The Battle Against Invasive Plants on Fishers Island

By Faith Coolidge
H.L. Ferguson Museum Board of Trustees
March 24, 2025

Invasive plants are taking over the island’s forests and wetlands, suffocating the native vegetation and upsetting the delicate balance of its ecosystems. These plants, such as phragmites (common reed), bittersweet and porcelain berry, grow aggressively and spread quickly. However, the most notorious of them all is Japanese knotweed. Known by many names—fleece flower, crimson beauty and false bamboo—knotweed has become a serious problem.

Introduced to the U.S. in the mid-1800s as an ornamental plant, knotweed has proven to be extremely resilient, fast-growing and difficult to control. Its extensive root system can even damage infrastructure like walls and building foundations. Once it establishes itself, it spreads rapidly through seeds, roots and cuttings, taking over riverbanks, hillsides and roadsides, where it crowds out native plants and disrupts the local ecosystem.

The H.L. Ferguson Museum’s Land Trust has several invasive plant removal projects underway on properties across the island it oversees. “We have come to realize, in our attempts to increase biodiversity on the Museum’s Land Trust properties, that it is not enough to remove invasives,” says Museum Trustee Terry McNamara. “A second step must be taken that entails replacing them with a native species.”

Indeed, replacing invasive plants with native species supports local pollinators and wildlife, helps to rebuild the ecosystem and restore the island’s natural habitat.

To achieve this goal, the Land Trust is growing native plants at a nursery located on Bell Hill Avenue. The first beds of the Bell Hill native garden were planted in 2022. “We started with six [4- by 12-foot] raised beds built by students from the landscape construction class at the Fishers Island School,” says McNamara. “The Trail Dogs [a Museum summertime paid position for teens] installed the irrigation system and we maintain the beds with the help of volunteers.” Under the supervision of FI School teacher Adam Murray, three more beds were added this year.

Raised beds in the Museum’s plant nursery on Bell Hill Avenue were built by students from the landscape construction class at the Fishers Island School.

Some of the 2024 Trail Dogs by the red maple in the H. Lee Ferguson Sanctuary. Bottom to top: Museum Trustee Terry McNamara, Nate Parker, Quinn Raymond, Patrik Sepko, Museum Land Trust Stewardship Coordinator Jessica NeJame, Molly Heeney, Jack Heeney.

Initially, the native garden was populated with bare-root shrubs and trees acquired from various state nurseries free of charge or at very low cost. “When they leafed out, they were transplanted to other Land Trust preserves, replacing removed invasive plants,” says McNamara.

The following year, native perennials like cardinal flower and echinacea were added to the mix. As plants grew and expanded, they were separated and distributed into other Museum Land Trust properties. Also, summersweet (Clethra alnifolia) is being grown from offshoots of existing native plants and raised to replace invasive Hydrangea paniculata, briars, and blackberries.

Museum volunteers are growing native plants for replanting to help in the battle against invasive species.

Seeds are collected locally from wool grass, swamp mallows and other native perennials, while seedlings, raised over the winter from locally collected seeds (grown by McNamara and Land Trust Manager Jack Schneider) are also added to the beds. “When these plants became large enough to ensure their survival in the wild, they are distributed to various properties,” says McNamara. “We hope to provide native plants, at no cost, to private property owners seeking to test the possibility of replacing nonnative plants currently in their gardens.”

Volunteers with any level of interest are welcome to join in the effort, adds McNamara. (Contact Jack Schneider for more information.)

A great place to explore native plants on the island is at the John Thatcher Native Garden. Managed by the Fishers Island Conservancy, this garden showcases a variety of native trees and shrubs in a beautiful setting. According to the Conservancy, native garden plants at Fishers include arrowwood viburnum, bee balm, goldenrod, foxglove penstemon, scarlet bee balm, and black-eyed susan, among others.

The H.L. Ferguson Museum’s Land Trust is not alone in the fight against invasive species. Indeed, several dedicated efforts are underway to combat their spread and restore the island’s native flora, including The Fishers Island Conservancy and a grassroots initiative called “Nix the Knotweed.”

Diana Fiske and Courtney MacDonald are out to “Nix the Knotweed” and are available to help Islanders recognize and properly remove the invasive plant shown here.

The Conservancy, for example, has launched an Invasive Plant Management Program, focused on identifying, mapping, and removing invasive species. Another grassroots effort combating knotweed is “Nix the Knotweed,” spearheaded by seasonal residents Diana Fiske and Courtney MacDonald. They work with residents to control the spread of knotweed using an organic, non-chemical control method while simultaneously educating the public about its dangers.

“We are mapping it across the Island and helping folks learn our method of control,” explains Fiske, who is happy to offer suggestions for best practices. “Nix the Knotweed” is always looking for volunteers for its identified areas of concern along the Rec path and by Dock Beach, she adds.

Diana Fiske of Nix the Knotweed demonstrates best practices for organic removal of the highly invasive plant.

1. Cut the plant to the ground (below the first node if possible) at three strategic times during the year (late May, early July, and mid-August) to force the plants to use a significant amount of stored energy (carbohydrates) to regrow between cuts. This weakens the rhizomes underground (the best way for it to colonize an area). Cutting before it goes to seed (the final mid-August cut) helps curb the spread by that method. Don’t cut more often than this, as that encourages new sprouts to grow outward from the main plants.

2. Make sure you remove ALL plant material from the area (especially the first year when taking down the heaviest growth), since a piece of stem the size of a thumbnail can reroot if left on the ground. On Fishers Island, you can take trimmings to the designated “Knotweed” pile at the back of the compost station.

3. Three cuts for at least three years before over-planting the area with native plants and shrubs. These will out-compete the knotweed (whose weaker shoots you will need to continue to cut seasonally). First, make note of what plants (including other invasive plants like garlic mustard and mugwort) come back naturally once the knotweed is weakened.  Keep culling these other invasive plants to give new or emerging natives a better chance of survival.

Do these three steps at least three times each summer to starve back the knotweed plants. Cut in May/June, July, and mid-August, before nutrients from leaves are sent back to the roots for next year’s growth. Watch to see if native seeds sprout after knotweed cuttings. Reseed and replant, starting in the second year. Continue to monitor and cut knotweed sprouts in future years, too.

Be careful not to throw live cuttings into leaf, compost, or wood piles or toss them into open space, as that will only lead to the spread of new plants.

A single-bladed brush cutter makes a clean cut on large stems. Do not mow knotweed or use a weed whacker, as both methods can create cuttings that will grow into new plants.

If you have questions about knotweed on your property, contact Fiske or MacDonald of Nix the Knotweed. You can also contact the Fishers Island Pollinator Pathway with questions.

The Land Trust of the Henry L. Ferguson Museum is dedicated to the acquisition and stewardship of undeveloped property of environmental importance on Fishers Island. Any property donated to the Land Trust is legally restricted so that it will be held in perpetuity as a wildlife sanctuary. While trails are maintained over certain properties to permit the study and enjoyment of sanctuary areas, no activities are permitted that would interfere with the primary purpose of maintaining their natural habitat. Total acreage protected from development under Land Trust auspices, either directly or through conservation easements now exceeds 377 acres, more than 13% of the Island’s land mass when you exclude the water surface area.