How to Manage the “Dirty Dozen”
1. Porcelain berry
Description: Public enemy number one! This woody vine resembles grape leaves early in summer, but mature leaves will develop deep lobes. The vines can be broken easily by hand, while grape vines are very difficult to snap without tools. As the name implies, porcelain berry produces brilliant berries in late summer and fall that resemble porcelain, and one of the reasons it was introduced into the horticulture trade until it became invasive.
Impact: Porcelain berry forms thick mats in open fields and forests, covering plants at such a rate that they can rapidly block out sunlight and kill the plants underneath. Birds will also eat the berries during their migration, but the fruits are not as nutritious as native plants and so birds have to make more frequent stops to refuel.
Management: A large, thick mat of porcelain berry can often be traced back to a single root, and killing the taproot is key. You can easily pull this year’s growth out of trees by hand, and cut down towards the base of the stem with trimmers or loppers. Either remove the taproot through digging or painting the cut stems with herbicide to kill the plant. Removing porcelain berry before it fruits will help reduce regrowth.
2. Black swallow-wort
Description: These vines have shiny dark green leaves that are oval-shaped when mature, with stems that tend to twine around one another. The plant has very small, purple flowers with five petals. Swallow-worts develop milkweed-like pods by August that turn brown and split open in the fall, releasing fluffy seeds that are dispersed by wind. Unlike milkweed, however, they do not produce milky sap.
Impact: Black swallow-worts grow rapidly in open fields and roadsides where there is sun, pushing out native plants. The plant secretes toxins that prevent other plants from growing near them, as well as being toxic to livestock and pets. Of chief concern is its effect on the monarch butterfly, which recognizes the plant as its primary plant host, milkweeds. When monarchs lay their eggs on swallow-worts, their caterpillars eat the plant and die. Swallow-worts grow in habitats shared by milkweeds, outcompete milkweeds and may lead to a decline in monarch butterfly populations.
Management: Control depends entirely on removing swallow-worts before they can reproduce seeds. Cutting down to the stem and painting with herbicide is effective, but cutting or mowing without applying herbicide afterwards will only encourage regrowth. With large infestations, foliar herbicides can be used in early June and again in August. Seed pods should be removed and either burned or bagged, not composted. Management of black swallow-wort may be necessary for multiple summers to deal with younger shoots, but it will be much easier to handle after the initial treatment.
3. Oriental bittersweet
Description: This deciduous vine has rounded or teardrop-shaped leaves with toothed edges that become glossy in summer and yellow in fall. The younger stems often appear to have spots or bumps. They produce green berries by July that turn yellow by August, orange-red by fall, and persist into winter.
Impact: Like porcelain berry, oriental bittersweet grows densely, preventing new plants from growing. It can girdle, strangle and eventually kill the trees they cover. Oriental bittersweet has replaced nearly all our native American bittersweet in New England.
Management: Killing the plant before it fruits is important. Younger vines can be removed by hand; look for the orange taproot to ensure you’ve removed the entire plant. Older vines need to be cut down or sawed to the base of the stem and painted with herbicide. Cutting back the vines before they fruit during the summer and applying herbicide in the fall or spring is also effective.
4. Autumn olive, Russian olive
Description: The olive’s leaves can be seen from a distance by their silvery sheen. Autumn olive has green on top and silver underneath, whereas Russian olive is entirely silver. All leaves appear scaled when up close. Young stems are silvery with brownish or orange scales that make them appear speckled. Some stems may bear large thorns. They have fragrant, yellow flowers in the spring and develop speckled red berries in the fall.
Impact: Both olives alter the chemistry of the soil where they grow, changing the kinds of plants that can enter and grow in the community, and helping the olives to persist. They also shade out younger trees by retaining their leaves into late fall. The leaves are undesirable by herbivores, including deer, and although the fruits are eaten by wildlife, they provide little nutrition.
Management: Since the tree spreads widely by seed, removing the plants before they fruit will help limit their spread. Cutting or mowing without herbicidal treatment, however, will only encourage regrowth in autumn and Russian olives. Paint cut stems when possible. Foliar sprays can be used in early spring, when the plant is leafing, or in late fall to minimize damage to native vegetation.
