Landscaping with Native Plants
Exploring the Environmental, Social
and Economic Benefits Conference
Quantification of the Benefits of Native Landscaping Current Knowledge
Native landscaping, sometimes called natural landscaping, uses plants that were native to a given region prior to European settlement. These native plants can be arranged in formally designed gardens or planted into a more natural meadow. Because native plants, once established, can require minimal irrigation, mowing, and chemical treatments, native landscaping is presumed to offer substantial savings and environmental benefits compared with conventional landscape designs. Native landscaping is promoted as a means to improve the quality of the air, soil and water, help to prevent flooding, control erosion, and enhance biodiversity. Native landscaping is also seen as a tool for sustainable urban development, as a means for reintroducing the natural heritage of an area, and as a vehicle for connecting urban residents to the natural world and promoting a conservation culture.
But how well are these various benefits quantified in the scientific literature? To answer this question we held a conference, Landscaping with Native Plants: Exploring the Environmental, Social and Economic Benefits, in Chicago in December 2004. Researchers evaluated the scientific literature pertinent to the Great Lakes basin to determine the current state of knowledge on native landscaping and its environmental, social and economic interactions. lose to 200 researchers, government officials, development professionals, environmentalists, landscape architects, civil engineers, natural resource managers and others reviewed the evidence.
The conference sponsors included: DePaul University’s Environmental Science Program and Institute for Nature and Culture; Chicago Department of Environment; U.S. Environmental Protection Agency; The Gutgsell Foundation; Peggy Notebaert Nature Museum; and University of Illinois at Chicago School of Public Health and Great Lakes Centers for Occupational and Environmental Safety and Health.
This document details the information presented at the December 2004 conference. Specifically, we examine the public perceptions of native landscaping; the economic benefits of native landscaping; the impact of native landscaping on water quality, air quality, and biodiversity; the fate of pesticides and fertilizers in native landscapes; and native plants’ effectiveness in containing or removing contaminants in soil (phytoremediation) and sequestering carbon.
In each section we provide a contact for the researcher(s) that compiled the information upon which this document is based.
During the conference we found that though there are many anecdotal stories about the benefits of native plants in the landscape, there are very few rigorous scientific studies quantifying the benefits. Many questions remain unanswered. Based on gap analysis discussions during the conference, native landscaping research needs was identified to give a systematic approach to addressing these gaps in knowledge. These identified research needs are included in Attachment A.Attachment B contains case studies that were presented during the conference.
Designing for the long term
Although native landscaping designs may be quite different from traditional landscapes, they must exhibit and maintain a certain degree of order to ensure public acceptance. Native designs that look picturesque or vividly display human intention in their pattern, and suggest continual care for the landscape in their materials and condition, are likely to be accepted and sustained by the public. Designs that look too wild and overgrown to the untrained eye, especially smaller patches of prairie, can become vulnerable to compromise and replacement by conventional plantings. Native landscapes are often designed for their environmental benefits, but they must also be designed for public perception in order to be sustainable over the long term.
For further information about how to manage public perceptions of native landscaping, see:
Click here to see identified research needs for Public Perception.
There is a perception that native landscaping is more expensive than conventional landscaping. This, however, is often not true when tested against both actual experience and cost/benefit models. Further, many environmental services provided by the native plants are not captured in traditional economic models. This section provides information on the economics of native landscaping - installation and maintenance, total development costs, and stormwater management.
Native plants provide economic benefits and positive externalities beyond the cost of landscape installation and maintenance. For example, the environmental services provided by native plants may include mitigating stormwater runoff and its associated problems, and filtering fertilizers thus preventing them from reaching a water body. These services have real economic value that communities would otherwise have to pay for. For example, communities might have to pay to construct additional stormwater infrastructure or address excessive algae blooms. Some benefits of native landscaping, like reduced air pollution from less mowing, are difficult to capture as economic values. Therefore, when considering the economic benefits of native landscaping, it is important to keep in mind the environmental value of native landscaping, which is often unaccounted for when considering the costs of installation and maintenance.
