Wisconsin Manual of Control Recommendations: Ecologically Invasive Plants

Introduction

Invasive plants have become recognized in recent years as a major threat to the integrity of natural areas. These species have the ability to invade natural systems and proliferate, often dominating a community to the detriment and sometimes the exclusion of native species. Invasive species can alter natural ecological processes by reducing the interactions of many species to the interactions of only a few species. Introduced species may compete directly with native species for nutrients, sunlight, and space, and indirectly by altering the food web or physical environment. Invasive species may also prey on or hybridize with natives. Native species with limited population size or ecological range are particularly susceptible to displacement by aggressive exotic or translocated species. According to a 1996 report by the Nature Conservancy, invasive species have contributed to the population decline of 42 percent of threatened and endangered species in the U.S. Many also pose threats to agricultural areas, urban parks, yards, and roadsides. While only a small percentage of the 4,000 estimated exotic plant species in the U.S. cause problems, just 79 non-native plant and animal species have already cost the U.S. economy $79 billion.

These guidelines primarily refer to control methods in natural areas, but are not restricted to such areas. Native plants should be protected and encouraged to grow in urban, suburban, and rural lands, not just in protected natural areas. Individual involved in land management, from parks personnel and public land managers to road crews to private landowners, will find these recommendations useful in efforts to promote balanced and diverse ecological communities throughout Wisconsin, from state parks and scenic riverways to roadsides and backyards.

Invasive Species Defined

Most invasive species are ecological pioneers and colonizers which, once introduced, quickly establish themselves in ecologically disturbed communities. Invasive species typically displace native flora due to faster growth rates, efficient dispersal mechanisms, and tolerance of a wider range of conditions. Invasive species often lack natural predators and diseases which control populations in their native environments. As the diversity and populations of native plants decrease, so does the variety of habitats available for wildlife.

For the purposes of this manual, invasive species are described as either exotic (alien), translocated, or opportunistic. Exotics are primarily European and Asian species that have been intentionally or accidentally introduced to North America. Translocated species are native North American species whose pre-settlement range either did not include Wisconsin, or did not include certain areas of Wisconsin in which they are now a problem. Opportunistic species are native colonizers which can dominate certain natural communities and decrease species diversity. Some native species, such as cattails (Typha spp.), have historically inhabited Wisconsin natural communities in lower numbers, but have become more widespread with increased disturbance and sedimentation in wetlands. Such situations often occur when environmental conditions change, or when natural processes are restricted or eliminated (e.g. suppression of wildfires).

Purpose of Control Recommendations

Many exotic plant removal and control applications have been developed for agricultural, horticultural, and forest industries. They have focused on the use of intensive mechanical disturbance or chemical treatments. In natural areas, most of these applications are either not feasible or are detrimental to the ecological integrity of the areas. These guidelines address invasive species control applications in a manner sensitive to individual species and natural communities.

This manual summarizes the morphological characteristics, habitat requirements, life history, and possible methods of control for several common invasive plant species. Botanical terms used in the plant descriptions are not available on-line at this time. A reference section is included to acknowledge sources of information and to provide a reference to the literature addressing the problem of invasive species in Wisconsin. Sources for weed control tools, organizations, and other useful information are included in this section. Recommendations included here are limited to the control of some of the more problematic invasive plant species of Wisconsin. As control information on additional invasive species becomes available, and as users request information on other problem plants, these species may be added to the manual.

Feedback from the users of this manual will assist the Wisconsin DNR in updating the recommendations. Readers are encouraged to send comments, questions, or control recommendations to the address listed below:

Bringing the Ecosystem into Balance: An Introduction to Control Methods

Human settlement often disrupts natural cycles both in the immediate environment and in surrounding areas. Disturbance creates conditions that allow opportunistic species to invade the area. In a healthy ecosystem, natural disturbances such as fire and flooding generally keep exotic and translocated species populations in check, allowing disturbance-adapted native species to thrive. With this in mind, a manager's short term goal should be to control the problem species, and in the long term to restore the natural processes of the ecosystem.

Much of the control work being done is experimental, and often little data is collected. Because environmental and other conditions are variable over time and space, what works in one situation might not in another. This manual provides a range of techniques that have been reported to work in some situations. To determine what technique might best control an invasive plant in any given situation, individual site characteristics must be evaluated, and different techniques attempted. Monitoring results and continuing control work for several years is critical to the development of long-term control.

Eradication vs. Control

In a human-dominated environment, natural areas are biological islands under constant invasion by exotic, translocated, and opportunistic species. Permanent eradication of invasive species often requires an extensive expenditure of labor and resources. The time, resources, and risks involved in invasive species control dictate the feasibility of control measures. Eradication of invasive species may be preferable, but it is rarely pragmatic as a management objective; control of invasive species populations often proves to be a more practical goal. Individuals involved in land management should aim to reduce a weed's population to a level that does not affect the integrity of the native community in question. The reduction of invasive plant populations typically will allow native vegetation to exist and thrive.

Efforts to control invasive species require patience, as often more than one season of control is necessary to achieve desired results.

Methods of Prevention and Control

Methods designed to prevent or control invasive species are separated into four categories: cultural, biological, mechanical, and chemical. Determination of the most suitable control method will depend on several variables, including the the species involved, the nature of the invasion, surrounding environmental conditions, and the management objectives for the area in question. In some instances, several control methods may be combined (e.g. cutting followed by a chemical application to inhibit resprouting).

Cultural Control

Cultural control involves the modification of human behavior both within and around the natural area. Recreational and economic land uses that contribute to the introduction and proliferation of invasive species should be discouraged. A monitoring program should be implemented to identify species invasions before they become a significant problem.

Biological Control

Biological control uses a plant's natural phytophagous (plant-eating) enemies to control the species population. Highly host-specific predators must be used in order to reduce negative impact on non-target species. Biological control insects for leafy spurge (Euphorbia esula), purple loosestrife (Lythrum salicaria), and spotted knapweed (Centauria maculosa) have been released in the state on an experimental basis. Native weevils are helping control Eurasian water milfoil (Myriophyllum spicatum) in many lakes.

Mechanical Control

Mechanical methods include prescribed burning, mowing, cutting, girdling, and other methods that physically remove the target species. Control methods that imitate natural processes (such as prescribed burning in a fire-adapted community) are preferable to other tactics. Whatever the control method, it should be tailored to fit the environment, the targeted plant species, and the management goals for the community. Because disturbance provides fertile ground for invasive species, it should be minimized in the control strategy.

Chemical Control

Chemical use may be justified when invasive species are severely degrading the natural community, and when effective non-chemical control methods are not known or do not adequately curb invasive species populations.

Chemicals discussed in this manual are not listed by brand name. More detailed information on control chemicals is available.

NOTE: If herbicides are used, care should be taken to prevent contact with non-target species. Herbicides should be applied by working away from areas already treated. By law, herbicide application must abide by label instructions. A summary list of chemicals mentioned in this manual is provided.

Post-Control Work

As most invasive species spread by seed, it is likely that viable seeds will exist in the soil after most of the mature plants are removed. Likewise, for species that spread vegetatively or resprout after cutting, new shoots are likely to emerge. In addition, new seeds may be brought into the area by hikers, wildlife, wind, or water. Control sites should be monitored several times a year to prevent further population explosions of weedy species.

Removal of invasive species often results in disturbance of the soil or large areas of bare soil where native vegetation has been displaced. In many cases it will be necessary to replant the area to native species. When seeding is completed, it may be useful to plant a short-lived cover crop such as annual rye or oats to compete with the weedy species and control erosion as the native seedlings are becoming established.