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FAQs on Native & Exotic Plants & Native Landscaping by Brian Kermath 1995-2007 |
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Frequently Asked Questions about Native & Exotic Plant Species and Sustainable, Native Landscaping
1. What is a native plant and how can I be certain whether or not a plant is native to my area? 3. Why should I be concerned about exotic species? 4. Why are exotic species problematic? 5. Are native landscapes messier and less visually appealing than conventional landscapes? 6. Is biodiversity increased by adding exotic species to the landscape? 8. Will a native landscape help conserve water? 9. What should I do with the exotics I remove? 10. Will a native landscape attract pests? 11. Is a native landscape the same as a xeriscape? 12. Is it wise to avoid some native species? 13. Can a native species be invasive? 14. Can I use wetland species in my native landscape even though my site is not wet? 15. Will I have to amend the soils to support native plants? 16. Is native landscaping the same as ecological restoration? 17. Is it okay to include exotic species? 18. Should I be concerned about the source of the plant material I use? 19. Are native plants more difficult to establish and maintain than exotic plants? 20. Are native plants more difficult and costly to acquire than exotic plants? 1. What is a native plant and how can I be certain whether or not a plant is native to my area?
Simply stated, a plant is native to all the places that it naturally occurs. Problems in using this definition to determine whether or not a plant is native to a particular place arise, however, when considering: a) ecological dynamics and time; b) the semantics of the terms native, natural range, and origin; c) unknown past human influences; d) spatial scale; e) the lack of congruence between social spaces (e.g., counties) and ecological spaces (e.g., habitats or ecoregions); and f) our incomplete knowledge of plant occurrences, both past and present.
a. Ecological dynamics and timeEcology complicates matters of plant nativity because plant ranges and distributions are not static, but continually shift with ecological changes. Plants move into new, hospitable territories over time from the places in which they evolved. The outer limits of their ranges and the distributions within those ranges are in a constant state of flux, shifting in all directions with temperature, precipitation, and a multitude of other ecological factors. By way of illustration, suppose that the range and distribution of some tree species forms a polygon that resembles an irregularly shaped slice of Swiss cheese when represented on a map. Beyond the edges of the polygon and within the interior holes, the tree does not occur. Over time, however, the position and shape of the polygon and the location and shape of at least some of the holes change with ecological conditions. Some changes occur relatively quickly, perhaps due to the arrival or disappearance of a seed dispersing organism, while others take place more gradually, as with natural climate change. When we think about natural plant ranges for landscaping purposes, we tend to consider them for the present time only, which complicates the decision-making process, because we often ignore short-term ecological changes. b. The terms native, natural range, and origin
Although the definition offered above
The ginkgo tree (Ginkgo biloba) is a good example. The ginkgo today is restricted to only a fragment of its once wide natural distribution. But to say that it is native to North America because it once occurred there naturally is inaccurate. If we used that thinking, then there would be many plants "native" to the region that are no longer found there � including tropical plants that could no longer survive there. c. Human impacts on plant rangesOver the broad sweep of our occupation of the planet we have always affected plant distributions in our quest to survive. We have eliminated some plants entirely from their ranges, we have eliminated some plants from portions of their established ranges, and we have introduced plants into new regions. Indeed, we have been an important ecological agent on the planet. Most details of our past environmental impacts, however, are beyond our ability to ascertain with accuracy. Our earliest ancestors must have been quick to discover ways to start and manipulate fires, which we know had profound and lasting impacts on vegetation patterns. Research in Australia, for example, is beginning to reveal the important environmental changes wrought by aborigines some 35,000 to 50,000 years ago through their use of fire, which drastically altered vegetation patters, drying the climate and pushing many animals to extinction (Bowler et al. 2003; Johnson et al. 1999; Miller et al. 1999, 2005). Relatively intense agricultural practices dating back at least 12,000 years surely had important consequences too, as did the building of human settlements (from small villages to large cities), forestry, mining, and other human land uses. Even before the advent of agriculture or the rise of extensive civilizations, our hunter-gatherer ancestors sowed seeds here and there, intentionally or otherwise. Unfortunately, accurate records of our early impacts on specific plants remain sketchy at best. Thus, we cannot know the degree to which we influenced plant distributions during most of our time on the planet, at least not until about 500 years ago. This is not to say that we simply abandon the 'native plant' concept, because while our early impacts on plant distributions were indeed important, they differ significantly enough from more recent impacts for us to meaningfully discern distinctions between them and give us reason to use the concept for land management. First, owing to the smaller human populations and limited transportation means of the distant past, fewer introductions over time must have occurred compared with the more recent past. Second, most early human plant introductions must have taken place primarily between contiguous ecoregions, and rarely could have involved intercontinental migrations. Third, most of the early introductions occurred on landscapes with more geographically extensive, naturally functioning, intact ecosystems. These factors together made early human plant introductions more ecologically like non-human introductions. Just as Earth�s ecosystems come to balance