Installation and maintenance costsIn 2003, three Northern Illinois landscapers estimated installation and maintenance costs for turf-grass and native landscapes. All three concluded that turf grass is more expensive than native landscaping. Variations in conditions from site to site can create exceptions to this scenario. For residential sites that do not require irrigation systems, seeding turf can sometimes be cost-competitive with native landscaping. For large corporate campuses and residential developments with large common areas, native landscaping is more cost effective
|Native Landscaping||$3,400||$ 5,975|
Figure A: First-Year Installation Costs Per Acre (Natural Landscaping for Public Officials: A Sourcebook. Chicago: Northeastern Illinois Planning Commission, 2004)
|Native Landscaping||$1,600||$ 1,788|
Figure B: 10-Year Average Maintenance Costs Per Acre (Natural Landscaping for Public Officials: A Sourcebook. Chicago: Northeastern Illinois Planning Commission, 2004)
Prairie Crossing, a residential development in Grayslake, Illinois, calculated installation and maintenance costs for open spaces planted with turf grass and compared these with areas on the property planted with native prairie and wetland species.
Residential site designs that employ a suite of tools to address conservation goals are referred to as conservation developments. Conservation and conventional developments use different methods to manage stormwater. Conservation developments employ an array of environmentally sensitive development strategies and techniques such as: protecting natural features, including streams, wetlands, natural areas, and critical habitat; the use of natural drainage patterns; clustering built areas; the provision of common open space; and the use of native landscapes that are integrated into the stormwater management system. These are designed to reduce the on-site and off-site impacts of runoff by preserving and restoring natural hydrologic processes. Conventional developments, on the other hand, attempt to collect and move stormwater as quickly as possible to detention ponds, where it is stored and discharged at the legally allowable rate
Developments: Conservation vs. Conventional
Beyond installation and maintenance costs, native landscaping can contribute to larger total cost savings when used in conjunction with other conservation tools to develop a site. Landscaping is one component of a built system that interacts with other components of the system such as buildings and infrastructure. Certain landscaping choices can lower energy and stormwater costs, reduce infrastructure needs, avoid the need for irrigation systems and improve livability on the site.
In Southeast Wisconsin, the actual costs of conservation development were compared to engineering cost estimates for conventional designs at three residential subdivision projects. While native landscaping costs were higher than conventional turf in two of the three developments and costs were the same in the third, the total construction costs were less for conservation development at all three sites. The inclusion of native landscaping in the overall site design contributed to lower stormwater management costs for the conservation developments compared to the conventional estimates.
Cost models developed for the Conservation Research Institute compared conservation and conventional development for four types of land use on a hypothetical 40-acre parcel: commercial, moderate density residential, low density residential and rural residential. The use of conservation development methods was approximately 40 percent less expensive for commercial/industrial and low density residential sites, and slightly less expensive for rural and moderate density residential developments. Total per-lot development costs for conservation design also proved more cost effective for all four types of land use.
Accounting for Stormwater Management
Stormwater runoff can cause many serious problems that incur real costs to communities, including: flooding, pollution, groundwater recharge deficits, and damage to stream ecology. Reducing stormwater runoff is an issue of growing importance for urban planners, especially since many communities smaller than 100,000 residents must now comply with federal regulations for managing their stormwater. (Larger communities have been subject to these regulations since 1990.) Research suggests that native landscaping in the form of rain gardens, bioswales, and prairies, may provide a low- cost alternative to large-scale infrastructure solutions to reduce runoff.
Some communities are using stormwater management fee reductions as a way to tie economic incentives to the economic value of reducing the amount of stormwater running off of a site.
Stormwater fees are typically assessed based on the amount of impervious surface on a property and they represent the cost for the municipality to manage the stormwater generated throughout the city. Cities like Minneapolis, Minnesota and Columbus, Ohio are reducing stormwater management fees for property owners that implement effective stormwater best management practices (BMPs) that improve stormwater quality and decrease stormwater quantity generated on a property. Through the use of stormwater management fees and by providing fee reductions, municipalities recognize the costs associated with stormwater. Community members can then either pay for those costs or use BMPs that offset those costs.
