Invasive Plant Corner – An Introduction

Jeff Nelson, KNPS Board Member

Anyone who is interested in native plants will soon come across the term “invasive.” Invasive species and habitat loss are the main causes of biodiversity loss and destruction of native plant species worldwide. Although most of us have a general sense of what is meant when a species is deemed invasive, there is also a lot of confusion as to what an invasive plant is, what it does, and what should be done about it.

Amur honeysuckle (Lonicera maackii)

With this introduction, we are beginning a series of articles about invasive plants. In this first article we will define what is meant by an “invasive plant.” We will discuss why they are bad for our native ecosystems and discuss, in general, how they can be controlled. We will also point you towards online resources about invasive plants. In each subsequent article we will concentrate on a single type of invasive plant, giving information on how to identify the plant, where it came from, how it got here, the specific damage it does, how best to control it, and some native alternatives to plant when it has been removed.

What is an invasive plant?

So what exactly is an invasive plant species? In 1999, President Clinton signed an executive order creating the National Invasive Species Council as part of the Department of Interior with the mission “to sustain and expand Federal efforts to safeguard interests of the United States by preventing, eradicating, and controlling invasive species, as well as restoring ecosystems and other assets impacted by invasive species.” Within the executive order, an invasive species was defined as “a species that is non-native (or alien) to the ecosystem under consideration; and, whose introduction causes or is likely to cause economic or environmental harm or harm to human health.” This original definition encompassed economic harm as a factor so as to include many agricultural weeds. Many of those agricultural weeds cause little, if any, harm to native plants and native plant communities. A better definition for an invasive species from our perspective is:

“An invasive plant is one that is both non-native to the eco-system under consideration and able to establish on many sites, grow quickly, and spread to the point of disrupting plant communities or ecosystems.”

Notice that there are two parts to this definition and both must be true for a plant to be considered invasive. Often in groups on the Internet you will see people call aggressively growing native species, particularly ones that spread rapidly in a garden, “invasive.” Using the term in that way dilutes the meaning and can take the focus off of truly damaging non-natives. A good term to use for native species that quickly grow in gardens or other disturbed areas is “opportunistic”. An opportunistic native plant is “a plant that is able to take advantage of disturbance to the soil or existing vegetation to spread quickly and out-compete the other plants on the disturbed site. “

Of course, this discussion raises the question of what is a native plant? A good definition that I use is:

“A plant that is a part of the balance of nature that has developed over hundreds or thousands of years in a particular region or ecosystem. Only plants found in this country before European settlement are considered to be native to the United States. The word native should always be used with a geographic or ecosystem qualifier (e.g. ‘native to wetlands in the Southeast US’).”

Why are invasive plants bad?

The best listing of the problems resulting from invasive plants was recently published by the US Forest Service:

“What are the impacts of invasive plant species?
  • Invasive species have contributed to the decline of 42% of U.S. endangered and threatened species, and for 18% of U.S. endangered or threatened species, invasives are the main cause of their decline.
  • Invasive species compete directly with native species for moisture, sunlight, nutrients, and space.
  • Overall plant diversity can be decreased
  • Establishment and spread of invasive species can degrade wildlife habitat
  • Results in poor quality agriculture lands
  • Degraded water quality
  • Increased soil erosion
  • Decreased recreation opportunities”

How can invasive plants be controlled?

Although the damage to native plant communities by invasive species can be great, many plant invasions can be reversed, halted or slowed, and in certain situations, even badly infested areas can be restored to healthy systems dominated by native species. Every management situation will be different. If a landowner has invasive species on her land, several questions need to be considered before deciding on a plan of control:

  • What is the species that is invading the space?
    A positive ID of the invader is an absolute must before beginning to control the infestation. In subsequent articles we will give tips for identifying many of the most common invasive species in Kentucky.
  • What is the size of the infestation?
    One or two bush honeysuckle plants are dealt with very differently than hundreds of square feet of wintercreeper.
  • What is the native biodiversity where the invasive species is?
    If there are lots of native plants existing in the area around the invasive plant, your approach to control must be done carefully and precisely. If there is a near mono-crop of invasive plants the control techniques used can be more aggressive.
  • What is the nature of the habitat that has been invaded?
    Not all potential control techniques are suitable for every habitat. Is there one or more other invasive species present that could reinvade the areas from which you are removing a target plants? Are you working in a forest or woodland, or a field? If you disturb soils, are they highly erodible? Are you working on flat ground or on slopes? Are you removing invasive species from wet sites or areas near water? Fields may be reinvaded by another species more quickly than forest land. Upland sites away from water tend to be less sensitive than sites along creeks or wetlands. In general, but not always, aggressive methods of control are more tolerated on upland sites. If one chooses to use herbicide, only some may be used in and around water.
  • Do you have the resources available for the size of the project?
    Time, equipment, chemicals, personal protective equipment, etc.

Once the questions above have been answered you need to consider the actual techniques that will be used to remove the invasive species. Often more than one technique should be used. No one technique is right for every invasive species removal project. They all have pros and cons that must be considered in light of the particular project.

An excellent discussion of the techniques used in invasive plant management is the Weed Control Methods Handbook: Tools & Techniques for Use in Natural Areas. All of the techniques mentioned below are discussed in depth. A very useful part of the book is the in-depth discussion of 11 types of herbicide commonly used in management projects.

  • Manual and mechanical techniques
    Techniques such as pulling, cutting, and otherwise damaging plants, may be used to control some invasive plants, particularly if the population is relatively small.
  • Grazing
    Grazing can either promote or reduce weed abundance at a particular site. By itself, grazing will rarely, if ever, completely eradicate invasive plants. However, when grazing treatments are combined with other control techniques, such as herbicides or biocontrol, severe infestations can be reduced and small infestations may be eliminated.
  • Prescribed fire
    Prescribed fire is primarily done to promote the germination and growth of native species in an area. By itself, fire can control some invasive species, although it can also promote the growth of some invasive species.
  • Biological control
    Biological control is the use of animals, fungi, or other microbes to feed upon, parasitize, or otherwise interfere with a targeted pest species. Successful biocontrol programs usually significantly reduce the abundance of the pest, but in some cases, they simply prevent the damage caused by the pest.
  • Herbicides
    “Before using herbicides carefully consider the overall impacts of herbicide use on your conservation targets, other native species, and the ecological system. Base all decisions whether to control weeds, and whether to use herbicides instead of other methods, on the conservation targets and management goals for the site. In addition, the health and safety of applicators and others in the vicinity must be considered BEFORE pesticides are applied. Simply put, one should be confident that the proposed herbicide will do more conservation good than harm and not endanger the health of the applicators or others in the area.” – Weed Control Methods Handbook.

Online resources

There are many online resources for learning about invasive plants and invasive plant control. This list is of some that I have found useful.