out natural introductions, so too they did with the human introductions of the past. Although the planet will eventually balance out recent introductions as well, these introductions are taking place comparatively quickly, many over great distances, including many from one continent to another, and on progressively more degraded and fragmented ecosystems. Moreover, we must deal with the ecological problems that arise from our actions and we have the option of avoiding mistakes. In recognition of this manifold problem, a reasonable definition of 'native species,' that allows us to better manage the environment today and into the future, must be used. In the United States a plant is generally considered to be native if it occurred here before the arrival of Europeans. Although such a definition leaves doubt about the absolute legitimacy of the 'native' label, it is a reasonable definition for dealing with today�s issues and it is the best we can do given our limited knowledge of pre-Columbian American landscapes. d. Spatial scale and natural rangesThe spatial scale employed to determine plant ranges also complicates matters. What scale should be used? The continent, the back yard, or something in between? To elucidate the scale issue suppose you live in North Carolina in the continental sub region known as the Southeastern Coastal Plain. Suppose also that you identify three trees that interest you: the silver fir (Abies amabilis), the longleaf pine (Pinus palustris), and the sand pine (Pinus clausa). Upon investigation, you find that all three are native to North America and the United States, but that the silver fir is restricted to the Pacific Northwest from Alaska to northern California, and that the longleaf and sand pines occur in the Southeastern Coastal Plain. At this scale (sub continental), it would appear as though the two pines would be appropriate trees to use. Upon further investigation, however, you discover that the sand pine is restricted to Alabama and Florida, whereas the longleaf pine�s natural range stretches from Virginia to Florida and Texas, including North Carolina. At this scale, only the longleaf pine would be an appropriate choice. If you were to narrow the focus to your backyard, it would be difficult to determine whether or not any individual longleaf pine ever occurred on your small piece of land. Such a scale would be inappropriate. Obviously, a reasonable scale must be used. But what should the regional scale be? Typically, we rely on field guides, plant atlases, and a variety of agency brochures and planning guides, which are often organized by political units, or more generally speaking social spaces (e.g., states and counties). e. Lack of congruence between social spaces and ecological spacesAlthough social spaces (e.g., states and counties) are convenient starting points for thinking about plant nativity, they can be problematic, because they do not necessarily correspond with ecological spaces. By way of example, suppose you live near the western border of a county in an ecoregion that covers the western portion of your county and most of the adjacent county to the west. Suppose too that you are interested in a plant that is listed for the adjacent county, but not for your county. Even though the plant is not listed for your county, it may still be an appropriate choice given that it occurs nearby and within the ecoregion in which you live. It could be that the portion of your county that lies within the plant�s ecoregion merely represents one of the holes in the plant�s natural range. Given ecological dynamics and human impacts, it is possible that the plant did occur in your area in the recent ecological past, but before the area was inventoried. It is also possible that the plant may naturally move into the area in the near ecological future. Thus, because counties (as social spaces) are not necessarily congruent with ecological spaces, it is better to use natural regions when possible, rather than political units to determine native ranges. f. Incomplete knowledge of plant distributionsFinally, it must be understood that floral inventories are incomplete at best and under constant revision. It is likely that plants occur in more counties than existing floral treatments indicate, and it is common to find that different inventories report plant occurrences somewhat differently. One treatment may report a plant for a particular county, while another treatment may not. Future inventories will surely uncover plants in counties that existing inventories have missed. Similarly, future inventories will likely uncover the disappearance of currently extant taxa within given ecoregions. 2. What is an exotic plant? An exotic plant, also referred to as alien, introduced, non-indigenous, or non-native, is one that has been introduced by humans intentionally or otherwise through human agency from one ecoregion to another. An exotic that has escaped into the surrounding natural areas and is reproducing on its own in the regional flora is considered a naturalized species. Those naturalized exotics that become so successful as to spread in the flora and displace native biota are considered invasive. Plants that are not spreading, but have a deleterious affect on native biota may be viewed as 'pestiferous' or 'problematic,' if not 'invasive.' Some confusion about the meaning of exotic species exists in the popular language. The term 'exotic' simply means 'foreign,' 'strange,' or 'different looking,' whereas in ecology, the term is applied to organisms that are from somewhere else. In temperate climate regions, the term 'exotic' often conjures up images of 'tropical looking' plants, hence the generic label 'tropical' frequently applied to palms and similar looking plants. As such, plants that were introduced from somewhere else, but look like they belong, frequently are mistaken as natives, especially if they have been in use for several human generations and are readily available at local nurseries. On the other hand, a plant can look like it doesn�t belong, even though it may be a native. Central Florida illustrates the point well. For example, people often think of the ubiquitous azalea (Rhododendron x spp.) as 'native' and the Florida coontie (Zamia integrifolia) as an 'exotic' because of its 'tropical' appearance, yet the dominant azaleas used in Florida are Asian, whereas the coontie is in fact a Florida native. 3. Why should I be concerned about exotic species?