5. Japanese knotweed
Description: This plant has large, heart-shaped leaves and jointed, hollow stems that resemble bamboo shoots. Its ability to grow over six feet by summertime adds credence to being called “bamboo” by residents. They develop large clusters of white flowers by August, although they do not reproduce by seed.
Impact: Japanese knotweed grows so rapidly that they prevent just about anything else from growing near them. But the roots (rhizomes) are the real danger, as even a single splinter from the rhizome will regenerate the plant the following season. Shredding or composting dead knotweed, as well as driving over treated areas, can contribute to its spread throughout the Island.
Management: Management is possible, but will take multiple summers to reduce knotweed on your property to manageable levels. They key is reducing the plant’s storage of food in the root system. Cut or mow down plants in June to keep regrowth short. By August, apply foliar herbicides to reduce the plant’s ability to store energy in the winter. Plant material should be bagged or burned, not composted. Continue these steps each summer as you starve out the root system and create space for your plants.
6. Japanese honeysuckle
Description: A perennial, twisting vine with opposite leaves that can be deciduous or evergreen. New growth will have red, fuzzy stems, whereas older stems appear tan. The flowers are white, late spring into summer, then turn yellow and bear a sweet-smelling fragrance. Black fruits develop in the fall.
Impact: This invasive was introduced in New York in the early 19th century as an ornamental that didn’t become a problem until nearly two centuries later. It’s main impact is how fast it grows, twisting and climbing over native grasses, native shrubs and even into trees. If not controlled, a mature stand of Japanese honeysuckle can girdle, choke and kill woody vegetation. Although the berries provide a food source for birds, the nutritional quality of the fruits is poor.
Management: Young shoots (this year’s growth) can be removed by hand and should be removed before June. Large swaths can be controlled with a combination of mowing (once in early summer and again in late summer) and painting stems with a topical herbicide. Mowing alone can control the invasive after multiple years. Management should occur before the plant fruits in the fall.
7. European buckthorn
Description: An understory tree that can grow up to 20 feet and closely resembles plum and cherry trees. Branches are alternate and typically bear thorns on twigs. When cut, inner bark is yellow with a pink-to-orange heartwood. Leaves are broadly oval, glossy and have three-to-four pairs of upcurved veins. Flowers develop in spring as clusters of four-petaled, yellow or green flowers. Small black fruits develop late summer into fall.
Impact: European buckthorn forms dense thickets that crowd out or shade younger trees or plants, resulting in a loss of undergrowth in many forests. The fruits of European buckthorn, while attractive to wildlife, serve as a severe laxative and regurgitant that can make many species sick. During the fall migration, birds will consume berries of European buckthorn, only to vomit their meal and become less likely to survive their trip south.
Management: Cutting stems and applying a vascular herbicide has been effective killing stands of European buckthorn. Applications should be made in spring before the plant has time to grow, and plants should be removed before fruits develop.
8. Mile-A-Minute Weed
Description: A trailing vine easily recognized by its triangular, arrow-shaped leaves. It is even more recognizable by the barbs lining the underside of leaves and on stems, giving it the name “Devil’s tearthumb”. Flowers show up in summer in small, cup-shaped leaves (called “ocreae”) and quickly develop dark blue berries as early as June.
Impact: As the name implies, this weed grows rapidly, covering other herbaceous vegetation and shrubs. Plants covered by mile-a-minute can be smothered, distorted or broken under the vine’s weight, which will eventually kill them. The berries are consumed by wildlife, but provide little nutrition. The majority of the plant’s spread is through water, and mile-a-minute commonly appears in disturbed stream banks and wetlands downstream from previously invaded sites.
