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Native, Alien and Invasive Plants

I always advise my customers to use “native” plants in their landscapes.  Nothing makes me crazier than when a customer wants to use some palm tree native to Borneo in their landscape here in DC, MD or VA.  Although I’m writing with my horticulture hat on right now (versus my turfgrass hat), this concept goes back to what I’ve said all along.  You must choose the correct type or species of turfgrass based on the region in which you live.  But, with larger shrubs and trees, that gets a little more complex.  If you want mission success in your landscape, you need to know a little bit about where you live – what are the conditions; the soil, the amount of sun, the weather, moisture…all sorts of considerations.
In Virginia, we make it easy.  You live in one of three regions – the coastal plain, the Piedmont or the mountains.  Each one of these areas has specific considerations and therefore has specific plants that will do well in each of those regions.
Why use native plants?  What’s the big deal?  Well, in this blog post I’m going to discuss some of the issues surrounding native, alien and invasive plants.
Native type plants have evolved within a specific area and are dispersed throughout that area without human intervention.  They could be called “indigenous” to a geographic area.  Native plants form the primary component of the living landscape and provide food and shelter for native animal species.  Native plants co-evolved with native animals over many thousands to millions of years and have formed complex and interdependent relationships.  Our native fauna depend on native flora to provide food and cover.  Many animals require specific plants for their survival.
How long must a plant species be located in an area in order for us to consider it native to that place - two hundred years, since colonization, since before agriculture began?  For example, for those who consider two hundred years sufficient, Queen Anne's lace would be native - yet we know from historical records that this plant has a European origin.  We usually depend on local plants, the uncultivated plant life of a given region, to tell us which plants are native.  However, these inventories are sometimes inconsistent and are subject to repeated debate.  I guess only fossil records can really prove that a plant evolved in a particular place, but even fossils can be misinterpreted.
If we randomly pick a spot in time and say "plants here before this date are native," we may not be acknowledging that for centuries, indigenous peoples, traders, explorers, and botanists have had an impact on local floras with their activities.  Geopolitical and environmental boundaries also play a role in defining native plants.  To say a plant is "native to North America" or "native to Virginia" implies that it is suitable for growth throughout North America or Virginia, when in fact it may only occur naturally in limited microclimates or sections and thus only be suitable for growth in equally limited scenery situations. 
Using native plants in your landscape means using plants that are better suited for that region.  Native plants use less water, require less fertilizer and less pesticides.  As I alluded to earlier, native plants also increase the presence of desirable wildlife.
Ecotypes (sometimes called ecospecies) describe a genetically distinct geographic variety, population or race within a species, which is adapted to specific environmental conditions.  Typically, ecotypes exhibit phenotypic differences (such as in morphology or physiology) stemming from environmental heterogeneity and are capable of inter-breeding with other geographically adjacent ecotypes without loss of fertility or vigor.  Basically, ecotypes are the same species that are found in different habitats and have evolved specific adaptations to their differing environments – taxonomically equivalent to a “sub-species”.  Red maple, for example, is native from Florida to Canada, but some sub-species have adapted to dry or wet sites, cold or warm climates.  Although technically red maple is native to a large section of North America, one ecotype will not necessarily perform successfully in another site because it is not adapted to the site's conditions.  The larger the geographic range of the species, the more occasion there is for dissimilarity.  
Exotic plants, also known as non-native, introduced, “non-indigenous” or alien plants, are species that occur in cultivation or in the wild.  These plants may have been transported across boundaries by people and their activities.  Sometimes they are species introduced intentionally or accidentally, into a new region by humans.  Over time, many flora and animals have expanded their ranges slowly and without human assistance.  As people began cultivating plants, they brought beneficial and favored species along when they moved into new regions or traded with people in distant lands.  Humans became a new pathway, enabling many species to move into new locations.
According to The Flora of North America, one-fifth to one-third of the plant species encountered north of Mexico had their beginning in other continents.  Several exotic plant introductions, such as lily-of-the-valley, day lilies, and daffodils, have become naturalized, meaning that they have succeeded in reproducing and spreading to a restricted extent on their own.  Unlike invasive plants, most naturalized plants are not a severe threat to other species or to an ecosystem.  In fact, the ability to naturalize is often considered an advantageous feature in horticulture.  Wildflowers can be true natives or introduced plants that have been naturalized for over two hundred years.