In the Blackberry Creek watershed in Kane County, Illinois, researchers estimated the economic benefits of downstream stormwater management through the use of upstream conservation design practices. Analyzing within the 100-year floodplain, researchers estimated the benefits of flood risk reduction on property values and the cost of stormwater drainage infrastructure. When hypothetical conservation design practices, including native landscaping, were calculated, the researchers found that flood mitigation would create downstream property value benefits from $14,538 to $36,345 per acre for those properties no longer in the floodplain. There were lesser savings for those properties remaining in the floodplain, but experiencing reduced flooding. Conservation design practices would prevent the need to build new concrete culverts for stormwater diversion, saving $3.3 million to $4.5 million that would have otherwise been spent on culvert replacement or upgrades. When including both flood reduction and infrastructure savings are accounted for, the conservation development would be $301 to $681 less expensive per developed acre.
Click here to see identified research needs for Economics.
Montgomery [email@example.com], Associate Professor
Director of the Environmental Science Program, De Paul University, Chicago, Illinois
What does available data in the scientific literature tell us?
In preparation for the December 2004 Native Landscaping Conference, researchers evaluated more than 220 scientific articles on the hydrologic impacts of native plants and those used in conventional landscapes, such as turf grass. Of special interest were studies of the processes most affected by urbanization, such as (1) infiltration, (2) evapotranspiration (water that evaporates and is transpired or passes through the plant leaves), (3) interception (the storage of water by leaf surfaces before it evaporates or drips down to the ground), and (4) runoff.
Infiltration rates varied widely among native grasses and turf grasses, and between compacted and non- compacted soils. A 1996 study reported an infiltration rate of 7.5 inches per hour in native switchgrass, while a 1979 study of urban sidewalk grass reported an infiltration rate of 0.29 inches per hour. A 1970 study demonstrated that as soil becomes more compacted, which is typical of urban soils, the rate of infiltration decreases.
Evapotranspiration measurements varied widely between region and plant type. Northern semiarid grasslands had less than half the rate of evapotranspiration as turf grass, indicating that more water remained in the soil and less was lost via evapotranspiration.
In John Weaver’s classic 1968 study of the Midwestern prairie he measured interception by different types of grasses, cereal and forage crops. Native grasses such as little and big bluestem intercepted 47-81 percent of precipitation.
Studies show that surface runoff varied, from 3.6 inches per year in native grassland, to 1.3 inches per year in compact lawn turf. These numbers are not conclusive, however, since varying conditions such as the slope of the land, and the type of soil texture could strongly influence the amount of runoff.
Native plants conserve water because, once established, they do not need additional watering. Although it depends on climate, soil and grass type, on average, turf lawns require about one inch of water per week during the summer. This means that in the Midwest watering an acre turf lawn for 12 weeks uses 325,848 gallons of water. Watering a half acre turf grass lawn during the summer months requires 162,924 gallons of water. This is equivalent to filling a 30-foot round, 4-foot deep, swimming pool almost 4.5 times during the summer. Since native plants, once established, do not need additional watering during the summer months, they could be a tool for meeting local water conservation needs, especially in communities with water usage restrictions.
Native Landscaping as a Management Tool
Native landscaping can be one option in a range of stormwater best management practices (BMPs), which are used to promote infiltration, reduce runoff, and improve the quality of water.
One type of native landscaping is a rain garden planted with indigenous plants. Rain gardens are built to reduce runoff by capturing stormwater, allowing the water to infiltrate into the ground. There are not enough quantitative data to state conclusively that rain gardens improve the quality of the water that flows through them, although studies measuring the ability of rain gardens to reduce the amount of water running off of a site are underway.
