There are many stakeholders in the invasive plant arena, including the gardening public, the nursery and landscape industries, the forestry industry, those labeled as environmentalists, and government officials including legislators. Due to the nature of the business, nursery personnel find themselves in the midst of the invasive plant debate. The debate involves the freedom to import and sell nonnative plants, some of which may be potentially invasive, versus the control of the sale of potentially or known invasive plants. As an example of this control, Massachusetts and New Hampshire have banned the sale of burning bush (Euonymus alatus), Norway maple (Acer platanoides) and Japanese barberry (Berberis thungbergii). In 2010 Maryland legislators introduced House Bill 1360 (http://mlis.state.md.us/2010rs/bills/hb/hb1360f.pdf) which proposed to ban the sale of common nursery species such as sawtooth oak (Quercus acutissima), Japanese flowering cherries and hybrids (Prunus spp.), ornamental pears (Pyrus calleryana and cultivars), Japanese spirea (Spiraea japonica), burning bush (Euonymus alatus), Chinese and Japanese wisteria (Wisteria sinensis and W. floribunda), English ivy (Hedera helix), periwinkle (Vinca spp.), creeping bugleweed (Ajuga reptans), pampas grass (Cortaderia selloana and C. jubata), and Japanese silver grass (Miscanthus sinensis). However, this proposed legislation was not passed and an alternative invasive plant legislative approach (Maryland House Bill 831) became law in April 2011 (http://md.opengovernment.org/documents/1100909-third-reading).
Figure 1.
Native range of eastern redcedar (Juniperus virginiana).
SILVICS OF NORTH AMERICA, USDA
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House Bill 831 put into place the creation of an Invasive Plant Advisory Committee to advise the Maryland Secretary of Agriculture in a risk-assessment protocol that would categorize invasive plants as either a Tier I or a Tier II species. Tier I species would ultimately be banned from production and landscape use. Tier II species would not be banned, but would require invasive species labeling and landscape firms would be required to inform clients that Tier II species were included in a design or planting. A significant aspect of this legislation, with special interest to the nursery and landscape industries, is that the Committee establishes a science-based risk assessment protocol for invasive plants that will be used in developing the two-tier system. In addition to state government officials, the Committee is to be composed of one person from a landscape firm, one person from a wholesale or retail plant operation, two individuals with experience with invasive plants (or related experience), and a consumer.
Thus, nursery industry personnel must be proactive in their approach to selling potentially invasive species by developing a set of best management practices – or else legislators will be making the decisions on what plants to sell.
PHOTOS COURTESY OF ALEX X. NIEMIERA UNLESS OTHERWISE NOTED.
The purpose of this article is to elucidate the terminology, concepts and issues within the invasive plant arena. Knowledge of these facets will allow stakeholders to better understand and discuss invasive plant issues. We will also discuss the role of the ornamental horticulture industry and what the industry can do to employ best management practices in dealing with nonnative species and potentially invasive plants.
What is an “invasive” plant?
The main focus of controversy is that some nonnative plants, also termed “nonindigenous,” “exotic” or “alien” plants, are invasive and cause ecological or economic harm. One of the main impediments to a clear understanding of the invasive plant discussion is the numerous terms that often have multiple interpretations. The interpretation of “invasive” depends on one’s point of view and several definitions exist. The definition that carries legal weight is that set forth by the U.S. Government: an invasive species is “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.”
Thus, by definition, an invasive plant species is nonnative. Labeling a native plant growing in its native environment as “invasive” would be technically incorrect. For example, eastern redcedar (Juniperus virginiana), a conifer native to most of the eastern half of the U.S. (see Figure 1), has weedy tendencies, meaning it usually invades disturbed or exposed areas and abandoned fields. However, it is not considered an invasive species when growing in its native range. Then what is an “alien” plant? The U.S. Government’s definition of an alien species “means, with respect to a particular ecosystem, any species, including its seeds, eggs, spores, or other biological material capable of propagating that species, that is not native to that ecosystem.” The U.S. Government’s definition of a native species “means, with respect to a particular ecosystem, a species that, other than a result of an introduction, historically occurred or currently occurs in that ecosystem.” Ecosystem plays a part in the definition of alien and native species and is defined as “the complex of a community of organisms and its environment.”
