Gaining Control of Invasive Plants

Photo © Kerry Wixted

In a flash, invasive plants can alter plant communities in the natural areas, backyards, and other green spaces around us, leaving us feeling a bit helpless and overwhelmed at the sheer scale of their transformative power. Resistance may seem futile, but management is possible. The impressive picture above shows Japanese knotweed’s eye-popping efficiency as a nonnative invader. Aggressive species like this come equipped with a suite of growth and reproduction advantages aimed at efficient colonization; but, when they are non-native, they lack the common predators, diseases, and other natural forces of their native range that usually control or limit their spread. This can advantage them over native plants that are now under greater competition for the same resources. With the right approach, you can become the predator or pathogen that keeps invasive plants from assimilating your landscape.

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Invasive plant species may come from all over the planet but can typically be separated into a limited number of categories based on growth habits. Asking questions about the plant species you are dealing with, whether it is an annual or perennial, has fibrous or tap roots, woody or herbaceous stems, and knowing what the seasonal timing of growth and reproductive events are will reveal important things to consider when developing a plan for management. Knowing the categories an invasive plant falls into also lets you know which specific approaches to management are likely to work. Here, we help you to understand these characteristics and how to target your efforts at key phases of their life cycles to put the pressure on and gain some control of the processes that create invasions.

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Photo © U.S. Fish and Wildlife Service Northeast Region

Practices for controlling invasive species vary in scale and size and cover a range of costs and benefits. Some methods of habitat management are handled by trained and certified professionals, like the seasonal controlled or prescribed burn of Reed Canary grass demonstrated above, but most methods are accessible to the general gardener or land manager.

LEAF interns remove invasive species—Asiatic Bittersweet and Black Swallow-wart—from TNC’s Lewis Farm on Block Island in Rhode Island. These invasives threaten the native Milkweed plant, a treat and stopover point for monarch butterflies during thei

Photo © Karine Aigner @TNC

Mechanical control methods are the most common and include actions like mowing, pruning, and pulling. These can be labor intensive but are, generally, inexpensive. Chemical control methods are less laborious and can be highly effective but are more costly and have other risks associated with them. We’ll discuss some ecological tips to reduce the labor, costs, and risks for the methods you choose.

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Photo © Oregon Department of Ag

Biological control, although rare, is another effective method implemented in large-scale cases of nonnative plant invasion. This method uses biological elements like insect predators, fungus, parasites, or other pathogens of the plant from its native range to control the population. A popular example of a biological control is use of beetle and weevil species that eat parts of purple loosestrife (latin name) whose overpopulation in North American wetlands dramatically influences the local ecology where it growsopen_in_new. The image above shows one of the 4 species of insects that are used in the biocontrol–two beetles that eat leaves, a weevil that eats blossoms, and another weevil that lives in the soil and eats at the roots.

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Photo © Jacob Johnston

Years of extensive research and trials are required to obtain a successful biocontrol that is safe to release into the environment. The insects, like those selected to treat purple loosestrife, need to be capable of reproducing and maintaining a viable population but not pose a threat–like eating non-target plants–to the environment where they are released. Although successful in many parts of the country, the local results of this biocontrol, along with many invasive species controls, depend a lot on the conditions and surrounding influences of the site, and the restoration effort or management afterward.

TNC Kentucky staff look at preserve plans at Dupree Nature Preserve.

Photo © © The Nature Conservancy (Devan King)

Start with a Plan
Before digging in, it is important to determine the overall achievable goal given the size of the area and the degree of the invasion. A goal of complete eradication may only be possible in a small area with minimal invasion. As elimination is not usually practical, control of the existing population is the next desirable outcome. Control requires active measures to remove a large proportion of the invasive population, replant native plant selections, and then maintain negative pressure on the reproduction processes of the remaining invasive species to restore a balance to the resource competition in the area, creating a diversity of plants, habitats, and ecosystem services.

Conservancy staff and volunteers hike up to Cascade Head Preserve in Otis, Oregon to remove invasive Himalayan blackberry bushes.

Photo © © The Nature Conservancy (Devan King)

If the area is too large for the available effort, alternate options for the overall goal of management include containment and mitigation. In vast areas with heavy invasion, sometimes containment may be the the best option, depending on the available resources, to prevent further spread into neighboring areas. For example, a nearby infested area, with more work than possible, may threaten to encroaching on a pristine area and it is easier to remove the few that come to the area or leave the containment area. This requires the smaller task of monitoring and taking steps to remove any new or existing occurrences outside of the designated containment area. Above, the few invasive Himalayan blackberry bushes attempting to spread beyond the contained area are easily being removed. Mitigation may allow the continued spread of the invasive but will attempt to offset some of the damaging effects of the invasion through active measures.

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To decide the amount of effort and the overall goal of the management, divide the area into sections that require different types of control. Investigate the best control for each type of invasive species within those areas. You may have invasive shrubs in the hedgerow that require mechanical removal, a ground cover that is taking over the gardens, and garlic mustard filling the understory of the back woods. These all require different control measures with different timeframes and various goals for the outcome. Your plan should out line these actions and their timeframes.

Photo © Andrew Kornacki, USACE Public Affairs

As managers and stewards of the land, we have the opportunity (and the responsibility!) to be the control factor that puts growth limits on invasive species. With the right knowledge and the correct tools, we can be efficient and effective forces on nonnative plants in ways that limit their reproduction, their influence, and their inevitable spread. Regional, large scale eradication has not usually been possibleopen_in_new but maintaining native plant diversity in select areas, like backyards or local parks, can certainly be achieved.

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