In Burnsville, Minnesota, Barr Engineering Company is testing the ability of rain gardens to reduce the volume and improve the quality of storm water runoff entering nearby Crystal Lake. The company installed 17 rain gardens along one street, with existing curbs and gutters, in a 56.9-acre watershed. A street in an adjacent 81.9-acre watershed served as the control for comparative purposes. The gardens were constructed in fall 2003, with curb cuts added the following spring to funnel storm water from the street into the rainwater gardens. Two years before the gardens were installed, Barr engineers collected baseline data on storm water runoff rates and volumes from both the study and control streets. Water quality was also measured from a storm sewer pipe at the outlet of each watershed. Data will be collected for two years post-installation. Preliminary results show an 82 percent reduction in stormwater from the street with the gardens.
Click here to see identified research needs for Hydrology.
Researchers: David J. Nowak [firstname.lastname@example.org], USDA Forest Service, Northeastern Research Station, Syracuse, NY; Steven Rosenthal [Rosenthal.email@example.com}, US EPA, :Jospeph Vaughan [firstname.lastname@example.org], Laboratory for Atmospheric Research, Washington State University
Air pollution is a significant concern in many American cities. At the same time, trees – natural air purifiers - are being cut down and fields are being plowed to accommodate growing urban and suburban areas. This section provides information from the scientific literature on the impact of landscaping on air quality.
Mowing and Air Pollution
One advantage of a native landscape is that it does not need to be mowed regularly. A turf-grass lawn, on the other hand, requires frequent maintenance, resulting in emissions of air pollutants from lawn mowers, leaf blowers, and weed whackers. The U.S. Environmental Protection Agency has calculated that standard maintenance of 1000 acres of lawn will lead to the emission of 18 tons of VOCs per year, and that a gasoline-powered lawn mower pollutes as much in one hour as does driving an automobile for 20 miles. According to calculations by the Illinois Environmental Protection Agency, the use of lawn equipment in the Chicago Region alone accounts for 50 tons of VOC emissions per day in the summer. For comparison, even the largest sources of VOC emissions, like a large auto assembly plant, produce about two tons of VOC emissions per day; the cumulative use of lawn equipment produces 25 times that amount. In addition, evaporative emissions from gas cans, which are used primarily to support lawn maintenance activities, emit about 22 tons per day of VOC emissions in the Chicago Region.
Ecologic Benefits of Controlled Burns
Controlled burns have many ecological benefits, and are vital to the maintenance of some ecosystems, like prairies. Native plants are able to survive a controlled burn, while nonnative and invasive plants are killed without resorting to the use of herbicides. Burns increase plant biodiversity by preventing any one species from becoming dominant. They also prevent the invasion of shrubs and trees into the prairie.
Growing awareness of the essential role of fire in maintaining natural landscapes argues for increased use of fire as a management tool. However, there is also concern about the impact of smoke on human health and tighter pollution standards for constituents of smoke. These concerns coupled with the growing number of fire-maintained prairies within urban and suburban communities, necessitates examination of fire as a management tool.
There is a surprising lack of research on the air quality and health impacts from controlled burns in prairies. While there is existing research on the effects of fire in the Western United States and burning of agricultural land, it is unknown if this information transfers to the quick burning controlled prescribed fires used to manage prairie ecosystems and native landscapes.
Note: Information on air quality and plant emissions, pollution removal and carbon sequestration is currently being peer reviewed. It will be added to this document when available.
Click here to see identified research needs for Air Quality.
Researchers: Liam Heneghan [email@example.com], Associate Professor, Environmental Science Program, DePaul University, Chicago, Illinois; Mary Carol Hunter [firstname.lastname@example.org], Assistant Professor, School of Environmental Design, University of Georgia
Why is biodiversity important?
Biodiversity allows for a rich array of microbes, insects, plants, birds, and animals. Communities are interested in promoting and preserving biodiversity for several reasons. Ecosystems with greater biodiversity are considered by many to be more resilient to physical disturbances, natural disasters, and invasive species. Diverse ecosystems also provide ecological services that are expensive to replicate, like air and water purification, attracting pollinators, and providing natural material for advances in science and medicine. Ecologically rich areas also provide aesthetic value and reinforce a sense of place for urban dwellers, bringing a bit of nature into the city.
What factors determine biodiversity?
