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Invasive Threats

Invasive, non-native species are considered one of the greatest threats to our environment.
The damage they have already caused to natural ecosystems and the economy has cost governments, private land trusts, and landowners billions of dollars each year. 

Connecticut Invasive Plants Council

What are Invasive Plants?

Invasive plants are non-native plants that are disruptive in a way that causes environmental or economic harm, or harm to human health. In Connecticut, the Connecticut Invasive Plants Council has developed a list of non-native plants that cause (or have the potential to cause) environmental harm in minimally-managed areas.

Invasive plants are defined as having:

  • a high reproductive rate,

  • the ability to establish new plants and grow rapidly under a wide variety of site conditions, 

  • the ability to disperse wide distances, often by the spreading of vegetative fragments as well as seeds,

  • the lack of the natural controls on growth and reproduction that would be found where the invader is native.


What is the impact of invasive plants on the environment?

​In minimally-managed areas, invasive plants crowd out native plants. The presence of invasive plants alters the way plants, animals, soil, and water interact within native ecosystems, often causing harm to other species in addition to the plants that have been crowded out.  

Invasive plants deny food and shelter to native insects, pollinators, birds and wildlife which have a symbiotic relationship with native plants developed over centuries. Invasive plants are considered second only to habitat loss as a major factor in the decline of native species.

Control methods vary from mechanical (physical removal, introduction of fire or flooding), chemical (using pesticides, herbicides or fungicides) or biological (introduction of a natural enemy or pest which in turn requires continued monitoring of the introduced control). No method is fool-proof and the use of pesticides carries its own risks.

Connecticut Invasive Plants Council's List of 12 Invasive Plants that Threaten our Environment, Economy, and Human Health

Japanese Knotweed (Polygonum cuspidatum)
Japanese Knotweed (Polygonum cuspidatum)

Japanese Knotweed

(Polygonum cuspidatum)

Japanese Knotweed (Polygonum cuspidatum) is a shrub-like, upright herbaceous perennial that grows to 10 feet. It spreads vigorously from long, stout rhizomes and forms dense stands. It also produces winged seeds that are carried to new areas. A significant threat to riparian areas.

Control: Cut plants three times per year at ground level during growing season to starve roots and rhizomes.

Mile-a-minute vine.jpg
Mile-a-minute leaf.jpg

(Persicaria perfoliata)

Mile-a-Minute (Persicaria perfoliata) is an annual vine that can grow six inches per day, smothering other vegetation. Seed persists in soil for six years. Seeds are dispersed by birds, mammals and water.

Control: Hand pull plants and roots before fruiting in August. Repeated mowing or weed-whacking will reduce the plants reserves and prevent or decrease flowering. Weevils are effective for bio-control.

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Japanese Barberry Leaf.jpg

Japanese Barberry

(Berberis thunbergii)

Japanese Barberry (Berberis thunbergii) is a thorny shrub with a dense twiggy form, growing to five feet. Tolerant of a broad range of soil, moisture and light conditions. Seeds dis-persed by birds. Barberry leaf litter changes the chemistry of the soil, displacing many native herbaceous and woody plants. Provides optimum tick habitat.



Control: Pull or dig young plants, making sure to get the roots. Repeated cutting of large plants. Weed wrench ® is effective for uprooting.

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Oriental Bittersweet Berries.jpg

Oriental Bittersweet
(Celastrus orbiculatus)

Oriental Bittersweet (Celastrus orbiculatus) is a very aggressive vine that smothers other vegetation. It has twining stems that strangle shrub and tree limbs and the weight of the vine can uproot and topple trees. Birds are attracted to the berries and spread seed. The vine also spreads by root suckering.



Control: Pull small plants including roots. Cut larger vines close to the ground every two weeks to prevent resprouting and to deplete the root system. Properly dispose of seeds.

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Winged Burning Bush Leaf.jpg

Winged Burning Bush
(Euonymus alatus)

Winged Burning Bush (Euonymus alatus) is a deciduous shrub that tolerates a wide variety of soil, moisture and light conditions. The plant produces hundreds of seedlings that are spread by wildlife. This shrub forms dense thickets, displacing native wood and herbaceous species in many habitats including forests and coastal shrublands.



