the case for native plants
by Dan Walton
As the use of native plants in urban landscaping has increased, a number of articles have been written questioning their value. A few of the articles have appeared in trade journals primarily for growers of non-natives. Others have appeared in more general publications. No one seems to question the value of natives for restoration of landscapes that have been badly damaged by industrial activities, such as phosphate mining. The questions instead revolve around their advantages for use in urban and suburban landscapes, and whether the claims that have been made for their value are valid. Particularly questioned are the suggestions that natives, because they have evolved in their natural sites, require fewer inputs of water, fertilizer and pesticides. As an owner of a native plant nursery my bias is certainly toward natives, but I think it is useful to see whether more modesty about our claims for natives is warranted.
One of the first questions to be answered is what do we mean by a native plant. In Florida we mean plants which were here prior to the arrival of Europeans. The question then becomes where is here? The boundaries of any state, county, or municipality are human constructs. Since that is the case, can any plants grown outside of their current natural areas truly be called native? Is silver saw palmetto really a native plant in Sarasota because it occurs naturally on the southeast coast of Florida? Are trees that are found in North Florida to be considered natives in more southerly regions of the state? Plants grown in different parts of the state clearly differ in morphological characteristics. Do they also differ in their physiological characteristics sufficiently to reduce their survival when they are grown elsewhere? One of the criticisms levied against native plants is that in some instances their native status is more apparent than real. The question becomes how far can they be removed from their native habitat and still be considered natives. This is a contentious question even among native plant enthusiasts.
A major advantage claimed by native plant afficionados is that native plants have evolved to be able to thrive in their present natural locations. One of the arguments used against this assertion, however, is that urban and suburban backyards and streetscapes are a far cry from the woods, fields, wetlands, and beaches that are natural settings for native plants. Can one expect a native plant to necessarily thrive in a yard that was filled and may contain construction rubble buried around the house? Furthermore, these areas will also differ in their microclimates from the native habitats. In fact, we have very little data about the survivability and longevity of natives under such conditions except perhaps as anecdotal evidence. We can say, however, that in Florida plants have evolved under conditions that are often stressful such as high temperatures, high humidity, drought, flooding, and nutrient poor soils. In some instances such as on coastal sites, very few non-natives can survive because of their lack of salt tolerance. It may well be that because of these types of stresses, natives are better able to withstand a wider range of environmental conditions than plants that have not evolved under such conditions. Without data, however, perhaps we should be more modest in our claims.
Another advantage touted for native plants is that their evolution has involved exposure to insect pests and fungi that inhabit the region in which they occur, and that they have had to develop defenses enabling them to survive. Non-natives that have not had the benefit of such an evolutionary history are more likely to fall prey to such insects and diseases. The latter's survival, therefore, will require applications of pesticides and fungicides. The coevolution of plants and animals means that a dynamic equilibrium has been reached between the two but clearly the animals must eat to survive. Although a population of plants may survive in the presence of various predators, this does not mean that individual plants won't fall prey. In addition, there are very few plants that will be completely resistant so they may sustain some damage although not fatal. It is possible that natives grown under non-natural conditions, and therefore stressed, lose some of their natural resistance. It is also true that insects have evolved mechanisms to key in on chemical cues from their host plants, and often non-natives may be less prone to attack since they do not emit these cues. Indeed, one of the advantages of natives is that they are recognized by a variety of fauna such as birds, butterflies and other insects that do not recognize exotics.
The fact that we have exotic pests such as Brazilian peppers, melaleuca, Australian pine and carrotwood as well as an ever increasing list, is testimony to the lack of serious pests in their new locations. Conversely, it is also true that in a time of great mobility in which we live, the importation of foreign pests occurs regularly. That has occasionally led to disastrous results for natives: e.g., Dutch elm disease and the chestnut blight. The dogwoods in the southeastern U.S are being decimated by a foreign fungus and the oaks in California are in similar danger. It may well be that on balance natives do present a better naturally protected group of plants than those which are not native to an area. It may be our mind-sets that need changing. It is really not necessary to try to kill all of the pests that attack plants. Do we really need perfect plants in our yards?
Another assertion made about natives is that they require less fertilizer, if any, compared with non-native plants and therefore groundwater supplies as well as surface water run-off will be less contaminated by excess nitrogen and phosphorus. In most cases, however, there is little evidence that plants, native or non-native, won't benefit from fertilizer applications if one is interested in flower or fruit production or rapid biomass accumulation. The major difference is that in most cases native plants are not installed for any of the above reasons while non-natives frequently are. In fact, it seems likely that the major culprits in such excess fertilizer applications are lawns, and that substitution of shrubs and trees, whether native or non-native, would be advantageous. Perhaps it is not so much that natives require fewer inputs than non-natives, but that growers of the latter are better able to accept a less manicured and lush appearance of plants that are not subjected to continual pesticide and fertilizer application. Again, it may be the mind-set and not the plants that are most in need of changing.
Often asserted by native plant aficionados is that native plants require little or no watering. Firstly, one must acknowledge that natives most certainly need water for some time after planting. How much will depend on the species, its size, the time of year and the type of environment in which they are planted. Secondly, it is also clear that natives, as is the case for other plants, differ in their abilities to extract water from droughty environments and to survive under drought conditions. Merely planting natives will not necessarily absolve the homeowner from watering. As the old saying goes, the proper plant in the proper place is also necessary for natives. Members of the public unfamiliar with natives sometimes have the erroneous impression that xeriscaping and natives are synonymous. As is the case with pesticides and fertilizer, the major users of water in urban areas are lawns.
If reduced requirements for water, pesticides and fertilizers are attributes that are at least questionable, are there claims for native plant use that do not seem so? One claim for which there is good supporting evidence is that natives are important in maintaining populations of many types of native fauna including birds, butterflies, and a wide variety of insect pollinators. In Britain it has been found that native trees such as oak and hawthorn support several hundred invertebrates. The horse chestnut, widely planted in Britain, supports only 4 invertebrates in Britain but more than 100 in its native Mediterranean region. These invertebrates are food sources for birds and other wildlife and the replacement of native species with introduced ones can disrupt many food chains. Native wild flowers are often the food sources for butterfly larva as well as nectar sources for butterflies and many pollinators. As more land is developed, urban and suburban areas become more important for wildlife.
A second important role for native plants in urban areas is aesthetic and more subjective. We obtain our sense of place in many instances from the flora without even knowing the identity of the plants. One of the ways that we know we are in Florida and not in Ohio, other than the absence of winter snow, is the presence of massive live oaks and stately cabbage palms as well as other less well-known plants. Non-native plants have their place in our urban spaces, but surely the almost total replacement of our native flora with exotics is comparable to replacing all of our native birds with species of parrots because we enjoy their colorful plumage.
In summary, I believe that native plants have an important role to play in the urban and suburban environments. I have written this to suggest that we be a bit more modest in our claims about natives, not because many of these claims are necessarily wrong, but because in most cases we do not have sufficient evidence based on research to support them.