carbon dioxide that has already occurred and is projected to occur in this century,” he said.

Ziska says Vermonters should watch out for weeds that are already major problems in the South: kudzu, pale swallowwort, Japanese barberry, and giant hogweed, which causes phytophotodermatitis many times worse than poison parsnip. Ziska also says we can expect more poison ivy.

Several Vermont agencies have weed-control programs. The Vermont Agency of Transportation times its roadside mowing to kill poison parsnip before it goes to seed, for example. But according to Schmalz, at least in the Agency of Agriculture, Food & Markets, funding is tight and weed control must compete with other priorities.

Until recently, state officials didn’t have many weeds to worry about. Vermont’s cold winters kept many of them at bay.

“What we’re seeing is not just for weeds, but for a range of different animals and a range of different plant species, that one of the things that keeps species where they normally are is cold winters,” said Ziska. “As the winters warm, it expands the range that species can move into.

Not every species takes advantage of that. Oftentimes, the species that do take advantage of that are invasive species. Those are the species that tend to move very rapidly when there’s a shift in the climate.”

Kudzu, a fast-growing vine that covers forests and farm fields in the South, has already been found in Massachusetts and Ontario.

Kudzu in Port Gibson, Mo. Photo courtesy of Galen Smith, Wikimedia Commons
Kudzu in Port Gibson, Mo. Photo courtesy of Galen Smith, Wikimedia Commons

When Massachusetts state officials first visited the property in Needham that had a kudzu infestation, they estimated the plant to cover 150 square feet. When they came back a week later, the plant — what they believe is a single individual — had grown noticeably.

“Seeing it like that gives you an idea of how bad it could get if you let it go,” said Jennifer Forman-Orth, state plant pest survey coordinator with the Massachusetts Department of Agricultural Resources.

The site in Needham is just one of four kudzu infestations that various government agencies are battling in Massachusetts. Though they are small compared to the acres of kudzu-choked forests you can find in the South, these patches represent the forerunners of an expanding threat.

Kudzu, native to Japan, is among a number of noxious weeds that some scientists say will likely prosper thanks to climate change. A 2010 study by researchers at Princeton found that kudzu’s bioclimatic envelope — the conditions in which it can survive — will include Vermont by the year 2100.

Research also shows that kudzu has growth spurts when the air around it is enriched with carbon dioxide, according to the Northeast Climate Impact Assessment, a hefty report that outlines major predictions for climate, ecology, agriculture, and business for the Northeast.

Paul Marangelo, a conservation ecologist with The Nature Conservancy, said it’s hard to say which weeds will show up and when — and what effect they will have.

“It’s a little more theoretical than actual at this point,” said Marangelo. “It has to do with anticipating that as the climate warms, natural communities are going to change in unpredictable ways. Some of the changes in the climate might provoke more disturbance events like fires or some other phenomenon that disturbs natural communities. Those disturbances can provide opportunities for invasive plants to come in and get a foothold and start to grow.”

Invaders of concern, says Marangelo, include mile-a-minute vine and Japanese stilt grass, both of which cut off sunlight to surrounding plants.

“There are a bunch of species that are poised to make their ways up this way and they’ll probably do a little bit better in Vermont than they would have otherwise,” said Marangelo.

Not only are there more invasives in a warm climate, but non-native species appear to have a more plastic response to temperature changes. A team of researchers led by Elizabeth Wolkovich of the University of British Columbia studied hundreds of plant species in five sites, four of which were in the U.S. (including one in Concord, Mass.). They used information dating back to the 1850s, when Henry David Thoreau recorded when flowers bloomed for 10 years at Walden Pond. During the last 160 years, the climate of Concord has warmed by almost 2.5 degrees Celsius.

Wolkovich and her team found that across all sites, the exotic species flowered earlier than native species. More than that, they adjusted their flowering time depending on the temperature — in warmer years they flowered earlier, while in cooler years they flowered later. Native species tended to flower at the same time every year, regardless of the rising temperatures.

A group of researchers led by Charles Willis from Harvard University studied Thoreau’s woods in particular. In Concord, invasive species flowered 11 days earlier than native species and nine days earlier than non-native species that weren’t classified as invasive. The research team found that the inflexibility of the native species was linked to declines in the abundance of those species.

Impact on farmers

Vern Grubinger, a professor at the University of Vermont agricultural extension office, said that it’s difficult to pinpoint a single cause for the rise of invasive species. Some weeds and diseases get transported on farm equipment or manure. “It’s just hard to say. Is that temperature or more movement of plants and seeds?” asked Grubinger.

Rather than one force, Grubinger thinks it’s the synergy of multiple factors that is scary. “To me the biggest concern is the combination of the shifting change and this intense commerce, moving things around globally, that together elevate uncertainty and risk for new things arriving,” he said.