“Ecosystem,” however, is somewhat of a nebulous term relative to determining the native range of a plant species. Native range maps of plants in the U.S. have been established by historical botanical records, and give approximate boundaries for native plant ranges. Such maps (for example, Figure 1) are given for individual woody species in the USDA’s Agriculture Handbook 654, Silvics of North America (http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm). Despite these definitions, commonplace descriptions of the terms native and nonnative often vary. “Native” is often referred to in terms of continental, country or state borders. Black locust (Robinia pseudoacacia) is a tree species native to the central Appalachians and the Ozark Mountains, but is labeled an invasive species in New England states and California. Thus, labeling a plant as native to the U.S., or even a region of U.S., has limited meaning since a plant native to one state may be invasive in another state. Southern magnolia (Magnolia grandiflora) is native to the southeastern U.S. but is widely planted throughout the mid-Atlantic and nearby states; southern magnolia is technically a nonnative species in areas outside of its natural range.
What is plant invasion?
Plant populations are not static; they are constantly expanding and receding. Thus, plant population expansion is a natural phenomenon. However, human activity has greatly influenced plant populations. In some cases, plant population ranges are being significantly reduced, as in the case of habitat destruction. In some cases population ranges are being significantly increased, as in the case of widespread sale and transport of species, or by animal-, wind- or water-spread of seeds of those planted species. The typical occurrence of plant invasion occurs in three phases (see Figure 2). The first phase is the lag phase that is characterized by slow population growth. The second phase is the exponential phase characterized by rapid population growth. The third phase is the naturalization phase, in which the curve flattens out and the species has established a self-perpetuating population.
The duration of the lag phase, the time from initial introduction to the inception of rapid population growth, depends on numerous factors including species, environment, climate and human-mediated factors; this phase can occur in years or decades. Woody species typically have a lag period of many decades, and this can exceed 100 years. In contrast, lag periods for herbaceous perennials is much shorter than for woody species.
Invasive species vary in their aggressiveness. Some authors have categorized invasive species into three stages of invasiveness: 1) widespread but rare, 2) localized but dominant, and 3) widespread and dominant. Thus, to assemble all invasives into a single group would be impractical since environmental impacts of invasive species vary from relatively innocuous to quite damaging. Remembering that the definition of an invasive species is “an alien species whose introduction does or is likely to cause economic or environmental harm …”, the harm aspect differs depending on the invader. We will now discuss how invasive plant species can impact the environment.
How do invasive plant species harm the environment?
Although invasive species are an environmental problem, the proportion of nonnative plant species that become invasive is quite small. The term “harm,” synonymous with impact, is open to interpretation. Some researchers characterize impact on the basis of range, abundance and the per-capita or per-biomass effect of the invader. To fully understand the invasive plant issue, one must consider that invasive plant species have varying degrees of impact, ranging from relatively harmless to environmentally disruptive. The ranking of impacts of invasive plant species will be discussed in a later section.
Without a doubt, invasive plants can dramatically impact the environment; they can do so at the ecosystem level and at the community and population levels. In terms of the ecosystem level (large-scale effects), invasive species can affect:
- geomorphological processes, such as soil erosion and sedimentation rate;
- hydrological channeling processes, such as surface water channeling, water table depth and soil-water holding capacity;
- biochemical cycling, such as soil and water chemistry, and;
- disturbance (for example, fire, weather, human-mediated impacts) regime such as the type, frequency, intensity and duration of disturbance in the presence of invasive plants.
In terms of the impacts on community and population levels, invasive species can change stand structure, such as occurrence of new species or placement of new species in an area; recruitment of natives, such as physical barriers, allelopathy (chemicals exuded by plants that inhibit or kill competing species), or changes in microclimate, and; resource competition, such as plant species competing for light, nutrients, water or space.
Can invasive potential be predicted?
Predicting which nonnative plant species will become invasive is one approach to keeping potentially invasive species out of commerce, thereby eliminating the threat to the native environments. However, predicting which species will be invasive in a particular area is a difficult task due to the complexity of nature. There has been an abundance of work to determine the plant characteristics and ecological factors that lead to plant invasion. At present, the most reliable and powerful predictor of a species’ invasiveness is its record of invasiveness in other nonnative sites. Many prediction schemes have been developed to assess the potential of plant species to be invasive. These approaches to understanding the invasive potential have significantly increased the ability to predict which species will be invasive.
