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Weeding Out Invasive Plants

Kara Douglas photo

By Kara Douglas

Published in the Harpswell Anchor, June 2017

Asiatic Bittersweet, Japanese Barberry, Himalayan Balsam: An exotic list, and one that members of the Harpswell Invasive Plant Partnership (HIPP) have come to know well. As more non-native, invasive plants are identified along roadsides and on public properties, HIPP has taken the initiative to educate people and preserve native habitat.

The Maine Department of Agriculture, Conservation and Forestry (DACF) names invasive species “The second largest threat to biodiversity after habitat loss.” They’re not only referring to plants. Green crabs, northern pike and the emerald ash borer—an insect that has wreaked havoc on deciduous forests between Texas and New Hampshire—all make their list.

Speaking specifically on the topic of invasive plants, HIPP member Jeff Stann explains to a group of volunteers, “Invasives don’t support as many microbes in the soil, nor as many native insect populations. This deprives the environment of nutrients and reduces the food sources for native birds and wildlife species.”

HIPP was originally organized in 2014 in response to a rapid increase of invasive plants on Harpswell’s public lands. It remains a cooperative volunteer effort that involves the Harpswell Heritage Land Trust, the Harpswell Town Lands Committee, the Harpswell Conservation Commission, the Harpswell Recreation Committee, the Mitchell Field Committee, the Harpswell Garden Club and local residents.

The group has been busy doing surveys of public properties and major roads in town. Last year they began removal of invasives at places like Bailey Island’s Johnson Field Preserve, where a 10-foot wall of non-native bush honeysuckle once edged the shore.

Fast-forward to spring 2017 and HIPP volunteers are once again at Johnson Field for a second year of work. The honeysuckle has resprouted, but is smaller and perhaps easier to tackle a second time.

“Invasive plants usually don’t yield to one (removal) effort,” Stann assures the volunteers. He snips a piece of honeysuckle, looks closely, then shows the work crew. “Native honeysuckle has a solid stem. The stems of invasive varieties are hollow, like this one. They also leaf out before the native variety, so it’s easier to identify them in spring.”

Early leafing, fast spreading and tolerance of imperfect growing conditions are characteristics common to most invasives. This gives them an advantage over local plants that leads to crowding out and diminishing native plant diversity, which ultimately leaves a landscape incapable of supporting insects, birds, wildlife or soil health.

Human health is impacted by invasives as well. Susan Elias, a Research Associate at the Maine Medical Center Vector-borne Disease Lab and Ph.D student at UMaine Climate Change Institute and School of Earth and Climate Sciences cites research done by her lab in Maine and by researchers in Connecticut “indicating that deer ticks (Ixodes scapularis) were twice as numerous in invasive infested forests (especially Japanese barberry infestations) than in adjacent forests dominated by native shrubs (Lubelczyk et al. 2004, Elias et al. 2006).”

In an email interview, Elias writes, “Scott Williams and Jeff Ward with the Connecticut Agricultural Experiment Station published in 2010 that Japanese barberry infestations are favorable habitat for ticks because they provide a buffered microclimate that limits desiccation-induced tick mortality. In other words, the thick vegetation protects deer ticks from winds and hot sun, which would normally stress and even kill the ticks. Ward and Williams also found that control of Japanese barberry reduced the number of ticks infected with Borrelia burgdorferi (the agent of Lyme disease) by nearly 60% by reverting microclimatic conditions to those more typical of native northeastern forests.”

Elias names the presence of non-native, invasive plants as only piece of the tick-borne illness mosaic. With five separate tick-borne illnesses—not just Lyme disease—on the rise in the northeast and a myriad of climactic changes and species interactions setting the stage for this spread, Elias encourages communities to look at the health of their entire landscape.

She describes a scenario in which white-tailed deer populations are high and they browse the native forest understory plants, leaving space and favorable conditions for invasives to spread, forming thickets that the deer use for shelter. When female ticks that the deer carry drop from their host, they are in a hospitable, protected environment in which to lay their eggs.

The thickets, especially Japanese barberry, also provide shelter for white-footed mice and other small rodents that carry the Lyme disease bacteria in their blood. When ticks in their larval and nymphal stages feed on the mice, they can contract the Lyme bacteria. When ticks bite humans and pets, they can transmit the bacteria.

“Exacerbating the problem are warming winters,” Elias writes. “Winter in Maine is now compressed compared to several decades ago and there is no end to this trend in sight. Longer late falls and earlier springs give those adult ticks more time to feed and survive and lay eggs.”

The volunteers at Johnson Field tuck their pants into their socks, shower their legs and shoes with tick repellent and dig in. HIPP’s policy is to control invasives without further damaging water, soil, desirable plants and people. They utilize mechanical removal means first—cutting or pulling the plants, repeatedly mowing where applicable. Anything that will prevent the plant from producing fruit and seeds can help keep its population in check.

In January 2017 the DACF prohibited the sale of 33 invasive terrestrial plants in Maine. Nurseries have been given a year to comply with the new rules and homeowners are encouraged to check the list before making landscape plant purchases.

“First,” advises Stann, “don’t plant invasives and if you find them coming up on their own, early removal works best, before they produce fruits and flowers.

HIPP maintains a list of non-native invasive plants to look out for along with disposal guidelines. They can be found at hhltmaine.org under Harpswell Invasive Plant Partnership. The group plans to continue their work removing invasive plants along major roadsides and on public lands throughout the growing season.

“I think the good news,” writes Elias, “is that invasive vegetation can be removed, with the collective will of a community.”