we can employ other ways to control them.) Natives rarely need watering. They improve water quality and reduce flooding, serve as carbon sinks, and reduce the demand for nonrenewable resources. The US Environmental Protection Agency (EPA) explains that “native landscaping practices can help improve air quality on a local, regional and global level. Locally, smog (ground level ozone) and air toxics can be drastically reduced by the virtual elimination of the need for lawn maintenance equipment (lawn mowers, weed edgers, leaf blowers, etc.) which is fueled by gasoline, electricity or batteries. All of these fuel types are associated with the emissions of [many] air pollutants.”7 Reduced lawn and expanded native plantings produce healthier environments, and the absence of loud machinery brings peace and quiet. Regionally native plants create an authentic sense of place in the landscaping at home, in parks, and in other public places throughout our communities. While creating a future for the regional ecosystem, native gardens with diverse plants that attract a host of birds, butterflies, and beneficial insects also serve as convenient places to learn about plants and wildlife and simply enjoy nature.
There are other benefits. Gardens of native plants reduce opportunities for nonnative plants to overrun the landscape. Seed from native plants that is carried by wind and birds from our garden into natural areas does not degrade the environment. “Nonnative plant species pose a significant threat to the natural ecosystems of the United States. Many of these invasive plants are escapees from gardens and landscapes where they were originally planted. Purchased at local nurseries, wholesale suppliers and elsewhere, these plants have the potential of taking over large areas, affecting native plants and animals and negatively changing the ecosystem.”8
The statistics are alarming. In the United States, more than 100 million acres of land have been taken over by invasive plants and the annual increase has been estimated to be 14 percent.9 “The impact of all of these nonnative plants is creating novel ecosystems that are not supporting food webs, therefore not supporting biodiversity.”10 The huge numbers of nonnative plant species that have naturalized and flourish without human assistance can be seen by looking at the maps on the USDA PLANTS Database. Native plants once grew in the spaces now holding naturalized nonnative plants, and their absence deprives birds and butterflies of reproduction sites, shelter, and food. History also shows that many seemingly benign nonnative plants become invasive. Eliminating this possibility from our own gardens and landscapes is a significant advantage of going native.
Research of interest from an ecological perspective also serves to scientifically document the potential negative impacts an invasive plant can have on human health. Studies reveal that the exotic invasive shrub Japanese barberry provides habitat favorable to the blacklegged tick, exposing people to tick bites and the associated Lyme disease threat.11 Another study looked at how leaf litter in water influences the abundance of mosquitoes, which can transmit West Nile virus to humans and wildlife, and found that two of the most widespread nonnative, invasive plants, Amur honeysuckle and autumn olive, yielded significantly higher numbers of adult mosquitoes than the other leaf species. Nonnative invasive plants “are having very significant ecological impacts, displacing a lot of native species. And now we’re seeing that some of them also enhance the transmission of a dangerous disease,” said the researchers.12 Choosing native species is the ounce of prevention that is worth a pound of cure.
A plant’s origin is the key to its beneficial ecological role. Is the tree or shrub native to the Midwest, or an introduction from Eurasia? Is the plant a true native, or is it a nativar (a recent term combining the words “native” and “cultivar”)? People making gardening and landscaping decisions may feel too busy or distracted by other concerns to pay attention to the origins of their woody plants. This book aims to lighten that burden by providing this information. Awareness of the key environmental role played by native plants encourages landscaping that benefits birds and butterflies rather than a nursery’s preferences or the landscaper’s convenience. A plant’s origin is relevant when the town or city where you live conducts an annual tree sale for its parkways, or a local organization sells plants to raise revenue for a good cause.
It seems counterintuitive, but the majority of plants used in agriculture, forestry, and horticulture in North America are not native to the continent.13 One of the biggest—yet least recognized—impacts humans are having on urban habitats is a change in vegetation from predominantly native to nonnative species. “As the nursery industry evolved in the 1800s, exotic plants were imported from foreign lands. As a result, approximately 80 percent of the plants in the nursery trade today are non-native exotics.”14 And, about 80 percent of suburbia is landscaped with plants from Asia.15 The majority of woody plants in the United States that became invasive were introduced into this country for horticultural purposes.16
Studies indicate that a plant that is invasive in one region might be problematic in another region, particularly if the two regions have similar climates. For woody ornamental species, for example, being invasive elsewhere was the single best predictor of potential invasiveness in a new region of introduction.17 Unfortunately, there is no screening method to determine if newly introduced plants are going to become invasive. Estimates have put the economic cost of invasive plants in natural areas, agriculture, and gardens at $125 billion per year, and deem it to be “rising quickly.”18 Laws to prevent the sale of invasive plants are created only after the plants have already become a problem. Even then, outlawing nonnative invasive ornamental species is sometimes opposed because their sale is economically beneficial to the nursery industry and to the state.19
Decades can pass before a seemingly inoffensive nonnative ornamental plant becomes invasive. “In general, introduced plants [defined as not native to the area that have naturalized] are likely to invade or become noxious since they lack co-evolved competitors and natural enemies to control their populations,” states USDA PLANTS.20 The marketing period—the number of years nonnative horticultural plants are sold—has profound influence on their naturalization and invasion. As long as they are sold, nonnative plants will continue to naturalize and invade, notes a federal study.21 This pattern also applies to cultivars of nonnative plants.22 Even “sterile” nonnative invasive cultivars, though less fecund, eventually produce enough pollen and seed to reproduce, so cultivars of invasive plants remain invasive and are not “safe” for the environment.23
Plant choices, like clothing styles, have gone in and out of fashion. Plantings reveal information about the landscaping time period of a home or neighborhood, but there is usually a common denominator in the plants’ origins—Europe or Asia. Studies verify our personal observations. “Most of the ornamental species in parks and gardens are alien, e.g., lawn grasses, rose bushes, lilacs.” Introduced flora dominates “the visual impact of the flora in much of middle North America.”24 The persistence of consumers’ habits in choosing introduced species is revealed in popular garden catalogs that continue to advertise nonnative ornamental plants.25
Creating environmental benefits that never go out of fashion is a byproduct of choosing native species. Arguing that “for the first time in its history, gardening has taken on a role that transcends the needs of the gardener,” Douglas Tallamy, author, researcher, and professor and chair of the department of entomology and wildlife ecology at the University of Delaware, writes that gardeners have become “important players in the management of our nation’s wildlife.” “Gardening with natives is no longer just a peripheral option” but one that “mainstream gardeners can no longer afford to ignore.” “It is now within the power of individual gardeners to do something that we all dream of doing: to make a difference. In this case, the ‘difference’ will be to the future of biodiversity, to the native plants and animals of North America and the ecosystems that sustain them. . . . My argument for using native plant species moves beyond debatable values and ethics into the world of scientific fact,” he writes.26 “As quickly as possible, we need to triple the number of native trees in our lawns and underplant them with the understory and shrub layers absent from most managed landscapes.”27
The