Urban and suburban areas have many physical structures (e.g., buildings) and disturbances (e.g., mowing). When designing projects using native plants on large tracts of urban land, biodiversity can be supported, in part, by replicating the historical disturbance regime found in nature. For example, in the case of native prairie plants, this is often accomplished through the use of controlled burns. When seeking to improve biodiversity on smaller parcels of land in the built urban environment, designers should take into consideration many aspects of local ecosystem structure including ecological niches, the texture, size and shape of the native landscape, and their distance from natural areas.
Does native landscaping improve biodiversity?
A landscape with a high level of biodiversity is important because it provides multiple ecological services and aesthetic value. Designed landscapes in backyards, corporate campuses, and city parks are often created to emulate nature. These designed landscapes offer opportunities for cities to increase biodiversity. A review of scientific literature finds few formal quantitative studies addressing the biodiversity benefits of native landscapes.
If the plants we plant attract local insects, we can assume that the native birds will follow to feed on those insects. A study of ground arthropod diversity in and around Phoenix sampled 24-sites over six different land uses. The highest diversity was found in areas three habitats where native plant species dominated. Less arthropod diversity was found in areas dominated by non-native plants.
A second study examined biodiversity among certain insects, particularly spiders. Yards landscaped with non-native drought-resistant plants imported from Australia, Africa and South America had diversity of species roughly equivalent to industrial sites. The lesson here seems to be that if a community’s plant and animal life have not evolved together (as native flora and native fauna have), non-native plants may not appeal to local fauna (spiders, birds and pollinators). This seems to be true even if the imported plants perform the same function as natives, such as being hardy and drought resistant.
A research project in Birmingham, England demonstrated the importance of disturbance in impacting biodiversity. There, researchers compared carabid (ground beetle) biodiversity at 65 sites across an urban-rural gradient. They found that the time since last disturbance was a key predictor of carabid diversity. In urban areas, if there are more, or different, disturbances than would naturally occur, then biodiversity is expected to decrease. Abandoned sites, with the absence of land management, reduced disturbance, and wetlands had the highest biodiversity and also provided habitat for scarce species. In addition, the smaller the fragment and further it was from a habitat corridor, the lower the biodiversity.
Click here to see identified research needs for Biodiversity.
Native landscapes are thought to reduce the amount of fertilizers and pesticides from entering lakes, streams and rivers. Much of the science supporting these claims is borrowed from an extensive and growing body of literature in related fields—for example, from agricultural studies on the impact of buffer strips for farm nutrient management, and the use of native cover for sediment control in mine reclamation.
The Problem With Generalizing
Pesticide and fertilizer impacts depend on a range of conditions, including: soil type, density of the plantings, the types of planting used, the slope and size of the plot, and most importantly, the decision whether to use pesticides and fertilizers by the property owner. Much of this information is poorly documented in the scientific literature. This limits the conclusions that we can reach regarding whether the use of native plants impacts the effects of pesticides and fertilizers.
With these caveats in mind, researchers reviewed available studies comparing native landscaping, such as prairies and savannas, with conventional landscaping, i.e., a turf grass lawn. They distinguished between small-scale landscapes, measuring less than half an acre and large-scale native landscapes, which are characterized as areas up to 640 acres, such as parks, corporate campuses and brownfield sites.
The literature reviewed suggested the following benefits from native landscaping: (1) native plants may not require the fertilizers and pesticides needed to maintain conventional landscapes, i.e., turf grass; (2) native plants may reduce the impact of fertilizers through direct uptake of nutrients—nitrogen and phosphorous— from runoff that otherwise would contaminate the water supply; (3) via facilitation, native plants may create sub-soil conditions that reduces the levels of nitrate entering drinking water; and (4) through filtration, native plants may physically remove sediment particles and nutrients that bind to soil particles, i.e., phosphorus. Native plants also reduce soil erosion. Native prairie plants have extremely deep and thick root systems, and once established these plants anchor the soil, preventing wind and water erosion. Preventing soil erosion is very important for improving water clarity and fish habitat.
