Control Methods
Cultural control methods are consistent for all invasive plants. They include not planting invasive plants in the landscape, minimizing soil disturbance, avoiding bare ground, early detection, and a rapid response to remove invasive plants before they become established.
Biological control is the use of natural enemies, such as insects or pathogens, to reduce the plant’s growth, reproduction, and spread. Biological control methods do not exist for all invasive plants but are available for some species. The goal with biocontrol is long-term suppression, not eradication.
Mechanical methods can be used alone, or in combination with other management techniques. Mechanical control is manually removing invasive plants by hand pulling, cutting, digging, mowing, or smothering; the physical removal of plant material to reduce growth, seed production, and spread. Repeat treatment is often needed because many species resprout or emerge from a seed bank in the soil.
Chemical control is the judicious use of herbicides to kill invasive plants. Landowners can apply general use herbicides themselves, or hire contractors with the appropriate license to apply herbicides.
Always follow all laws and regulations required for applying herbicides. Always read the product label carefully to ensure safety and effectiveness.
An IPM approach is explained below, for five common invasive plants in Connecticut landscapes.
Asiatic Bittersweet (Celastrus orbiculatus)
Asiatic bittersweet is a woody perennial vine, introduced as an ornamental plant in the 1860s. Bittersweet is ubiquitous in Connecticut and much of the Eastern U.S. It is found climbing trees along roadsides, on forest edges, in urban landscapes, and in forest openings, old fields, railways, and trails. It can be very damaging or deadly to trees, as it strangles trunks and blankets canopies with foliage.
Mature bittersweet stems can reach over 10 inches in diameter and grow up to 60 feet tall. Stems are green when young and light brown as they mature. The stems twine to wrap around structures, tree trunks, or other vines (Figure 1). The leaves are alternately arranged, round to slightly oval, and bright yellow in the fall. Plants are dioecious (male and female flowers are on separate plants); only the female plants produce fruit, which persist on the vine through winter. The fruit is yellow in late summer; as the fruits ripen, they split open to expose the bright red interior (Figure 2). The fruit is eaten and spread by birds. Roots are distinctly orange and multiple vines are produced from one root system.
Bittersweet Management
Seedlings can be pulled from the soil easily, and small plants can be dug out, ensuring most if not all roots are removed. Pulling and digging is easiest in the spring, when the ground is moist. Repeated pulling of new shoots may be necessary, as the root system of mature vines is vast and new shoots can regrow from small root fragments. Cutting vines is only effective if resprouts are cut repeatedly over several years until the root system is depleted. One cut vine can produce many new vines from a mature root system.
Mechanical removal using a mower or brush hog can also encourage root re-sprouting and must be repeated to reduce root system reserves. In most cases, mowing and cutting are unlikely to eradicate bittersweet. Physical efforts to eradicate this plant without herbicides may become laborious and overwhelming.
The most effective method to manage bittersweet is an integrated approach that includes cutting the vines along with a follow-up herbicide application. This can consist of a foliar spray after the plant has regrown for at least eight weeks. Alternately, immediately after cutting vines, the cut stumps can be treated with an herbicide using a Buckthorn Blaster or sprayer. This cut stump method applies herbicide to the freshly cut stump where the vascular tissue is located.
Herbicide treatments are best completed in summer, after July 1st to encourage the movement (translocation) of herbicide into the root system. Cut vines entangled in the tree canopy should not be pulled from trees, as this can cause damage to the tree, or cause large tree limbs to fall. The vines will decompose over time.
Japanese Barberry (Berberis thunbergii)
Japanese barbery is an introduced deciduous woody shrub commonly used as an ornamental landscape plant. In natural or unmanaged areas, this plant is found as a forest understory shrub, or established along a wooded edge, often forming dense thickets that displace native plants (Figure 3). Japanese barberry can tolerate deep shade to full sun, and both dry or moist soil.
It has arching branches that can root upon contact with the ground. The stems have sharp spines, which makes handling the shrub with bare hands unpleasant. Flowers and inner bark are distinctly yellow. The small, spoon-shaped leaves have smooth edges. The fruit ripens from green to bright red and persists on the plants through winter (Figure 4). Fruits are eaten and dispersed by birds. Deer avoid eating this plant.
Japanese Barberry Management
Japanese barberry is not a deeply rooted plant, but the roots extend laterally. Mechanical removal of barberry is best done in the spring when the soil is moist and plants have not set seed. Pull plants by hand or dig out, being careful of the spines. Manual pulling is best suited for small populations, in areas where chemical methods are undesirable or not an option. Repeated cutting is unlikely to eliminate barberry plants, as the stumps will readily resprout. Continued efforts to remove regrowth must occur several times a season over several years to eventually reduce plant vigor enough to kill the plant.