Control: Pull or dig small plants, making sure to remove the entire root. Large plants can be cut at ground level but repeated cutting will be necessary.

Japanese Stiltgrass.jpg
Japanese Stiltgrass Leaf.jpg

Japanese Stiltgrass

(Microstegium vimineum)

Japanese Stiltgrass (Microstegium vimineum) is an annual grass that forms dense stands and is very shade tolerant. Spread by seed and by rooting at joints along the stem. A single plant can produce as many as 1,000 seeds and are viable in the soil for three years.

Control: Cut, pull, or mow at end of July. Dispose of roots and shoots.

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Norway Maple Leaf.jpg

Norway Maple
(Acer platanoides)

Norway Maple (Acer platanoides) is a large tree that spreads by numerous, germinating seeds. Tree can dominate a forest by creating canopy of dense shade that prevents regeneration of native seedlings. Tolerates hot dry conditions and poor soils. DO NOT BUY OR PLANT
Control: Pull seedlings when soil is moist and dig out larger plants with roots. Cut down large trees and grind stumps or girdle tree in spring.

Mugwort Leaf.jpg

(Artemisia vulgaris)

Mugwort (Artemisia vulgaris) is a perennial weed that spreads aggressively through extensive rhizomes and by seed. It forms mono-specific stands anywhere soil is disturbed. Thrives in sun but tolerates shade.
Control: Mow or cut to ground every 2-3 weeks for 2 years. Pulling may result in more plants since it regenerates from its extensive rhizomes. Cut to prevent seedheads from forming.

Autumn Olive.jpg
Autumn Olive Leaf.jpg

Autumn Olive
(Elaeagnus umbellata)

Autumn Olive (Elaeagnus umbellata) is a fast-growing woody shrub or tree that produces abundant fruits. Wildlife spread the seed after eating the fruits. This plant grows in disturbed areas such as clearings, open fields and forest borders.

Control: Pull or dig young plants, making sure to get roots. Cut large plants at ground level when in flower to prevent seed production. Repeated cutting will be required.

Multiflora Rose.jpg
Multiflora Rose Leaf.jpg

Multiflora Rose
(Rosa multiflora)

Multiflora Rose (Rosa multiflora) is a thorny shrub that can form dense thickets and can also climb like a vine. It spreads by root suckering, tip layering, and by seed dispersal when wildlife consume it fruits. Often found along roadsides and fields.


Control: Hand pull or dig, removing entire root to prevent resprout. Repeated mowings during growing season for 2-4 years can be effective.

Porcelainberry Berries.jpg

(Ampelopsis brevipedunculata)

Porcelainberry (Ampelopsis brevipedunculata) is a vigorous climbing vine that blankets shrubs and small trees, weakening and killing them by blocking sunlight. Fast growing with prolific seeds, it prefers moist habitats and disturbed areas, thriving in a variety of light conditions. Seeds are spread by birds and mammals.


Control: Hand pull vines in the fall or spring. Cut large vines at ground level and cut regrowth as needed.

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Black Swallow-wort Leaves.jpg

Black Swallow-wort
(Cynanchum louiseae)

Black Swallow-wort (Cynanchum louiseae) is a perennial trailing vine. It spreads through rhizomes and wind dispersal of seeds. It is tolerant of a wide range of moisture and light conditions. A threat to Monarch butterflies when eggs are deposited on it’s leaves since caterpillars cannot eat this plant and perish.



Control: Pull or Dig up large root masses, April to July. Bag and dispose of roots and seed pods.

Crazyworm notice 11-15-2019 gramey ed ri

Crazy Snake Worms

New invasive worms to Connecticut. Avoid spreading Crazy Snake Worms (or Jumping Worms, Amynthas spp.) in your gardens, yards, and forests.


Be on the alert for these new pests spreading into New Haven County. This is serious. This complex of at least three worm species can kill or damage trees and plants by decreasing growth, health, and vigor. This also includes our forest plants and trees. These worms are prohibited in New York State.

More Information

Japanse Knotweed
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