The
simple answer is that "exotic species negatively impact biodiversity."
Consider the following statistics from distinguished biologist Ed
Wilson�s landmark book, The Diversity of Life (1999:xix) on wild
growing plants late in the 20th century: 11 % in the U.S.
were exotic; 44 % in Hawaii were exotic; 43 % in the British Isles
were exotic, and 28 % in Ontario, Canada were exotic. In Florida about
a third of the wild-growing plant species are naturalized exotics (Florida Exotic Pest
Plant Council; Pimentel et al. 1999;
Richards et al. 1997:2). Of all invasive woody plant
species
in the
U.S. some
85 % were originally
imported for the landscaping and nursery trades ( In a recent article published in the journal BioScience, David Wilcove et al. (1998) report that exotic species have played a significant causal role in almost half of the species threatened with extinction in the U.S. By comparison, they connect only 20 % of imperiled organisms to pollution. Distinguished conservation scientist, Walter Reid (1995), has stated that exotic species represent the fastest growing threat to biodiversity. Narrowing the focus, some 80 % of the endangered species in the Cape area of South Africa are threatened due to the presence of exotic species (Armstrong 1995). In Florida, the disruption of natural ecosystems by the presence of exotic organisms is second only to development according to the Florida Exotic Pest Plant Council. As consequential as the ecological impacts are, the economic toll often strikes home with more resonance: a short list of exotic organisms in the United States, for example, costs the country $138 billion per year in losses and control efforts (Pimentel et al. 1999). Exotic weeds cost Australia over $3.3 billion per year (Combellack 1989). 4. Why are exotic species problematic? When an organism arrives in a new environment, it either adapts and becomes established, or it fails and eventually disappears. If it succeeds in its new home, then it must necessarily squeeze something else out. Think of the process in terms of spatial opportunity costs � where one plant grows, another cannot. In this way, individual native plants are eliminated or prevented from succeeding one by one, as individual exotic plants take root. While the elimination of individual native plants reduces genetic diversity, the problem doesn�t end with individual plants. The concept of ecosystem packing tells us that an ecosystem in its climax state can harbor only so many species (Wilson 1992:181), and because Earth�s climate has been relatively stable for the past 10,000 years, most healthy ecosystems are thought to be packed, or nearly so (Gleason and Cronquist 1964:92).1 The idea holds that as new species establish in a packed ecosystem, naturally or otherwise, the ecosystem becomes overloaded and eventually some species will disappear through the process of competitive exclusion. Although this process takes place naturally, it generally does so when an organism moves into a nearby habitat, where its natural predators are already present or are able to move in with them. Contrary to the natural process, many exotics, especially those that come from great distances, often find that their new homes are free of their predators, thereby giving them a comparative advantage over natives, which can lead to the competitive exclusion of natives by exotics. For species with extensive ranges over large, continental areas, it is likely that rather than vanishing altogether, the species will disappear from the invaded region only. The elimination of a species from a portion of its range is referred to as a local extinction or an extirpation. The presence of exotic species within the ranges of geographically limited taxa, however, may result in the total extinction of one or more species. People tend to understand the gravity of complete extinction, but extirpations tend to occur almost invisibly. As long as a species endures somewhere, lost populations appear irrelevant. This is unfortunate, because when a sub-population of a species is lost, genetic material, potentially important to the long-term viability of the species and evolutionary potential, is lost. Because the time needed for ecosystems to adjust to new arrivals may be long (decades to centuries) and can vary significantly from place to place and by the species involved (Ruesink et al. 1995), the potential problems posed by naturalized exotics may not appear for several human generations or longer, leading to the perception that naturalized exotics are benign in the environment and that they actually increase species diversity. Over time, however, opportunities for presently non-invasive species to become invasive arise with ecological changes. The new arrival could eventually fail, but if it succeeds, it will do so at the expense of one or more of the existing species. Furthermore, because complex symbiotic relationships characterize ecosystems, organisms in addition to those that are directly displaced are affected. For example, predation rates among robins (Turdus migratorius) are higher when they nest in exotic amur honeysuckles (Lonicera maackii) and buckthorns (Rhamnus cathartica), as opposed to when they nest in native hawthorns (Crataegus spp.) and viburnums (Viburnum spp.) (Schmidt and Whelan 1999). Degraded and fragmented ecosystems are more vulnerable to invasions than are more functional and more intact ecosystems for several reasons. First, as natural areas are reduced and fragmented the total length of their edges in relation to their area increases, producing the "edge effect." These edges are not merely lines, but rather bands that have an area that becomes ecologically different from their interiors. Edges are also the entry zones for exotics to become naturalized (Cadenasso and Pickett 2001). Second, many natural area fragments are ecologically rich protected areas, and it is precisely in ecologically rich habitats that some exotics thrive (Stohlgren et al. 1999). The same ecological qualities � abundant nutrients, water, etc. � that favor richness, also are conducive to the proliferation of exotics. Third, isolated natural fragments behave like islands and we know that islands are more vulnerable to invasive organisms than are larger, intact continental areas (Simberloff 1995). 