MIDCOAST — Have you been hiking lately and wondered what that plant is taking over the forest? Or maybe you are looking out your dining room window at a wall of “bamboo.” It is very likely that you are looking at an invasive plant. Invasive plants pose serious risks to the biodiversity and functionality of Maine’s ecosystems as well as its forestry and agriculture industries. Many landowners have invasive plants on their land or nearby and need help identifying the plants, understanding how to control them, and preventing infestation.

What is an invasive plant?

• Is not native to the region
• Has spread or has the potential to spread into minimally managed habitats
• Causes economic or environmental harm by developing self-sustaining populations that are dominant or disruptive to native plant and animal species

Invasive plants threaten what we value about Maine’s natural and working landscapes, preventing forest regeneration and increasing costs for farmers. They negatively impact recreational experiences and reduce the habitat value for mammals, birds, and pollinators. Invasive species like Japanese barberry and multiflora rose can form thorny, impenetrable thickets in forests and agricultural fields.

Invasive species are the second-greatest threat to global biodiversity after loss of habitat. Invading plants out compete native species by monopolizing sunlight, water, nutrients, and space. They change animal habitat by eliminating native foods, altering cover, and destroying nesting opportunities. Some invaders are so aggressive they leave no room for our native plants.

How can you control them?

There are two basic approaches to the address invasive plants: physical and chemical. Physical removal can be approached either by manual removal with hand digging or mechanical removal with the use of a machine. Another type of physical approach is cultural, also referred to as interplanting. The goal is to plant native species in and around the invasive species to suppress further spread. The last type of physical removal is the use of biological methods. For instance, there is a beetle that feeds exclusively on Purple Loosestrife. However, this method requires a permit and is only an option for a few species.

The other approach to removal is chemical. This can be done by spraying the leaves during the growing season. Care should be taken to only spray the invasive plant as all plants — good or bad — will die if the herbicide comes in contact with leaves. Foliar (leaf) spraying should be avoided when the plant is in bloom, as pollinators visiting the flowers of treated plants could be impacted. Another chemical approach is basal bark treatment, which limits spraying to around the base of the plant only. The last approach, cut-stem treatment, offers a high success rate of eradication and/or suppression of the invasive plant and little to no impact on any other species. After physically cutting and removing the above ground portion of the plant, apply a solution of the recommended herbicide directly to the freshly cut stump. The herbicide will be absorbed systemically.

Ten invasive species to look out for:

Himalayan Balsam

Himalayan Balsalm Photo by Knox-Lincoln SWCD

An early detection species to be aware of is this annual plant that can grow up to nine feet tall in one season. Another common name is “Pink Touch-me-not,” as the seeds are released by spring-action when they are ripe. Blooms are bright pink and tubular shaped. Leaves are long and pointed along the edges. One plant can produce up to 800 seeds annually, which remain viable for up to two years. They are shallow-rooted annual herbaceous plants, making them very easy to hand pull. There is a native “Orange Touch-me-not.” As the name implies, this species has orange blossoms and leaves have wavy edges.

Japanese Barberry

Japanese Barberry Photo by Knox-Lincoln SWCD

This is a dense and thorny shrub formerly grown in the horticultural industry for landscape plantings. Popular varieties were “Rosy Glow” and “Crimson Pygmy,” both showcasing purple leaves. The shrub is widespread throughout Maine. It can create a dense understory in forested landscapes and can overtake unmanaged pasture lands. Scientific studies have also shown that the dense and thorny masses create ideal protection for ticks. Areas with large populations of Japanese Barberry often correlate with tick populations.

Shrubby Honeysuckle

Shrubby Honeysuckle with fruit. Photo by Knox-Lincoln SWCD

A large shrub brought to New England for its fragrant ornamental flowers. It is common to see these shrubs in older well-established landscape plantings as well as abandoned agricultural fields and in open woodlands. Flowers bloom in June and range from white to yellow to pink. Stems are hollow. There is a native honeysuckle that blooms in late April/May and has solid stems making it easy to distinguish between the two.

Multiflora Rose

Multiflora Rosa Photo by Knox-Lincoln SWCD

An aggressive rose with long arching thorny branches. Flowers form in clusters, range from white to pale pink, and bloom June to July. If given the opportunity, it can form thickets, create dense edge borders between fields and forests, and is tolerant of many soil types. Leaves are compound with seven to nine leaflets per leaf. A unique identification technique is to look at the petiole of the leaf (leaf stem). Multiflora rose has fringed petioles; native roses have smooth petioles.

Common and Glossy Buckthorn

These two lesser-known invasive plants are large deciduous shrubs or small trees reaching up to 20 feet in height. Common Buckthorn has dull green deeply veined leaves with fine teeth along the edge, while Glossy Buckthorn, as its name implies, has glossy deeply veined leaves with smooth leaf edges. Leaves alternate along the stem. Flowers are barely noticeable greenish yellow followed by red, purple, and black fruits. Each species can flower and fruit simultaneously. Buckthorns can easily be mistaken for alders, winterberry, or dogwood seedlings when younger, allowing for infestation to go unnoticed for several years. Common Buckthorn is generally found in more upland habitats. Glossy Buckthorn is generally found in more wetland habitats.

Japanese Knotweed

A perennial, often referred to as “bamboo,” forming dense thickets up to nine feet high in a single growing season. The plant aggressively outcompetes all other species creating a dense monoculture by way of rhizomes (horizontal underground stems). It is easily identified by the large leaves and zig-zag hollow stems. Flowers bloom in August and September. It is commonly found along roadsides, waterways, and disturbed sites.

Burning Bush

Widely sold in the nursery trade until recently banned, this is a popular plant for its bright crimson fall foliage. Flowers are insignificant on this vase-shaped shrub which will readily spread into open forest habitat and roadsides. It can tolerate dense shade and will sprout by root suckers or seed earlier and more often than native plants.

Asiatic Bittersweet

Asiatic Bittersweet Photo by Knox-Lincoln SWCD

An aggressive woody vine made popular due to the attractive orange and red fruit and has been used to make ornamental wreaths. This vine will grow along the ground as well as overtop trees up to 50 feet high. These vines have the capacity to smother entire plant communities.

Garlic Mustard

Garlic Mustard Photo by Knox-Lincoln SWCD

This is an herbaceous biennial growing up to 3 feet high. First-year plants looks quite different than second-year plants. First-year plants remain as basal rosettes with wavy round-toothed leaves. The second-year plant produces tall stems with small white flowers. Leaves on the stem are pointed and angularly toothed. All the plant parts when crushed smell like garlic, hence the common name. These plants are easily hand-pulled but can spread aggressively in open and forested habitats when left unmanaged.

Norway Maple

Norway Maple Photo by Knox-Lincoln SWCD

A highly adaptable tree that outcompetes native sugar maples amongst others. It has escaped cultivation and can create monoculture tree stands if left unmanaged. Norway and Sugar maples look similar as seedlings and mid-sized trees. The easiest distinguishing identification feature is to take a leaf and break it off the stem at the base of the petiole (leaf stem). If it oozes a white sap, it is a Norway Maple. If it oozes a clear sap, the tree is a Sugar Maple.

For more information about these and other invasive plants as well as best management practices for each species, visit the district’s website at knox-lincoln.org/invasive-plants.