Management: A biological control agent, the mile-a-minute weevil, has reduced the competitive ability of the invasive plant in much of its introduced range. However, additional treatments are necessary to reduce the spread. Removing seedlings by hand in the spring can reduce later growth. Larger stands can be sprayed with a foliar herbicide, or, to protect nearby native or desirable plants, cut mile-a-minute stems back to the ground and paint stems with a vascular herbicide. Remove and bag fruits when possible. Do not compost mile-a-minute vines, if seeds are present.
Description: Kudzu is a climbing vine with three leaflets, each leaf over three inches across, sometimes lobed and hairy. As they are related to peas, the vines can sometimes resemble oversized pea vines. Flowering occurs in late summer and appears as clusters of purple, fragrant flowers that give way to brown, hairy seedpods.
Impact: “The plant that ate the south” is an apt name for this species, as it grows over shrubs, trees and man-made structures at a rate of one foot per day. Plants beneath the vines are smothered or break under its weight, and the vines can also girdle trees over a season. Mature stands of kudzu blanket landscapes and leave very little space for any other plant to survive.
Management: Kudzu vines can be traced back to a crown, which many be several inches in diameter. Cutting at the base and painting or injecting a vascular herbicide into the crown is an effective means of control. Applications should be done in spring, followed by repeated cuttings of any new material in summer to help starve out and kill the crown. Multiple summers may be necessary to control mature stands of kudzu (i.e., crowns over 5 inches in diameter). Cuttings can be burned (or fed to cattle on farms.)
10. Norway Maple
Description: Norway maple is a large, deciduous tree that is often confused with sugar maple. It is the most widely planted street tree in North America. Norway maple leaves produce a milky sap when broken, and their fall color will always be yellow vs. clear sap and red-yellow coloration in sugar maple. Flowers develop in late spring as yellow-green clusters and in the fall as winged fruit (samaras).
Impact: Norway maple is a slow invader, growing as a shade-tolerant tree in many forests. However, it forms a dense canopy that will out-shade anything beneath it, preventing wildflowers, forbs, shrubs and saplings from growing in its proximity, except for other Norway maple trees.
Management: Remove seedlings in the spring by hand when the soil is wet; this ensures that the root system does not break off and regenerate. Larger stands need to be cut down when possible, and stumps painted or injected with an herbicide. Plant material can be burned or composted.
11. Sweet autumn clematis
Description: Also known as sweet autumn virginsbower, this plant is a climbing woody vine. Flowers are produced in late summer into fall, with white, open, four-petal flowers. Seeds develop afterwards with silvery, long hairs. There are over 250 species of clematis, many of which are colorful and bloom twice in one season.
Impact: Like many non-native vines, clematis grows over undergrowth, shrubs and young trees, smothering plants underneath and preventing them from obtaining sunlight.
Management: Sweet autumn clematis can be cut back in late August, when it is easily recognized for its showy flowers, and before the plants develop fruit. Cutting stems and painting with a vascular herbicide is recommended. Repeated cuttings over the summer to starve out the root system can also be effective.
12. Black Locust
Description: Black locust is a shrubby, thorny plant that has a tendency to sucker if broken or cut. It has alternate, pinnately compound leaves (leaves on both sides of a stalk) that are untoothed, with a pair of thorns at the base of each stalk. Flowers develop in late spring as fragrant, white clusters that are attractive to pollinators, and develop into seedpods by fall that persist into winter.
Impact: Although native to the US, landscaping practices have encouraged the plant’s spread outside of the Appalachians and into much of the eastern U.S., where it alters the ecosystem. This is largely due to the fact that the plant increases soil fertility by fixing nitrogen in the soil. As native plants are accustomed to low levels of nitrogen in the soil, increased nutrients in the landscape may encourage other invasive plants to grow and out compete the natives. The plant does provide food and pollen for wildlife, although it is not known whether or not these species would develop better on native plants within their native range.
Management: Black locust tends to grow back quickly and with more branches whenever a branch is broken or cut, so herbicide is necessary to control the plant. Cutting down to the stem and painting with a vascular herbicide is effective. Remove seedpods before composting any plant material.
By Adam Mitchell, Associate Wildlife Biologist®
PhD Candidate, Department of Entomology and Wildlife Ecology, University of Delaware