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Common garden invasives: What you can do

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While summer provides a bounty of fruits, veggies and flowers, it also brings invasive species.

Common invasive species that you might find in your garden or landscape often started out as decorative landscape plants. Plants like knotweed, goutweed, burning bush and Japanese barberry were once imported for their attractive and vigorous garden characteristics.

For example, knotweed has a lot of characteristics that likely made it attractive to plant in the past. It has showy flowers and seedheads, yellow-orange autumn color and red showy stems. It easily forms hedges and grows in almost any condition. Ironically, this growth habit is exactly what makes knotweed invasive today.

Knotweed is an expert at spreading. Just a little piece of a root or stem can sprout into a new plant. Its opportunistic spirit makes it very good at establishing in eroded areas, such as river banks. It looks very obvious along river banks, forming dense hedges and stifling diverse native plant growth.

Knotweed, an invasive garden plant, can be controlled by cutting back plants throughout the season to weaken them or smothering with layers of black plastic or wood chips. Photo courtesy of Bonnie Kirn Donahue

If you have knotweed in your home garden, there are a number of ways to manage it. The first is mechanical, or cutting back the plants by hand. This should begin in spring and continue through the growing season. After multiple years, the constant cutting should weaken the plants.

If cutting knotweed, the fresh stalks and roots should be thoroughly dried and browned in the sun before composting. Lay the cuttings on a tarp or plastic and ensure that the cuttings do not touch the edges, as it can spread easily.

Cuttings can alternatively be sealed in thick, black plastic bags and left in the sun to solarize, but this requires hot, sunny weather to be effective.

Other options for management in the home garden include smothering the plants with layers of heavy black plastic and wood chips.

Bishop’s weed or goutweed is another garden menace that spreads easily by pieces of rhizomes. It grows low (one to three feet tall), in large, uniform swaths. Each leaf is made up of nine leaflets and can be a solid or variegated green.

Bishop’s weed or goutweed, which can reach heights of up to three feet, has solid or variegated green leaves with each leaf consisting of nine leaflets. Photo courtesy of Leslie J. Mehrhoff, UConn/bugwood.org

Management for bishop’s weed is very similar to knotweed, including consistent hand pulling and smothering. Proper disposal is also essential, so make sure every piece gets into a trash bag or onto a tarp to dry and solarize.

Another common garden invasive is burning bush. It has fantastic autumn color, with leaves turning a vibrant red, and tiny, bright red berry-like pods that grow on the underside of the stem. The stems have ridges and make almost a square shape, which makes the bush easily identifiable by touch.

Japanese barberry is an aggressive shrub with clusters of one-inch leaves that are often located on the upper side of the stem, thorns and tiny red berries that droop on the underside of the stem. The plant comes in many cultivars with foliage that can be yellow to green to red and purple. Japanese barberry spreads by roots and seeds, quickly taking over.

While burning bush is prized for its vibrant fall color, its aggressive spreading habit makes it a poor choice for landscapes. Photo courtesy of Leslie J. Mehrhoff, UConn/bugwood.org

Burning bush and barberry should be cut back before fruiting to help prevent spreading the seed. The plant and roots should be carefully dried on plastic or tarps before composting or tossing in the brush pile.

As you can tell, some of the common invasive garden plants have attractive qualities, but their aggressiveness has enabled them to spread into the wild, stifling native plant communities and habitats. You should also keep an eye out for plants in your own garden that technically may not be invasive, but may act aggressively.

Interested in what more you can do? Try to grow more native plants in your garden. Doing this will help build habitat for native insects and pollinators, and help prevent future decorative plants from spreading.

For more information on invasive species and tips for managing them, check out https://www.vtinvasives.org.

by Bonnie Kirn Donahue, Extension Master Gardener, UVM

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