Grubinger is concerned that climate change brings so many new concerns to farmers that they might get overwhelmed. “Farmers can’t deal with all of this all at once. They need to prioritize. In the case of some of these invasives, it may mean that you have to change your crops if you’re not willing to do the management that’s necessary.”

The necessary management might be costly. According to the Northeast Climate Impact Assessment, weeds respond more positively to increases in carbon dioxide than crops do, typically. Worse, increased carbon dioxide concentration in the atmosphere appears to reduce the effectiveness of certain herbicides, like Roundup. Southern farmers consistently lose more of their crops to weeds. So, more weeds plus less effective means of control equals more losses. And even though all plants put on more growth with an atmosphere richer in carbon dioxide, as far as scientists can tell that’s not enough to compensate for the negatives.

Japanese stiltgrass
Japanese stiltgrass

According to Ziska, $58 billion worth of crops are lost to invasives, disease, and weeds each year. A full quarter of U.S. agricultural production is lost to invasive species each year.

The link between climate change and invasion for many species is not always clear. Take Japanese knotweed. This hollow-stemmed ornamental plant spreads by underground stem and when it’s torn apart by floods, pieces of the stem wash into new areas. These grow into monocultures of knotweed, crowding out native species. The floods of Tropical Storm Irene certainly spread knotweed further afield, but it was spreading anyway.

Though many non-native species never change their new habitat substantially, a few do. The species that do, called invasive species, can wreak ecological, economic, or public health havoc in their new home. Invasive species may be relatively tame in their home habitats, but when released into a new place without natural predators, they get out of control.

Some invasives don’t change a whole community, they merely remove a member. Dutch elm disease wiped out the fountain-shaped elms that formed arcing galleries along Vermont’s riverbanks. Chestnut blight took out the American chestnut, so that now it exists largely as resprouted shrub-like trees.

That doesn’t mean invasives are always detrimental. Ariel Lugo, director of the International Institute of Tropical Forestry, has published a number of scientific papers suggesting that invasive species may in some cases be ecologically beneficial. Invasive trees helped speed reforestation in Puerto Rico, for example. In the desert Southwest, an endangered bird called the Southwestern willow flycatcher now nests in the invasive tamarisk tree that took over its riverside habitat. Sometimes, urban biological diversity is made up largely of non-native species.

Regardless of whether they harm or help, by definition invasives are game-changers, and they’re on the rise.

Forman-Orth says Massachusetts is dealing with the kudzu infestations early so they spend less later on. “Massachusetts has limited resources to take care of invasive plants — that’s true for every state,” she said. “We’re focusing on early detection and rapid response to smaller infestations that lets us utilize our limited resources in a smarter way.”

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Audrey Clark

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  • Linda Quackenbush

    Commerce is the biggest reason for the surge in invasive species! Ballast purging and imported goods of foreign shipping companies are the biggest reason for the increase of foreign invaders. The Black plague was spread as a direct result of international commerce through ships packed with goods including bolts of silk that were infested with rat fleas. NAFTA and CAFTA is spreading the importation of disease and foreign invaders…

  • Connie Godin

    Kudzu in Mass, unreal, terrible. I’ve noticed the towns in VT just cut the knotweed therefore spreading it. If this happens to all our invasives it dosen’t bode well.

  • Kelly Stettner

    As Forman-Orth states, EARLY DETECTION and proper identification can by the real key to a rapid response on smaller populations of these problem plants, which translates to less money spent and less mental exhaustion in the long run. Note to EVERYONE: learn what these plants look like, how to most wisely manage them on your property, and who to turn to for help.

    A “war on weeds” mind-set is useless, but a long-term management philosophy can help us all to strategize and prioritize and work collaboratively.

    Invasive plants are likely to never be “controlled” 100%, but we can learn how nature works and figure out how to stop making the problem worse. So many resources available, just keep looking and you’ll find local organizations willing (EAGER!) to help.

  • John Murray

    For many species, including humans, the climate may be changing faster than we can keep up. http://clmtr.lt/cb/wVA0bJd

  • Peter Conlon

    On the western side of the state, poison parsnip is spreading very fast, overtaking fallow fields in a couple of seasons. Meanwhile, sumac seems to be thriving and taking over roadsides after they are cleared for power lines, then it heads into other cleared areas. There does not seem to be effective control for either species.

  • Joanie Maclay

    Poison parsnip….wild caraway….or known by any number of names…
    This as I know it Poison Parsnip seems to be everywhere this year..especially by VT roadsides. Most has gone to seed providing for a larger crop next year. I have been terribly allergic to this “stuff” and had my first encounter while around 9 years of age. Many, many years ago.
    Not fun!!

  • Bruce Post

    Actually, I think the headline should read: “Invasive humans on the rise in Vermont.”

  • John Murray

    As climate change accelerates, the well-being of thousands of species could be in real danger. http://clmtr.lt/cb/wVA0bJd