5. Are native landscapes messier and less visually appealing than conventional landscapes? The misconception that native landscaping is a call to let nature take over is unfortunately all too common. In fact, native landscaping does not in any way abandon aesthetics. Just about any landscape look can be replicated with native plants. Of course, native landscaping does call for the use of more ground covers and greater diversity in general. On a somewhat related subject, native landscaping does not ignore practical human needs like safety and recreation. 6. Is biodiversity increased by adding exotic species to the landscape? This question presents one of the most common misconceptions about biodiversity. It is fairly common for people to think that biodiversity simply means the number of species present in a particular place. If this were true, and if alien species were not capable of displacing natives or affecting ecological functions, then the addition of exotics to the local plant community would indeed increase biodiversity. But biodiversity is more than the sum total of species, and is often discussed in terms of: 1) species diversity, 2) genetic diversity, and 3) ecological diversity. Species diversity can be further explained as species richness and evenness. Richness is simply the total number of species in a given area. Evenness is a measure of how well represented each species is. As an example, suppose that two similar areas contain 1,000 individuals of trees from the same 10 species. If in area one each species had 100 individuals, then evenness would be high. If in area two, nine of the species had only ten individuals each, and the tenth species had 910 individuals, then evenness would be low, even though richness would be the same. While it may seem somewhat mundane to discuss these distinctions, it is often important to conservation decision-making. Suppose for example that the 910 individuals from the second area were from a species with great abundance outside the area, and that some of the remaining species were rare. If decision makers had to decide which area to preserve and which area to develop, then it would be better, all other things being equal, to preserve the area with high evenness, because it has more individuals of the rare species. Genetic diversity is a measure of the genetic variation that exists among the species present. When exotics become established, even if natives are not extirpated, native genes are lost as individual natives give way to individual exotics. If the lost natives are from a subpopulation with evolutionarily significant or economically important traits, then much would be lost. Thus, native genetic diversity is reduced even if native species are not entirely extinguished. Ecological diversity can be defined as the variation of the functional and structural components of ecosystems. Functional components include such things as symbiotic relationships (e.g., pollination, seed dispersion, predation, and parasitism), evolutionary processes, energy flow, biogeochemical and water cycling, photosynthesis, and ecological succession. Structural components include niches, geomorphology and topography, soils, biomass, and the vertical and horizontal structure of the flora. 7. Can I use a plant from a nearby habitat, even if it does not occur in my area and the habitats are different? Because the habitats are close by, you can assume one of two possibilities: 1) nature has introduced the plant to your region, but the plant failed to establish, or 2) your region is in one of the "holes" in the plant�s range. You are, therefore, probably not at any great risk of introducing a potentially disruptive plant to the surrounding natural areas. By way of example, I used the firebush (Hamelia patens), at the Stetson University site and elsewhere in the city of DeLand in western Volusia County, Florida. The firebush naturally occurs in warm, humid areas of the Americas from Paraguay and Bolivia to Mexico, the Antilles, and Florida. Volusia County lies at the northern end of the plant�s native range. Within the county, the firebush is restricted to the Atlantic coastal plant community. Presumably, winters are too cold for the plant to survive north of Volusia County and inland from the coast within the county where winter low temperatures are generally lower than they are along the coast. Given the plant�s natural distribution, it is reasonable to assume that nature has introduced the plant into western Volusia County, but because of the plant�s inability to survive cold winter temperatures, it has failed to establish away from the coast. With these ideas in mind, I used the firebush in DeLand understanding that it would probably "die back" in the winter and believing that it would not invade surrounding natural areas. As a point of interest, most of the firebushes I used in DeLand have indeed died back during most winters, only to re-sprout in the spring. 