They look beautiful in the nursery and thrive in the New England climate, but many popular plant species are really outsiders that now threaten the native plants of the region, and may permanently alter the landscape if left unchecked. That was the central theme in a recent talk by naturalist Peter Alden at a Warner Free Lecture held Friday, April 11, in Volunteers Hall. The presentation was a joint effort of the Warner Free Lecture series volunteers and the Garden Club of Harvard, and included a short talk by summer pond resident Rick Dickson about the water chestnut weeds that currently threaten the health of Bare Hill Pond.

Alden, the author of more than 15 books, including the Audubon Society’s regional field guides, gave an entertaining and sometimes pointed talk about some of the problems facing the environment today: human-accelerated climate change, the rising number of threatened and endangered species, and the invasion of plant species once imported from northeast Asia for their beauty and ability to naturalize freely. The vast majority of these plants entered the country about 50 years ago, and includes such standards as Norway maples, oriental bittersweet, Japanese knotweed, and multiflora rose. The burning bush is another invasive species, Alden noted, that the federal highway department planted literally thousands of for many years. While many of these plants are now banned, the botanical cat is already out of the bag: many of them are now taking over the environment of the native plants they were meant to co-exist with. Looking at slides of the conservation areas in his native Concord, Alden pointed out the vines that are choking many of the trees.

“They’re getting squeezed like a boa constrictor,” he said. Further, he said, a recent comparison of a plant species list kept by Henry David Thoreau in the 1850s reveals that Concord has suffered the loss of about 25 percent of its native flowers.

Alden believes that the problem can still be eradicated, but only if towns act quickly. He recommends a neighborhood-by-neighborhood effort to rid the landscape of invasive species using a combination of herbicides—and just plain pulling them out of the ground. The oriental bittersweet is a particular threat to the native landscape, Alden explained, despite its popularity as an ornamental.

Referencing Thoreau’s meticulous records of the flora and fauna of the mid-19th century, Alden explained that climate change is influencing both the time that flowers bloom, and bird migration patterns. Flowers now bloom 10 days earlier than they did in 1850, and birds now return to New England seven to 10 days earlier in the spring. What this means is uncertain, Alden acknowledged, but it is a clear indication that the planet is warming up and significant changes are taking place.

A more hopeful note was sounded by Dickson, who has spearheaded an effort to clear Bare Hill Pond of the water chestnut weeds that have periodically covered large swaths of the water. By using a combination of methods, most notably the “green reaper,” a large harvesting machine that scoops up the weeds, Dickson has been able to clear away the vast majority of the invasive plants over the last several years. It is an ongoing problem, however, and he continues to use a “weed gate” that keeps the water chestnut from invading further, as well as the old-fashioned method of pulling the weeds by hand. Volunteers have been crucial to the success of this effort, he noted, and new ones are always welcome to come on board to help maintain the beauty and health of one of Harvard’s favorite places.