Not So Innocent Invasion

Invasive plants may look harmless, but can damage woodlots
By Patrick White

There's no shortage of science fiction movies where invaders attack from outer space. These Martians are usually terrifying in appearance and all-powerful.

Right now, woodlots in the Northeast are under attack from a seemingly less terrifying enemy - invasive plants. These invasives have the power to completely alter forest ecosystems, but unlike creatures in a movie, they look innocent enough. That innocuous appearance can fool some woodlot owners. "On the surface, they look like every other plant out there. There's no skull and crossbones or anything like that on them," says Chris Mattrick, a botanist with the U.S. Forest Service who works in the White Mountain National Forest in New Hampshire.

In that sense, invasive plants might seem less threatening than invasive pests, though both can devastate our forests. "You can make an awesome picture of an evil-looking Asian longhorned beetle, but it's hard to create the same image with a barberry plant," Mattrick points out.

Fortunately, knowledge of the problem is growing. "Invasive plants can harm forests on a number of different levels, depending on the species," explains Mattrick. First is the simple crowding factor of these plants due to their competitive advantage. "Invasives are, by nature, non-native species. So they generally don't have any natural enemies or pests that keep them in check," he says. "These species can expand ad nauseam, because they don't have to fight off native fungus and diseases."

Nor do they suffer as much from larger pests, like deer, who prefer to browse on native vegetation and often leave invasives untouched. "So you get these forests that are all Japanese barberry, Japanese stiltgrass, bittersweet and things like that," says Mattrick.

Just as bad is the fact that some invasive plant species can actually chemically alter the forest soil. "They can adjust the soil in which they grow to suit them. There's research on Japanese barberry, for example, that shows over time it will actually raise the pH level in the soil," says Mattrick. That not only gives the barberry an advantage, but also harms native plants that performed better in the original soil pH. "So the barberry, which produces lots of fruit, expands and expands and expands, and you reach the point of critical mass where there's enough barberry to significantly alter the soil chemistry in the forest," he adds.

One of the worst invasive plants from a woodlot owner's perspective is glossy buckthorn. "It's a small tree/shrub, and it casts such a dense shade that nothing will get established underneath it," explains Chris Mattrick.
Photo by Rob Routledge/www.forestryimages.org.

Another competitive advantage that some invasives have over native plants is that they are "generalists," explains Mattrick: "They don't care whether they grow in sun or shade, in wet soils or dry soils. Unlike our native plants, which usually have a niche that they occupy, invasives have a very broad spectrum of habitat."

Some invasive plants were brought to this country accidentally, while others were imported as horticultural specimens "and planted all over the place," says Mattrick. No matter the method of their initial introduction, many of these invasive plants continue to spread their reach and currently impact many areas of the Northeast, he explains. Japanese stiltgrass and mile-a-minute vine, for example, got introduced in the south and have been moving north. "Then there are more northern species that are moving south, like wild chervil," Mattrick adds.

There are different species in different areas, but generally speaking, you're likely to see the top 10 occur over and over throughout this region, states Mattrick. "When you look at things regionally, the southern areas are more infested than the northern areas are. And the more disturbed - or developed - areas are more infested than the less infested areas."

Land use history plays a role in the current prevalence of invasive plants, says Mattrick. He cites as an example the White Mountain National Forest, which was heavily logged in past years but currently has relatively few invasive species. The Green Mountain National Forest in neighboring Vermont has many more, thanks to the history of agriculture in that state, where much of the land was once plowed and used for sheep farming. "Invasives are what I call 'disturbance hounds.' They love ground disturbance. That's how they get a toehold, and once they get a toehold they spread pretty readily," he states.

Mattrick cautions that one of the worst invasive plants from a woodlot owner's perspective is glossy buckthorn. "It has been shown to outcompete sugar maple regeneration. So once it gets into the forest, it forms a very thick layer in the understory. It's a small tree/shrub, and it casts such a dense shade that nothing will get established underneath it," he explains. "So even shade-tolerant species like sugar maple can't take the shade cast by glossy buckthorn."

Japanese barberry is a problem in woodlots for the same reason - it forms such dense thickets that native species cannot compete. "In high pH soils in southern areas, it's very, very abundant," says Mattrick. Garlic mustard is an herbaceous plant that also poses a danger in woodlots, he adds. "It's a species that will just carpet the forest floor, and thereby have an effect on seedling recruitment," he notes.

While there's a growing awareness of the danger these plants can pose, it can be a challenge to figure out what action to take if you notice invasives in your woodlot. Mattrick says, "It can seem very hopeless and daunting, if you look at too big a picture. And that's the message I try to tell people: Don't look at the state or your town, because if you look at the big picture it can get very overwhelming very rapidly, especially in places like Connecticut and Massachusetts and portions of New York and Pennsylvania." Rather, he advises, "Look at your land and figure out what you can do to effect change on your land."

