Invasive Plants in New Hampshire

What is an Invasive Species?

An invasive is a plant or animal that is not native to a particular ecosystem, whose introduction does or is likely to cause economic or environmental harm, or harm to human health. It is capable of moving aggressively into an area, monopolizing light, nutrients, water, and space to the detriment of native species.

Invasive plants disrupt our native ecosystems facilitating cascading effects throughout all trophic levels. Outcompeting native plants for resources such as space, moisture and nutrients, invasive plants ultimately reduce the overall biodiversity of an ecosystem. Invasive plants have a leg up on native plants for several reasons; they lack natural predators, are fast growing, they produce many seeds, are very adaptable to a wide range of conditions, and have longer growing seasons. Studies have shown that invasive plants can result in the degradation and loss of wildlife habitat (including that of rare species), degrade water quality, alter nutrient cycling in the soil, and reduce forest production

It is illegal in New Hampshire to collect, transport, sell, distribute, propagate or transplant any living or viable portion of any listed prohibited invasive plant species including all of their cultivars, varieties, and specified hybrids. -NH Department of Agriculture, Markets & Food

Visit the NHDAMF for more information:

Often without even realizing it, people are becoming accustomed to seeing invasive plant species in our forests, roadsides, and backyards. Below are some of the most common invasive plants found throughout New Hampshire that the RCCD has seen on some of our restoration projects. The next time you step outside see how many invasive plants you can identify!

A Few Species Specific Examples

Japanese barberry (Berberis thunbergii)

Photo Credit: Getty Images

  • Forested/woodland sites invaded by Japanese barberry have higher occurrences of ticks including the black-legged tick (vector for lyme disease) than those habitats not yet invaded. This is due to the high evapotranspiration rate.

  • Japanese barberry invasion can alter soil microbial composition and increase nitrate concentrations. High nitrate concentrations may result in higher nitrogen losses or may make these sites more susceptible to invasion by other weedy plants.

  • Japanese barberry foliage is toxic to deer

Glossy buckthorn (Frangula alnus)

  • Glossy buckthorn shade native plants and suppress tree regeneration in forest understories

  • Glossy buckthorn seeds can persist in the soil for up to 6 years

Black Swallowwort (Cynanchum louiseae)

  • Swallow-wort creates dense monocultures and displaces native vegetation

  • Swallow-wort’s leave and stem tissue contain chemicals that are toxic to moths and butterfly larvae that feed on them. As a member of the milkweed family, Monarch butterflies can confuse the two and lay thier eggs on the swallow-wort plants (ultimately a death sentence for the butterfly larvae).

Japanese knotweed (Polygonum cuspidatum)

  • Japanese knotweed is incredibly fast growing, ~ 8” per day

  • Consists of large woody rhizomes that penetrate the ground up to 10’ deep and laterally can exceed 40’

  • Japanese knotweed has allelopathic properties (chemical compounds that are released by certain plants to eliminate vegetative competition) allowing it to displace native vegetation

  • Can regenerate from rhizome fragments as small as ½” in length (Cutting or mowing knotweed will only make matters worse!)

  • Japanese knotweed rhizomes can remain dormant for up to 20-years!

  • Can grow through concrete and pavement causing issues with infrastructure, utilities, drainage, septic systems, walls, and foundations

Identifying Invasive Species

  • Autumn Olive

    Autumn olive (Elaeagnus umbellata) is fast growing shrub native to eastern Asia. Grows up to 20 feet tall.

    The upper side of the leaf is dark green and the underside consists of silvery gray scales.

    Light yellow flowers in clusters of 5 to 10 that bloom in bloom in mid to late spring

  • Black swallowwort

    Black swallowwort (Cynanchum louiseae) is a perennial herbaceous vine that can grow up to 6 feet tall.

    Leaves are dark glossy green arranged oppositely.

    Light purple star-shaped flowers (June-September)

    Seeds are held in pods similarly to milkweed that disperse in late summer.

  • Oriental bittersweet

    Oriental Bittersweet (Celastrus orbiculatus) is a deciduous, woody, perennial vine native to China, Japan and Korea

    Bark is tan-brown with a white pith (as shown in the image above).

    Roots are bright orange

    Leaves are alternate, glossy, round or oval leaves that are 2-5” long

    Small green flowers in the spring

    Fruit is yellow and turns bright orange-red in the fall.

  • Japanese barberry

    Native to Japan, Japanese Barberry (Berberis thunbergii) is a deciduous, multi branched shrub 2-5 feet tall.

    Stems include small whorled opposite leaves and thorns.

    Flowers are small, yellow and typically bloom in May.

    Fruit turns red and glossy and will persist into winter

  • Japanese knotweed

    Japanese Knotweed (Polygonum cuspidatum) is an fast growing herbaceous plant native to Asia

    Stems are hollow and jointed (often mistaken as bamboo)

    Triangular leaves appear alternately on the stem.

    Large woody rhizomes that penetrate the ground up to 10 feet deep and up to 40 feet laterally

    Small white flowers (August-September)

  • Multiflora rose

    Multiflora rose (Rosa multiflora) is a thorny perennial shrub with arching stems or canes that is native to Japan, Korea, and eastern China. Can exceed 15 feet in height and 10 feet in width.

    Compound, oval, toothed leaves with “feathers” at the base.

    Clusters of white or pink flowers (June-July)

    Small rose hips turn red in fall and persist into winter

  • Spotted knapweed

    Spotted knapweed (Centaurea biebersteinii) is a biennial member of the sunflower family and is native to Eurasia.

    Rosette leaves are deeply lobed once or twice on both sides of the center vein, with lobes oblong and wider toward the tip.

    Pink -purple flowers (June-October)

  • Glossy buckthorn

    Glossy buckthorn (Frangula alnus) is a tall, deciduous shrub native to Eurasia that grows up to 20 feet tall and 15 feet wide

    Leaves are dark-green, glossy and arranged alternately with distinctive veins.

    Bark is grayish to brown with slightly raised lenticels (bumps)

    Greenish/yellow flowers (May-June).

    Fruit forms in mid to late summer, ripening from red to dark purple or black in the fall.

  • Common buckthorn

    Glossy buckthorn (Frangula alnus) is a deciduous shrub that can grow up to 20 feet tall and 15 feet wide and is native to Eurasia.

    The bark is grayish to brown with raised white lenticels (bumps).

    Leaves are dark green and oval shaped with toothed edges and distinctive leaf veins. Leaves are sub-opposite.

    Flowers are greenish-yellow (mid May-June)

    Fruits turn red mid to late summer and purple or black in the fall.

Resources:

GUIDE TO NH'S UPLAND INVASIVE SPECIES

NH Department of Agriculture Markets & Food’s maintained guide for the identification of invasive plant species in NH

Invasive Exotic Species of North America

Center for invasive species and ecosystem health.

NH Department of Agriculture, Markets & Food Invasive Plants Division

Identification-Regulations-Control-BMPs

UNH Extension

State wide offices staffed by specialists, educators, support personnel and volunteers working in 5 main areas including Food & Agriculture, and Natural Resources.  

EDD Maps

Find-Map-Track

EDDMapS is a web-based mapping system for documenting invasive species and pest distribution

NH Fish & Game Picking Your Battles

A statewide strategic prioritization plan for the control of upland, wetland, and intertidal invasive plant species.

NH DES Invasive Species Program

Prevention and rapid response to exotic aquatic species that threaten New Hampshire’s freshwater systems.