Management and Control of Woody Invasive Species

It depends very much on the site being considered!

In a natural areas context, woody invasive species are managed to increase the diversity of the plant community, resulting in better habitat for native wildlife. In forests, invasive woody species are managed to free up resources, including space, sunlight and water, for native shrubs, herbaceous plants and understory trees. Managing invasive plants can also improve the regeneration of canopy trees and the overall longevity of the forest. In prairie or grassland systems, invasive woody species are managed to prevent transition into a shrubland. In wetlands, woody species are managed to maintain movement of water through the habitat and to improve native plant diversity.

On public lands used for recreation, woody invasive species are managed for many of the same reasons as natural areas, but managers may also seek to control dense invasions to improve access to trails and other features, to decrease wildfire risk, and to reduce the populations of tick species that often thrive in dense underbrush.

On timber land, woody invasive species are managed to maintain access for maintenance and harvesting equipment and to encourage the regeneration of valuable timber species. Many timber species will not germinate under the dense shade of an invaded understory.

In gardens and other landscaped properties, woody invasive plants are managed to prevent them from spreading on the property and to neighboring properties. Also, the gardening community is becoming increasingly aware of the benefits of native plantings to pollinators and wildlife, and many are actively working to keep invasive plants out.

On utility rights-of-way, woody invasive species are managed to protect the infrastructure from damaging roots (in the case of underground infrastructure) and tall branches (in the case of overhead infrastructure). On transportation rights-of-way, woody invasive species are managed to improve sightlines. 

On and near agricultural land, certain invasive woody plants are managed to prevent the spread of crop pests.

No sugar coating here: managing woody invasive species is difficult. Why is that?

Pulling woody invasive plants out of the ground, roots and all, is the only completely effective way to ensure the plants are gone and won’t come back. Uprooting woody plants is obviously much easier when they are still seedlings. In an ideal world, all land managers would know how to identify invasive plant seedlings, look for them frequently, and pull them out on a regular basis. Invasive species professionals call this process of scouting for invasives and treating them while they are still new to an area “early detection and rapid response.” As woody plants grow larger and as the density of invasive plants increases on a site, physical removal becomes difficult to impossible, depending on the resources available. 

Another complicating factor is that killing the above ground portion of woody invasive plants while leaving the roots in tact (known as top-killing) simply does not work in the long term. The vast majority of invasive woody species can produce new growth from the root crown after top-killing and can eventually regrow to maturity. Among a few species, injuring or removing the original stem sends a signal to the roots to send up shoots from the entire root system. With these species, top-killing actually makes the invasion much worse.

The challenges described above are part of the reason that many land owners and managers include application of herbicide as part of management. Physical methods like mowing, cutting, or grazing of invasive woody species can be very helpful to improve site access and to create some immediate ecological benefits by providing more light to other plants. Herbicide is often applied as a follow-up measure to kill the root system of top-killed plants, preventing them from regrowing. 

The decision about whether or not to use herbicide can be a difficult one, and ultimately can only be made by the land owner or manager in consultation with other stakeholders. The following guidelines generally apply to all herbicide applications.

• There are four factors that influence the risk to the environment and to human health when applying herbicide. They are:
  1. The properties of the herbicide(s) selected
  2. The application method selected
  3. Applicator experience
  4. Applicator adherence to the herbicide label(s)
• Special care is needed in deciding whether or not to use herbicide, and then, if yes, choosing herbicide and application method. Extreme care should be taken if applying herbicide in an area where contact with surface water is likely, where soil is saturated, where groundwater table is high, or in areas where endangered, threatened, or other high value species are known to be present.
• In the U.S., state herbicide applicator training and certification is legally required for all commercial applicators and for anybody seeking to use restricted herbicides, but training and certification is a good idea for all regular herbicide users, even when not required by law.
• Always, always, always follow the herbicide label because “the label is the law” and contains the following key pieces of information:
  1. Applicator safety measures
  2. Land use types for which the herbicide is intended
  3. Mixing instructions
  4. Best practices for minimizing off-target damage
  5. Appropriate concentration and application rates by application method
  6. Maximum annual application limits
     • More is NOT better
• Herbicide application timing should be planned carefully with the target species biology and herbicide mode of action in mind.

Biological control of invasive species is the strategic release of a biological organism, often a fungus, bacterium or insect, to damage an invasive species. Both in the U.S. and Canada, any potential biological control organism has to go through a vigorous research and approval process to ensure that it is suitable and safe to release into the environment. At present, there are no approved biological control organisms for any of the woody invasive species covered by the WIGL Collaborative. The United States Forest Service Northern Research Station is currently engaged in field trials of a native strain of fungus to control tree-of-heaven, which are on-going. There was research conducted in Minnesota on potential biocontrol organisms for common and glossy buckthorn, summarized here by the Minnesota Department of Natural Resources, but it did not yield promising results and is not being continued.

Treatment of any invasive woody species is never a “once and done” proposition. Even if a manager successfully removes all target plants in one round of treatment, which is unlikely, it is still important to monitor for new invasive species seedlings. Long-term success requires adaptive management: implementing a control strategy, monitoring it carefully for effectiveness and then following up with additional measures as needed. The amount of time and effort necessary to eradicate or control a population of woody invasive species depends on many factors, including the age of the plants, the overall size and density of the population, the resources available for control, the characteristics of the site and more. Generally, managers should anticipate control of a population of mature invasive woody plants consisting of more than a handful of plants to take at least 3-5 growing seasons. 

There’s a saying that nature hates a vacuum, and at least when it comes to the plant kingdom, it is largely true. In situations where invasive plants are managed and the soil is opened up to sunlight, something is eventually going to grow in that space. The question is will it be something good or will it be another invasive plant? The answer to that question depends largely on what’s in the seed bank (viable seeds existing in the soil) and the external seed sources nearby. In situations where there is likely a lot of invasive plant seed in the seed bank or where neighboring properties are still invaded, there is high potential for re-invasion of treated sites. Managers can help prevent re-invasion by purposefully planting desired plants to compete with invasive seedlings. In a natural areas or native garden setting, managers will likely choose native plants that are a good fit for the site conditions (see our Landscape Alternatives for lots of great examples). On timber land or other agricultural land, managers might plant commercially valuable trees or other crops. There’s a whole wide world of plants out there, and only a fraction of species are invasive.