On April 13, 2016

Is that a weed?

Science Pub lecture explores the perennial question

By Julia Purdy

Even though snow is still likely to be in the forecast, now is the time gardeners and homeowners are gearing up for the spring ritual of inspecting gardens and yards and planning how to carry out projects for this growing season. It was not surprising, therefore, that a talk on weeds—benign and otherwise—was well attended, despite a cutting wind that accompanied brilliant sunshine.

The event was an illustrated talk on April 3 titled “Weeds: the Plants We Love to Hate,” presented by Mary Droege, instructor in the Natural Sciences Department and manager of the biology lab and greenhouse, Castleton University. The program was the fifth and final one in the 2016 Science Pubs series, organized by the Castleton Free Library Friends.

Hosted by Mountain Top Inn in Chittenden, the lecture took place in a small, modern barn with the dramatic panorama of Chittenden Reservoir far below and the Green Mountains just beginning to show the rosy tones of budding hardwoods. After attendees had secured liquid refreshment at the portable bar and extra chairs had been fetched, the audience settled in to hear about not only invasive plant species but also the historical and social contexts for issues surrounding weeds.

Ms. Droege introduced her topic with some entertaining cartoons poking fun at the ubiquitous dandelion, which she called the “Star Wars stormtroopers” of the field and lawn. She explained that the dandelion is nonnative, brought from Europe by early settlers as a medicinal plant, along with mullein, burdock and plantain. They were very useful plants, not considered “weeds” at all, she noted.

In asking “What do we call a weed?” she noted that Queen Anne’s lace, chicory, yellow flag iris, hawkweed and a plant commonly called “phlox” have become standard favorites of the New England landscape, even though they are also nonnative invasives. In some states, however, they are listed as noxious weeds. The definition of a plant as a pleasant part of the landscape or a weed not to be tolerated “depends on who’s asking” and often changes over time, she said.

Part of the intolerance toward “weeds” comes from industrial agriculture of the Midwest, where “anything that isn’t your one crop is a weed,” she explained. “Roundup-ready” crops, for example, allow farmers to kill every crop except the one that is being cultivated.

That’s where a major problem lies, she said. The wholesale killing off of weeds has serious unintended consequences, especially for pollinating insects such as bees and butterflies. But, she added, “Nature bats last”—more and more agricultural weeds are developing resistance to Roundup.

Turning to the home grounds, Droege spoke about the plants we love, “the ones we don’t want to hate”: ornamentals such as Asiatic bittersweet, purple loosestrife, yellow flag iris and burning bush (winged euonymus). Though we may choose them for color and interest, these nonnative invasives should be discouraged and are actually illegal to buy or transport in Vermont. Several in the audience reported being unable to buy barberry shrubs, for example. And not all nonnative plants, such as bulbs and roses, are invasive, she added.

Moreover, some aggressively invasive native “weeds” play a ecologically critical role. Goldenrod and milkweed are important for providing pollen for bees and incubating the Monarch butterfly. Staghorn sumac, which forms thick groves by spreading underground, supplies wildlife mast, with its seeds often the first food available to birds returning in the spring. It also offers the best fall color (in Droege’s opinion), and a refreshing sun tea.

So what makes a plant invasive? she asked. It is usually nonnative, which means it is no longer subject to the “natural balance” of its home environment, including diseases and predators that keep it in check. An invasive species is also often a prolific reproducer, multiplying exponentially and thriving in disturbed soil. The legal definition of nonnative invasives includes their harmful impact on the environment—to organisms, animals, plants and even ecological processes. Invasives often displace the native plants that support the insect biomass that feeds the birds.

Destructive nonnative organisms can also include insects and fungi. She cited the examples of the emerald ash borer (not in Vermont yet, but “close”), the Asian longhorn beetle (goes after maples), Dutch elm disease, and the fungus blight that decimated the once widespread and commercially valuable American chestnut.

But biological science can be used to turn the tables on invasives, she explained. For example, purple loosestrife, which for decades has resisted all attempts to eradicate it, can now be controlled biologically by importing the beetles that feast on it in its homeland.

There were several questions and comments about the Norway maple, which is, officially, a nonnative invasive plant and is on the Vermont quarantine list. It’s an example of an invasive that has been imported with the best of intentions as a pollution-resistant street tree. Others include Japanese knotweed, dame’s-rocket, garlic mustard and multiflora rose. Droege listed human activities that have unintentionally established invasives: embankment restoration, providing resource for wildlife, crops and pasture grasses, ornamental and landscaping choices.

Summing up, Droege recommended everyone to reevaluate what we consider a weed. Do you just not like the way it looks (like dandelions and plaintain), or is it really an invasive that causes environmental harm? she asks. Droege urged folks to plant native plants whenever possible and to immediately remove the most highly invasive plants in their landscaping, especially if they are near forests and wetlands. Investigate further, she counseled, using credible sources and the many excellent websites available, including www.vtinvasives.org, before finalizing garden and landscaping plans this spring.

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