Do Native Landscapes Reduce the Impact of Fertilizers and
Possibly. The maintenance of native plants could potentially impose a lighter chemical burden on the environment than conventional landscaping, but this depends largely on the choices that people make when maintaining their landscapes.
Researcher: Mitchell Pavao-Zucherman [email@example.com], University of Arizona
Carbon circulates in an endless cycle between Earth’s atmosphere, the oceans, plants and soil. There is a fixed amount of carbon in Earth’s system, and it is stored in (or sequestered) and exchanged among various sinks, or pools in forms ranging from atmospheric carbon dioxide to organic matter in the soil. Sequestering carbon in the soil, and thus removing carbon dioxide from the atmosphere, is a strategy for offsetting carbon emissions and addressing climate change. Sequestration of carbon in the soil also promotes soil quality and health. Plants facilitate the sequestration process by removing carbon dioxide from the atmosphere during photosynthesis and storing it as biomass, which later decomposes to organic matter in the soil.
Why Carbon Sequestration is Important?
As concern grows about the potential harmful impact of rising atmospheric carbon dioxide concentration, so does interest in “carbon sequestration” -the net removal of carbon dioxide from the atmosphere via photosynthesis and its storage in vegetation and soil - as a mitigation strategy to offset carbon emissions and abate climate change. While carbon sequestration is not a stand alone solution to rising atmospheric carbon dioxide and climate change, it is a useful strategy that can complement other solutions.
Soil carbon sequestration is one potential tool for combating climate change, because soil and plants offer a large carbon sink. Researchers have suggested that globally, soils have the potential to sequester 24 percent of the total emissions from fossil fuel combustion. Earth’s soils contain roughly three times as much carbon as all plant biomass.
How Soils Store Carbon?
Carbon is stored as soil organic matter in soil clumps, or aggregates, for different lengths of time. These aggregates range in size. Many variables influence the formation of aggregates, including temperature, rainfall, and soil fauna and mineralogy, as well as land management. In smaller aggregates the organic matter bonds physically to the minerals in the soil; some soil organic matter pools can store carbon for centuries.
Prairie Restoration and Carbon Sequestration
At FermiLab, west of Chicago, tallgrass prairie plots have been established every year since 1975 on cropland that had been cultivated for more than a century. Researchers are studying the effects of prairie restoration on carbon cycling and sequestration over time.
Researchers found that soil aggregates increase rapidly following prairie restoration—35 times faster than carbon accumulates in the soil. However, after ten growing seasons, soil carbon concentration was only 45 percent of virgin prairie, which underscores the importance of preserving remaining virgin prairie fragments. Practices that promote the sequestration of carbon include residue management (leaving plant biomass in place to decompose); soil amendments like manure and peat; minimizing disturbances to the soil; maintaining root biomass in the soil; and other practices that mimic the natural ecosystem. Prescribed fire in prairie restorations, for example, keeps the canopy relatively thin and warms the soil. Warmer soil promotes decomposition, which in turn promotes carbon sequestration.
Native Landscapes and Carbon Sequestration
It is fiendishly difficult to generalize about the relationship between native plants and landscapes and soil carbon sequestration. Not only do variations in urban soils have an impact on carbon sequestration—so, too, does the urban environment itself, which is fundamentally different from undisturbed systems. Cities are heat islands that typically have higher temperatures than the surrounding areas, with more air pollution, denser cloud cover, more precipitation, more exotic species, greater fragmentation, and higher disturbance regimes than rural or undisturbed systems. Due to variations in urban soil types, research done in one place in the city may not apply two blocks away. At least one study from USDA conducted in and around Baltimore, Maryland suggests that plants in cities are more productive than plants in more rural areas. Researchers planted lambs’ quarters in three environments: urban, suburban and rural. The urban environment provided 21 percent more atmospheric carbon dioxide and higher temperatures than the rural site, thus resulting in twice as much plant biomass in the city plants. Does this higher productivity enhance transfer of carbon to the soil? That question remains to be answered.