Chemical treatments can be used alone or in combination with mechanical methods. Always follow label directions when using herbicides. An herbicide with active ingredients of either glyphosate or triclopyr are effective on Japanese barberry. Application methods include foliar spraying, basal bark, or cut stump. A basal bark treatment is an application of an oil-based herbicide, usually a triclopyr ester, targeting the bottom 12 to 18 inches of the trunk.
Garlic Mustard (Alliaria petiolata)
Garlic mustard is an herbaceous biennial in the Brassicaceae family. It was introduced in the 1800s by European settlers, who believed the plant to have culinary and medicinal value. Garlic mustard spreads by seed; each plant may produce thousands of seeds. This plant will carpet shady forest understories and edges (Figure 5). It contains allelopathic chemicals that can prevent other plants from growing.
In its first year, garlic mustard grows vegetatively as a basal rosette. In the spring of its second year, it flowers. The leaves of garlic mustard are rounded kidney bean-shaped in the first year and heart-shaped during the second. The leaf margins are toothed. The leaves have a garlicky odor and taste. Flowers are white with four petals. Seeds are formed by late spring to early summer in seed pods called siliques, which shatter when mature, widely dispersing the seeds. Seeds can remain viable in the soil seed bank for five to 10 years.
Garlic Mustard Management
Management strategies for garlic mustard are straightforward compared to those for other common invasive plants. The plants are easily pulled by hand. This is the preferred method of removal for small or newly established infestations. Hand pulling should be completed before or immediately after flowering, before seeds develop. Plants removed after siliques (fruit capsule) have started to form have been documented to produce viable seeds. Plants forming seeds can be bagged and placed into the trash. Non-seed forming plants can be left on site to decompose. Garlic mustard infestations have been shown to decline over time (five-15 years) when left undisturbed.
For large garlic mustard infestations, when more immediate results are required, foliar herbicide applications with glyphosate or triclopyr are effective. Foliar applications of rosettes can be completed early in the spring before surrounding nontarget species are growing, or in the fall, when nontarget species are dormant. Depending on how long the population of garlic mustard has existed, repeated herbicide treatments or follow up removal of plants by hand pulling is necessary as new seeds germinate from the soil seed bank.
Burning Bush (Euonymus alatus)
Burning bush was introduced into the United States in the 1800s as an ornamental shrub known for its bright red fall color and adaptability to diverse growing conditions (Figure 6). Burning bush is still an extremely common landscape shrub. The abundant, persistent fruit of burning bush is eaten and dispersed by birds. Burning bush can tolerate full shade; it can take over forest interiors, edges, and the woodland understory. Leaves are elliptical and oppositely arranged. An alternate common name for this plant is winged euonymus, due to the presence of corky wings that grow on mature stems (Figure 7).
Burning Bush Management
Small infestations or young plants are easily managed by hand pulling, digging, or using a weed wrench. The plant is unlikely to regrow from root fragments if most of the root system is removed. Like other invasive plants, burning bush will leaf out before and retain leaves longer than many native plants, which extends the window for potential chemical control using glyphosate or triclopyr herbicide applications. Foliar, basal bark, and cut stump applications are effective treatments to manage burning bush populations.
Brush cutting or mowing are not effective unless followed by a cut stump or foliar herbicide application, to reduce the potential for stump sprouting. The seed bank reservoir may be persistent and follow-up monitoring is required.
Multiflora Rose (Rosa multiflora)
Multiflora rose is a deciduous, woody perennial rose native to Asia. It was introduced into the United States in the 1800s, for use as an ornamental in the landscape, and as a rootstock. It was promoted for its erosion control value and as a living fence for livestock.
The leaves are compound with serrated edges (Figure 8). It spreads both by seed and vegetatively, through arching canes that root upon contact with the soil. Multiflora rose can form dense thickets, scramble up trees, fences, and buildings. Clusters of white fragrant flowers mature into small, red hips by late summer. The thorns on this species are curved, with thorn tips pointing towards the base of the plant. Multiflora rose grows best in full sun but can tolerate partial shade. It grows in fields, forest edges, roadsides, and residential areas.
Multiflora Rose Management
Young plants or small populations can be physically removed by hand pulling, being careful to avoid the sharp thorns on the stems. Repeated cutting or mowing, three to six times per season over several years, may starve the plant and eliminate the root reserves. However, repeated cutting or mowing is unlikely to kill well-established multiflora rose plants. A targeted herbicide application using a foliar, basal bark or cut stem treatment of triclopyr or glyphosate is effective.