8. Will a native landscape help conserve water? Yes, if one selects the right kinds of plants for the particular sites. Of Course, exotic plants are also available that can help reduce water needs, so it is important to understand that the case is not being made that natives are necessarily better than exotics on this point. It is also important to point out that if you plant many plants, regardless of whether they are natives or not, on a site that is not currently being irrigated, then irrigation requirements will increase during the establishment period. Such was the case with the Native Landscape Project at Stetson University. During the period between August 1992 and April 1998, the site was not irrigated. When we undertook the first major planting campaign for the project in April 1998, we had to begin irrigating. At that time, only city tap water was available at the site, which of course led to a relatively large water bill during the months until reclaimed water was brought to the site. Now that the plants are established, irrigation is no longer needed. 9. What should I do with the exotics I remove? This is a tough one, because I hate to see plants destroyed. But, given the mission of native landscaping, it may be necessary to sacrifice some plants. That said, as I have gone through the process of removing exotic species from native landscape projects, I have had to deal with not only my concerns for the individual exotic plants, but also the concerns of other community members. Given the concerns, I sometimes offer non-naturalized exotic specimens to the community. Some of the plants we removed from the Stetson University project, mostly sterile, hybrid azaleas were indeed relocated to sites off campus. 10. Will a native landscape attract pests? Although claims have been made that native landscapes attract unwanted pests, evidence supporting such claims is lacking. To be sure, landscapes, whether they consist of natives or not, that provide more of what animals need to survive will attract more wildlife than landscapes that lack food, cover, and water. Similarly, landscapes, whether native or exotic, that are treated with pesticides will tend to have fewer pests than untreated landscapes. 11. Is a native landscape the same as a xeriscape? Not necessarily, though perhaps. The term "xeriscape" was coined in the western United States and applies to landscaping designed to conserve water. The root of the term, "xeri," comes from the Greek word xeros, which means "dry." In ecology, "xeric" is applied to dry conditions, which derive from low precipitation amounts, as in arid environments, or from soils with extremely low moisture retention properties especially in areas with deep water tables. In the plant world, a xerophyte is a plant that is adapted to dry conditions, a hygrophyte is adapted to wet conditions, and a mesophyte is adapted to conditions in between. In arid regions xeriscaping depends largely, though not exclusively, on the use of xerophytes. Because the xeriscaping movement has pushed way beyond its arid origins, its proponents no longer focus exclusively on xerophytes, but rather on plants that will not require large volumes of supplemental irrigation once established. The term "water-wise landscaping" is used by some synonymously with xeriscaping. Because the goal of xeriscaping (and water-wise landscaping) is to conserve water, one can plant natives or exotics as long as water use is minimized. The goal of native landscaping, on the other hand, is holistic. If properly established, it requires no more water than nature provides once the plants are established, and so meets the requirements of a xeriscape or a water-wise landscape. Native landscaping goes further, however, in that it seeks to contribute to biodiversity conservation, fostering an ecologically based sense-of-place, and eliminate fossil fuel and toxic and synthetic chemical inputs as well. 12. Is it wise to avoid some native species? Yes. Most native species can be incorporated into one design scheme or another, depending on site characteristics and the specific goals of the design. That said, it should be noted that some species present obvious hazards, some are less aesthetically pleasing than others, some are so prolific that they are hard to control, some may be difficult to cultivate, and some may be appropriate in some places, but inappropriate in others. For example, one should not plant trees with aggressive surface roots near dwellings, concrete surfaces, or underground utility lines (e.g., electric, gas, or water). Native weeds can be unsightly and difficult to contain. The native landscape project at Stetson University again serves as an example. I planted several pignut hickories (Carya glagra) on the site, but did not plant them close to roads or parking areas, because of the potential problems posed by their large nuts. I also kept them a good distance from the buildings because they can become large trees with heavy limbs that can cause property damage if and when they fall. I also planted slash pines (Pinus elliottii) and longleaf pines (Pinus palustris), but did not plant them near parking areas because of their sticky exudates, which can harm vehicle finishes. 13. Can a native species be invasive?