For further information:
The FermiLab project aims to use the information learned about carbon cycling in restored prairies in order to develop restoration practice guidelines for enhancing the natural carbon cycle and improving carbon sequestration (see http://www.uic.edu/labs/meler/fermi.htm).
The USDA has two urban soil programs that are relevant to the sequestration of carbon in soils in native landscaping. (1) The Forest Service’s Northeastern Research Station in Syracuse, New York, established in 1978, investigates the effects of urban forests and their management on human health and environmental quality. http://www.fs.fed.us/ne/syracuse/ .
The National Resource Conservation Service has a program entitled “Urban Soil Issues.” Its goals are to provide nationwide leadership in soil science to meet the needs of urban customers. The define urban broadly, so their activities will likely apply to suburban and exurban landscapes http://soils.usda.gov/use/urban/ .
Click here to see identified research needs for Pesticide/Fertilizer and Climate Change.
Phytoremediation is the use of plants to remove, alter, or contain contaminants on a site. Phytoremediation occurs when contaminants in either the soil or groundwater are drawn into the root system of a plant. Once in the root system the contaminants are absorbed, sequestered, degraded, or volatilized into the atmosphere. This section provides information from the scientific literature regarding the use of phytoremediation using native plants as a natural tool for cleaning up polluted soil and groundwater.
Scientists have studied many plant species to determine their ability to remediate specific contaminants. Plants are selected according to climate and contaminant cleanup objectives.
Non-native plants are often employed for specific tasks. Cleanup of heavy metals, for example, requires genetic traits of tolerance and uptake selectivity. Relatively few plants fit the bill, and those that do are contaminant-tolerant exotic species that have evolved on soils rich in the metal of interest. Two examples are Pteris vittata, a fern from Asia that is good for extracting arsenic, and Thlaspi caerulescens, a European plant that accumulates zinc. When non-natives are used for phytoremediation, precautions are taken to prevent them from becoming invasive. Sunflowers and other crops, coupled with chemical soil manipulation, can also phytoextract heavy metals.
Native plants work best when the functions required for remediation are tied to the plants’ effect on soil fertility, microbial populations, water consumption, or to their effectiveness as ground cover, since native plants often have a positive effect on these attributes. As an alternative arrangement, native plants may be planted after non-natives have finished a cleanup, to make the transition to a native landscape.
For example, at Argonne National Laboratory near Chicago, non-native trees are performing the specialized task of cleaning up contaminated soil and groundwater. The lab planted 800 hybrid poplars and willows in 1999. These tree species were selected because of their fast growth rate and deep roots that could reach a 30-foot-deep aquifer contaminated with trichloroethylene and other solvents. By the fall of 2000, the solvents were absorbed into the tree tissue, demonstrating that phytoremediation was occurring. Argonne’s goal is to restore the site to a native landscape after cleanup is complete, progressively planting native oak in the areas now dominated by willows and poplars.
Prairie plants have dense root systems, and native plants are good at degrading hydrocarbons in the rhizosphere, the zone surrounding the roots of plants. Decaying roots provide nutrients and energy to biodegrading microorganisms. Dense root systems also stabilize the soil and protect it from runoff and wind erosion. At the Bunker Hill, Idaho Superfund site in the Coeur d’Alene River Basin, the second largest Superfund site in the nation, revegetation with native and nonnative plants is helping to prevent soil that is contaminated with a variety of heavy metals from eroding into the river system.
Phytoscapes are landscapes that are designed to deliver phytoremediation benefits. Atlantic Richfield Company is pioneering the use of native-prairie and wetland plants to prevent small spills from spreading, and to clean up spills around its neighborhood gas stations.
Some native plant species not only tolerate but actually remediate gasoline in the soil. While gasoline- tolerant species are now being used to prevent small spills from building up, and to clean up leaks that have already occurred, intolerant native plant species are being used as leak detectors. Like canaries in the mines, the death of an intolerant “sentinel” plant indicates that there has been a gas leak. These intolerant plants are especially useful because they can detect leaks sooner than conventional detection methods.
Click here to see identified research needs for Phytoremdiation.