Using lists of known invasive and noninvasive plants, prediction models have correctly identified 80 to 90 percent of invasive nonnative species. One of these models identified a single question, “Is the species is a weed elsewhere?” as correctly identifying 92 percent (57/62) of the invasive plant species in Florida. This one simple question also correctly identified 92 percent of Florida noninvasive imports. The methodology for the prediction of invasive plants, however, has not been integrated into a package that is easily used by those who are not well-versed in ecology. A need also exists for prediction schemes to include, among other variables, the role humans play in overcoming the effect of random natural events on immigrant plant populations.
Prediction based on biological characteristics can reliably foretell if a plant will establish and spread. However, prediction is less reliable in forecasting the impact a species will have on an environment. Because invasiveness and ecological impact are not necessarily linked, some scientists are in favor of categorizing those invasive plant species that have a profound effect on biodiversity, about 10 percent of invasive plants, with the term “transformer species.” The notorious saltcedar (Tamarix spp.), originally introduced into the U.S. as an ornamental, in windbreaks, and for erosion control, is an example of a species that has significantly altered many western U.S. riparian ecosystems. Transformer species, because of their impact, would receive the majority of resources for containment, eradication and control.
Figure 2.
The process of invasion exhibiting the lag phase (slow growth), exponential phase (rapid growth) and naturalization phase (selfperpetuating) of population growth.
What is the role of the ornamental horticulture industry in the invasive plant subject?
Since the 1700s, the U.S. nursery industry has produced and sold tens of millions of landscape plants; many were nonnative species, that have graced our landscapes with inestimable environmental, aesthetic, economic and social benefits. However, the nursery industry has also imported plant species that have been quite harmful to our environment. By 1994, 235 woody nonnative woody species had become naturalized in North America; 85 percent of these were introduced by the landscape trade for aesthetic or functional purposes, such as erosion control. Of the 300 invasive plant species (all types) in the U.S. (except for Hawaii), 50 percent of them were imported for horticultural purposes.
The nature of the nursery industry fosters the potential for nonnative species to become invasive. The industry mass propagates, transports and sells plants throughout the U.S. The frequency of introduction of invasive species, known as invasion pressure, is one of the most important factors that contribute to the invasion of an area. One recent study looked at the role of the ornamental horticulture industry in invasion pressure by relating the naturalization rate of invasive plants to the number of years an invasive species was sold in the nursery trade. Results showed that the rate of naturalization increased as the period of sale increased. For example, only 1.9 percent of plants naturalized that were sold for one year, whereas 30.9 percent of plants naturalized that had been sold for 10 years or more.
Once sold, garden plants are cultivated. Cultivation is an important process in overcoming the destructive forces of random natural events. Ultimately, cultivation favors the invasion process. Additionally, plant characteristics that make desirable garden plants, such as a fast growth rate, abundance of fruit, and tolerance of poor growing conditions, also favor naturalization and invasiveness.
A bit of history
The importation and use of nonnative plants has a long history in the U.S. Thomas Jefferson (circa 1800), commenting on his services to the U.S., noted, “The greatest service which can be rendered any country is to add a useful plant to its [agri] culture.” In 1827, under the administration of President John Q. Adams, the following was put forth in a Circular to a portion of the Consuls of the United States: “The President is desirous of causing to be introduced into the United States all such trees and plants from other countries not hitherto known in the United States, as may give promise, under proper cultivation, of flourishing and becoming useful.”
The U.S. government first funded plant importation in 1839 through the Agriculture Division of the Plant Patent Office. In 1862, one of the duties of the first Commissioner of Agriculture, Isaac Newton, was “to collect, as he may be able to, new and valuable seeds and plants; to test, by cultivation, the value of such of them as may require such tests; to propagate such as may be worthy of propagation, and to distribute them among agriculturists.” Thus, plant importation has been a long tradition in the U.S. However, the topic of nonnative plants has become a controversial and contentious subject.