Yes,
though not in the same way as an exotic. By way of example, consider an abandoned farm field that is left to nature. Once
cultivation ceases and the field is left to natural processes, plants migrate into the field
from the surrounding woodlots and fields in a process known as
ecological succession. The first plants to
establish generally are pioneer species, or early successionals. Such
plants typically reproduce prolifically and are often short lived. While these
plants may be described as invasive in the sense that they invade
fields, and they may be
problematic in places where succession is not allowed to proceed, they tend to lose out
to climax species where succession is permitted to proceed to the
climax state. Thus an invasive native plant species may be problematic for
land managers
14. Can I use wetland species in my native landscape even though my site is not wet? If you are like most urban residents, you do not have a wetland habitat on your lot, but that does not necessarily bar you from using wetland species. This is especially true if you live in a humid area where the water table is relatively close to the surface. Many plants that are restricted to wet areas in nature will survive if planted on upland sites. Such is the case with many trees and shrubs in Florida, and indeed in much of the coastal plain of the eastern U.S. The red maple (Acer rubrum) and bald cypress (Taxodium distichum), for example, are naturally restricted to wetland environments, but both will establish on upland sites. Some wetland species, however, will not survive in upland sites unless they are irrigated throughout their lives, especially during dry times. This is probably because their root systems will not penetrate deep enough regardless of how they are cared for. Such plants may be inappropriate for urban sites anyway for two reasons. First, their shallow root systems often produce large surface roots that can be troublesome for foundations and pavement. Second, trees with shallow roots can topple more easily from wind than trees with deeper roots when they are planted away from the windbreak of the forest. 15. Will I have to amend the soils to support native plants? Maybe, but it depends on your soil and the kinds of plants you use. If your site has soils typical of your area, then you should be able to find plenty of plants with wide enough environmental tolerances to do well on your site without soil amendments. Of course, there are plants with very specific requirements that would need soil amendments, if your soil could not support them. Many species of Ericaceae (the heath family) in Florida, including blueberries (Vaccinium spp.) and azaleas (Rhododendron spp.) require acidic soils. Obviously, if your site has alkaline soils, then you would have to amend them for some Ericaceae species to thrive. The soils in urban areas are often very different from what they were prior to development. Oftentimes they are compacted, leached of nutrients, contaminated, and under altered hydrological conditions. All of this is to say that what once grew there may not now. This does not mean that natives are inappropriate, however. 16. Is native landscaping the same as ecological restoration? No, though it is closer to ecological restoration than conventional landscaping. The goal of many ecological restoration projects is to return degraded habitats to their natural states, which often, though not always, means to a climax state, wher "climax state" is the theoretical final or "mature" phase of ecological succession and consists of the biotic community that would come to dominate the area if given enough time without major disturbances. In some areas, climax communities can be dominated by only a few species. It must be understood, however, that an ecoregion is not necessarily a biotic community in its climax state, but rather a mosaic of natural land covers in various stages of ecological succession. To include only members of the climax community would exclude taxa that may only be present in earlier successional stages. Because we manage ecological succession in urban landscapes, there is no reason why you cannot, or should not include taxa from all successional stages. Moreover, because the hydrologic conditions of many urban areas are very different from what they were before development, it would be very difficult, if not impossible to replicate pre-development conditions. That said, regardless of the current hydrologic and edaphic conditions, regional native plants adapted to the new conditions exist and could be planted. 17. Is it okay to include exotic species? Only you can decide the degree to which you will "go native" in your landscape. Native landscaping can be viewed as part of a holistic, sustainable approach to managing urban environments. The objectives of native landscaping are to maximize landscaping�s contributions to biodiversity conservation, while minimizing unnecessary inputs of water, fossil fuels, and inorganic chemicals. It is also a way for people to express their appreciation for, and understanding of the natural heritage of the place in which they live, and to thereby cultivate an ecologically based sense-of-place. The current dominant landscaping paradigm essentially makes no meaningful distinction between exotics, and natives. It would be nice if native landscapes became the norm and exotic gardens became the exception. That said, you might find it difficult to avoid using some exotics for