A small percentage of naturalized exotic plants become invasive.  Invasive plants are those that reproduce quickly, displace many of the other species in their domain, and are difficult to eradicate.  Invasive plants are species that cause health, economic or ecological damage in their new range.  More than 30,000 species of plants have been introduced to the United States since the time of Christopher Columbus.  Most were introduced intentionally, and many provide great benefits to society as agricultural crops and landscape ornamentals.  Some were introduced accidentally, for example, in ship ballast, in packing material and as seed contaminants.  Of these introduced species, fewer than 3,000 have naturalized and become established in the United States outside cultivation.  Of the 3,500 plant species in Virginia, more than 800 have been introduced since the founding of Jamestown.  The Virginia Department of Conservation and Recreation currently lists more than 100 of these species as invasive.  In the United States, invasive species cause an estimated $120 billion in annual economic losses, including costs to manage their effects.  Annual costs and damages arising from invasive plants alone are estimated at $34 billion.  Purple loosestrife in the northern US and kudzu in the southern states are classic examples of invasive plants that profoundly affect the landscape.  In Fairfax County Virginia, bamboo is an extremely damaging and problematic invasive plant.  Invasiveness can range from the minor nuisance of garden plants, such as Lamb's ears, that tend to edge out their neighbors, to the other end of the spectrum, where the melaleuca tree from Australia is literally drying out Florida's marshes to meet its high water requirements.  Melaleuca is an example of an exotic species that is considered a serious invader because it enters disturbed lands, colonizes them and changes the ecology of them in detrimental ways.
One of the main reasons people promote native plants is to avoid the devastation that invasive plants may bring on landscapes and forests.  Exotic introductions that do become invasive, like kudzu, multi flora rose, and honeysuckle are a costly menace nearly impossible to control, much less eradicate.  Why do a small percentage of plants exhibit invasive tendencies, while the majority of plant introductions are benign or beneficial?  The answer lies in the combination of two factors: traits that invasive plant species share and traits of the site that make it susceptible to invasion.  No plant is inherently invasive under all circumstances.  Although more often it is exotic introductions that invade, native plants can also become invasive pests.  Native grape vines like fox grape form suffocating thickets over shrubs and rapidly climb trees, threatening to out-compete their hosts for light.  Though native to eastern and central North America, wild grape is an indisputable pest.  Other native plants that are often invasive include blackberries, poison ivy, wild onions and cattails.  Just as with exotic introductions, it is a small percentage of native species that cause problems.
Native plants have an important role to play in modern landscaping.  Arguments that are made in favor of native plants include lower maintenance, regional uniqueness, biological diversity, and wildlife habitat.  One theory holds that native plants are easier to care for because they have evolved in a place over many years, developing resistance to climatic extremes, insect feeding, disease pathogens and other stresses of the local environment for which non-native plants may not be prepared.  This may be true in some cases; however, it is important to note that native plants placed under stressful conditions fare no better than exotic ones if the plant is not carefully matched to the site.  Some exotic plants actually perform better and require less maintenance because of the qualities they were selected for, and because their insect predators and disease pathogens are frequently not imported with them. 
Another factor to consider is the interaction of native plants with the “built up”, urban or suburban non-native environment.  In an urban setting, for example, there is no planting site that approximates what would have been there prior to urbanization.  The original landscape in both cities and suburbs often has been changed so completely that the microclimate, soil type, soil hydrology, and insect populations no longer are what they were when the native plants of the area evolved.  To put a native tree, for example, in a median strip planting on a downtown street because it is native to the surrounding countryside would be foolish unless the tree is known to tolerate the heat of the asphalt, car exhaust, salt from the snowplows, a limited root zone, intermittent flooding, and periodic drought.
If native plants are used simply because they are native, without proper regard to site conditions, the results may be insufficient.  The most critical issue is not native plants versus exotic plants - it is appropriate versus inappropriate plant selection, given the constraints and opportunities of the site.  The more closely a plant's characteristics match the site's characteristics, the better chance for the plant’s survival and vigor.  If a native plant meets those requirements, by all means use that plant.
While we can and should strive to use plants long found in our region, perhaps the term native plants should be used with some modesty.  Using diverse plantings will create beauty and prevent the susceptibility to disease that can come from large scale single-species plantings, avoiding disasters such as that of the American elm in the mid-twentieth century.
In the quest for a diverse, healthy landscape, which may be a mix of native and exotic species, references are available both to help find the right plant and to avoid the trouble makers.  A little research before selecting plants can save time, money, and aggravation.  Or, you can consult with someone like me.  Reference guides may warn that a plant is invasive under certain circumstances, but they may not, and nursery catalogs frequently won't.  Phrases like "a very vigorous grower" can be euphemisms for potential invasiveness.  Don’t be fooled.  Treat such phrases as red flags.  Be sure to look in more than one plant reference to gather more than one perspective on any species you have in mind, especially if you suspect it may be invasive.  After invasive potential is ruled out, the physical limitations and possibilities of the site should be the first and most important consideration in the exciting process of selecting new plants for our landscapes.

2 Comments to Native, Alien and Invasive Plants:

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Robin Baxter on Monday, July 20, 2015 3:09 PM
Useful information!
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Jake on Friday, July 24, 2015 10:02 AM
This is a great article. I love the history and biology. This is my favorite one yet!
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