The very nature of the ornamental horticulture industry – selling, transporting, and cultivating species – has the potential to foster the invasion process. Lag time (the time from introduction to the inception of exponential population growth) is most likely shortened by the mass propagation and distribution of potentially invasive species. Another phenomenon termed “invasional meltdown,” in which the interactive activities of nonnative species facilitate each other’s invasive capacity, may also shorten lag time or serve to enable invasion. An example of invasional meltdown occurred in south Florida where 20 species of fig (Ficus) were commonly used as landscape species; these species did not establish self-sustaining populations due to the lack of a pollinating insect. Upon the introduction of an exotic wasp that served as a pollinator in the early 1970s, one of those fig species, Ficus microcarpa, has established self-sustaining populations.
What should the ornamental horticulture industry do?
How does one make decisions about nonnative plants that are invasive or potentially invasive? Information on the Internet can be overwhelming, confusing and inconsistent. Vendors and consumers of nonnative taxa (any taxonomic unit such as a genus, species or cultivar) are generally unaware of a taxon’s invasive potential. There are a few good web sites that include ranking systems of invasive impacts based on scientific observation. NatureServe, a nonprofit conservation organization, has a current data-based assessment of nonnative plant species. On the NatureServe web site (http://www.natureserve.org/), click on the butterfly icon (NatureServe Explorer tool). Then type in the name of the plant in question in the Species Quick Search box. Then click on the U.S. Invasive Species Impact Rank (IRank). NatureServe has assessed 452 plant species. Each species is given an overall impact ranking based on ecological impact, current distribution/abundance, trend in distribution/abundance, and management difficulty.
Other sources that have developed rankings relevant to Virginia residents are the Virginia Native Plant Society in conjunction with the Virginia Department of Conservation and Recreation (http://www.dcr.virginia.gov/ natural_ heritage/invsppdflist.shtml) and the Southeast Exotic Pest Plant Council (http://www.se-eppc.org/). These groups have ranked species according to their perceived and observed threats to the environment.
The above-mentioned ranking systems assess nonnative plants that are already naturalized in the U.S. Determining the invasiveness of new introductions is more difficult. Another complication of the invasive plant problem is that a nonnative species may be regionally invasive. For example, a species that is a problem in the eastern part of Virginia may not be a problem in the western part of the state due to temperature or ecological constraints. At present, the only good options for gardeners concerned about the invasive potential of nonnative plants is to stay aware of the invasive status of plants by accessing the aforementioned sites.
The nursery industry should develop a best management practices (BMP) approach to invasive plant species. A key component of this BMP approach would require each nursery to determine which of the species they sell that has a NatureServe impact rank. If a plant is ranked, then the nursery should label these plants with the rank; such information should include the region of current invasion, as well as the Ecological Impact, Current Distribution/Abundance, Trend in Distribution/Abundance and Management Difficulty subranks.
Native plants, and nonnative landscape plants that have been proved to be noninvasive, can be used in place of invasive species that negatively impact natural areas. Regional plant societies such as the Virginia Native Plant Society have developed lists of plants (http://www.dcr.state.va.us/dnh/native.htm) native to the state and physiographic provinces. Additionally, there are multiple books that provide instructions on how to propagate and grow plants native to specific regions (http://www.mdflora.org/booklist.html#landscaping). When selecting indigenous plants, one should remember that native plants may not necessarily be better adapted to a particular region than nonnative species. Considering how dramatically we alter the ecology of our residential or commercial habitats – soil, vegetation, sun/shade, temperature, and water regimes – a native plant may not be better suited to a garden environment than a nonnative plant. However, there are many aesthetically pleasing native plants that thrive in garden settings and provide environmental benefits such as nectar and fruit for native birds and butterflies.
Finally, if a nursery operator or gardener would like to use a native plant as an alternative for a specific nonnative plant, there are books and web sites that have this information. One such reference – Native Alternatives to Invasive Plants – comes from the Brooklyn Botanical Garden. See the sidebar, “Internet Sources for Invasive Plant Information,” for a list of Internet resources of native plant alternatives.
In sum, a responsible nursery operator will judiciously sell plant species native to the region or nonnative species that do not invade and negatively impact natural areas (daffodil, for example), rather than nonnative plants that harm the environment or have the potential to do so.
Alex X. Niemiera, Ph.D., is an associate professor in the department of horticulture, Virginia Technical Institute, Blacksburg. He can be reached at niemiera@vt.edu. Betsy Von Holle, Ph.D., is assistant professor in the department of biology, University of Central Florida, Orlando. She can be reached at vonholle@mail.ucf.edu.