a few of reasons. First, if you plant a vegetable garden, you most certainly will cultivate non-native plants, regardless of where you live. Consider some of the common garden-variety food plants including tomatoes (from South America), lettuce (from West Asia), cabbage (from Eurasia), and watermelons (from Africa). Most of these types of plants have little chance of escaping cultivation, however, as they are domesticated taxa. Second, if your landscape plan includes turf grass, you will likely wind up with an exotic grass, because the turf grass industry depends on relatively few grasses. If you are going to have a turf area, select a grass that is not likely to escape cultivation. Your local agricultural extension office can help with the decisions. If you are going to use exotic shrubs and trees, then select horticultural varieties that do not produce viable seeds. Third, it may be appropriate, especially in public places, to maintain exotics that were important elements of the place�s cultural heritage or those that serve for applied education. Fourth, certain desirable natural functional ecological elements that were lost due to development may only be replaced through the use of an exotic species. 18. Should I be concerned about the source of the plant material I use?
Yes, for at least four reasons. First, you do not want plants that have been taken from the wild, unless you know for sure that they were done so legally and in a way that does not harm the wild communities.
Second, mix it up genetically. If you are buying more than one individual of the same species, try to get individuals that came from different parent plants. This might help ensure that if disease strikes, not all individuals will be affected.
Third, buy local stock if you can. By this I don�t just mean buy locally grown plants, but rather buy plants that came from the native gene pool. In other words, if you live in Florida and are buying live oaks (Quercus virginiana), make sure that the seed source is traceable to native Florida trees and not trees from another state. If you live at the edge of a plant�s native range it may be especially important to obtain individuals that come from your area. The redbud (Cercis canadensis) and dogwood (Cornus florida) in central Florida illustrate this point well. Both trees exhibit considerable genetic variation across their extensive ranges, both are widely used in landscaping, and horticultural varieties of both exist. Unfortunately, I believe, in the process of producing the most desirable cultivars, the nursery trade has introduced varieties from the middle part of the plants� ranges to central Florida that, while nice elsewhere, are inappropriate for central Florida. As a result, many of the plants do not perform well in the region, and the perception now is that the species are inappropriate. I have, however, seen many beautiful individuals of both species. It is my belief that if the industry recognized that regionally appropriate varieties of the two species existed, that we would see quality plant material in the region. Fourth, buy organically grown material if you can. Because native landscaping calls for more responsible land management in general, it makes sense to buy the most environmentally friendly plant material available. Although it is generally difficult to find organically grown landscape plants, do try. The system will not change without demand. 19. Are native plants more difficult to establish and maintain than exotic plants? The simple answer is no, but of course it all depends on the specifics. There is no research to my knowledge that indicates that native plants are generally more difficult to cultivate than exotics. Of course, many exotics are popular for the simple reason that they are easy to propagate and maintain, but the same is true of many native plants as well, and many temperamental exotics are used regularly. It is important, however, to point out that the assumption underlying a native-dominated sustainable landscape is not that natives are necessarily superior to exotics because they are better adapted. There is little doubt that in many instances exotics may out-perform natives for a host of reasons relating especially to edaphic (or soil) conditions, water availability, resistance to pests and disease, and growth rates. But, to address sustainability adequately, one must look beyond plant performance. If decisions were made merely on plant performance, then we could envision a day when a homogeneous palette of genetically modified plants would come to dominate urban landscapes. 20. Are native plants more difficult and costly to acquire than exotic plants? The answer to this question depends in part on the specific plants in question and where you reside. Some states have more native nurseries than others and some produce a wider variety of native plants than others. Florida has a well developed network of nurseries (see Association of Florida Native Nurseries) that specialize in native plants and many of the nurseries that offer exotic species also offer natives. In Florida, the live oak (Quercus virginiana) is a popular and readily available native tree that can be found at native nurseries as well as nurseries that offer many exotics. Notes
1 Of course not all ecosystems are packed and there is a degree of elasticity in ecosystems that varies with the species involved and the ecological conditions.
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