Focusing on what you can do in your backyard makes battling invasive plants a more manageable situation. The next step is to determine what types of control techniques to employ to combat the invasives. "There's a whole range of choices from mechanical to chemical to biological - there's all kinds of things out there that can be used to control these species," says Mattrick. "But it does take some time - there's no one-time treatment. Invasives will have a tendency to come back, so you have to be vigilant."

Some landowners opt to suffocate the invasive plants. If the infestation is small, a heavy cover of UV-stabilized plastic sheeting can kill it off, but plastic must be left in place for at least two years. Hand-pulling can also be effective, though it's important to get as much of the root system as possible, which can sometimes require digging. Continually cutting the plants (three or four times per year for as many as five years, Mattrick advises) can also eventually rob the plant of the nutrients required to regenerate. Herbicides can be applied to the cut stems in high concentration to aid in control.

Perhaps the most effective method for large-scale control is foliar-applied herbicides. Even chemical control often impacts only what's aboveground, so seeds remaining in the soil will remain able to grow. "Herbicides have been very effective in controlling Japanese barberry, even when applied on large-scale infestations. It's very susceptible to that," Mattrick explains. Garlic mustard, on the other hand, is more challenging to control chemically. "It's a biannual species. In small infestations, really the best thing to do is just hand-pull it before it flowers," he notes. Larger infestations that cover acres may require a preemergent herbicide.

Invasive Plant Resources

In addition to state and federal forestry agencies, there are many government and private organizations that can provide information and guidance on controlling specific invasive plants. Here are just a few in the Northeast.

Center for Invasive Species and Ecosystem Health (www.invasive.org)

Cornell University (www.invasiveplants.net)

Invasive Plant Atlas of New England (www.eddmaps.org/ipane)

Maine Invasive Species Network (www.umaine.edu/invasivespecies)

Massachusetts Invasive Plant Advisory Group (www.massnrc.org/mipag)

New York Invasive Species Clearinghouse (www.nyis.info)

State of Pennsylvania (www.dcnr.state.pa.us/forestry/plants/ invasiveplants)

U.S. Department of Agriculture (www.invasivespeciesinfo.gov)

Vermont Invasives (www.vtinvasives.org)

Buckthorn can also be effectively controlled with herbicides, but it's a matter of scale and how much time and effort you want to put into it, says Mattrick. Without downplaying the environmental impact that herbicides can have, he says that the particular herbicides - such as glyphosate (Roundup) and triclopyr (Garlon) - typically used to control invasive plants "are pretty low-threat herbicides."

He urges landowners to consult with local foresters or other experts before attempting any control, chemical or otherwise. "Anybody who is thinking of using herbicides should consult with their state agencies, because there may be permits or licenses required," Mattrick notes. "There's a lot of people out there who do this work and know a lot about invasive plant species. There are resources available - you could save yourself a lot of time and money, because you might be trying to control something using the wrong herbicide."

The whole process can seem daunting, and Mattrick says he sees people go through several stages when it comes to controlling invasive plants. "At first, you're an idealist and think, 'We can get rid of all the invasives in New England if we just try hard enough.' Then you become a pessimist and think, 'Oh my gosh, we're never going to get rid of all this stuff.' Then you reach a point where realism sets in and you say, 'OK, we're not going to get rid of all invasives, but we can protect certain locations if we target our efforts.'"

That's why Mattrick reiterates his advice for landowners to target their own backyards - their own woodlots. "Focus on that and let somebody else do the next woodlot, and let somebody else do the area across town."

There are many resources to help, he adds. Land management organizations and government agencies are undertaking a lot of control and education efforts to combat invasive plants (see sidebar for a partial list of resources). "There are a lot of courses and workshops and programs to reach foresters and farmers and landowners and wildlife enthusiasts," says Mattrick.

Grants are available through the Forest Service and Natural Resources Conservation Service, including through the Wildlife Habitat Incentive Program (WHIP) to help landowners control invasives. "Many times you can use the funding to improve wildlife habitat by controlling invasives," he points out.

The most important thing people can do is learn what invasives are and get them early, Mattrick emphasizes. "When you see them, go for that control early, because smaller infestations are much easier to control. We have pretty good luck with small to medium-sized infestations," he says. "Larger infestations take commitment and they take time."

Patrick White is a freelance writer based in Middlesex, Vt. Over the past 10 years, he has covered a wide range of agricultural operations around the Northeast. He is always on the lookout for unusual stories.