Monthly Archives: December 2021

Q/As About Native and Non-Native Species

Lesser celandine may look pretty, but this aggressive invasive species can overwhelm native plants in Virginia.

Many environmental articles refer to either native or non-native species. Some folks have questions about exactly what the term “native” means, what it encompasses, and how it applies to neighborhood landscaping and managing natural areas such as parks. Some answers follow:

Q: What is a native species?

A: A native organism is one that evolved and naturally migrated to its current range and is biologically recognized by other species in the region. For example, the ancestor of the iconic American bison originated in Asia. Through natural circumstances, the ancestral bison crossed the land bridge into North America around 200,000 years ago and continued traversing between Asia and North America. These early bison spread across much of the continent (some maps include Northern Virginia in their range), evolved into new species, and coexisted with some organisms. Other species adapted to the new animals and became part of diverse ecosystems. In this dynamic process, new species arise as others move out or face extinction at the same rate. Another example, the horse, has ancestors that originated in North America and migrated to Eurasia.

A species’ natural range depends on factors such as geology, climate, altitude, and the other organisms present. Political borders rarely define a species’ range but do make it easy to explain that range. For example, the Colorado blue spruce (Picea pungens) is “native” to the U.S., but its natural range is only areas in the Rocky Mountains. When planted in the Washington metro area, this tree deteriorates in the hot, humid summers and clay soil, surviving for several years to a few decades. Wild specimens can thrive for centuries in their natural habitat. Describing a species’ native range at the county level is a more meaningful measurement. Two web sites with county-level plant ranges are and

Q: A nursery worker told me that liriope and periwinkle naturalize. Do “naturalize” and “native” mean the same thing?

A: No. Naturalize is a term for introduced plants that can reproduce without human support. Periwinkle (Vinca minor and V. major) and liriope (especially Liriope spicataand L. muscari) are introduced weeds that invade woodlands and other natural areas. Few, if any, native species benefit from them.

Q: The bison came from another part of the world and spread, but that was OK. What’s wrong with the plants and animals designated as “non-native invasive?”

A: The key is that the new bison species spread over thousands of years, which happens through natural evolutionary and dispersion processes. Imagine a jet liner cruising at 600 mph. To smoothly and safely land, that jet needs 25 minutes; the less amount of time, the harder the landing. If that speed and altitude change happens in a split second, it means that the plane crashes. In biological communities, a rapid change sometimes results in more species dying off than others evolving to fill the vacant niches.

People fanned the proverbial flames of this distribution, especially through mail order catalogs and other trade. Infected stock spread disease such as CBF. More invasive plant sales meant more places the plants could escape and spread through seeds, runners, or other reproductive means. If native flora and fauna are unable to control the newcomer, the result is an unchallenged population explosion that alters and may degrade the natural environment.

A big problem with non-native invasive species like this Bradford pear is its seeds, which spread, germinate, and grow uncontrollably outside of cultivation. Any birds capable of eating the small fruits dotting the branches will disperse the seeds in their drop-pings. Each fruit typically contains two to four viable seeds (inset).

Q: But what you call a “non-native invasive” plant is beautiful. What’s the harm in allowing it to survive, evolve, and add to the biodiversity within the parks?

A: There are many native — often more beautiful — alternatives for each invasive plant. Remember, artificially broadcasting exotic species into an established, balanced, natural ecosystem is not evolution or a case of survival of the fittest. Instead of increasing biodiversity, it can cause severe imbalances. In the invasive plant scenario, animals relying on native species cannot use these new invaders. For example, let’s say that a person is given a salad filled with tomatoes, lettuce, peppers, cheese, hard-boiled egg, and anything else they like that provides all the nutrient and calorie requirements for a day. Each day, another of the same salad is served. Now, let’s say that an inedible oak leaf is substituted for one of the components — maybe a lettuce leaf. After removing that oak leaf from the salad, the rest is devoured. Each subsequent day, additional oak leaves replace more of the salad until one day the edible food drops so low that the person now starves. This same analogy occurs as non-native invasive plants displace native flora. The wildlife dependent upon those native plants either languish or leave.

Q: Many of the plants that are called “invasive species” are sold in garden centers. They have value. People pay money for them. How can they be all that bad?

A: Just because something is legal to sell and use does not make it right or mean that it is good. An example is tobacco. Consuming these products, as directed by the manufacturer, can create severe problems for the user as evidenced by warnings on the packaging. Too many times, private citizens to professional landscapers gravitate to certain non-native invasive species whenever their project needs a cheap, quick fix. Such actions lack comprehensive, long-term consideration of these plants. The weeds’ abundance degrades the health of natural areas. If family, friends, or neighbors request a non-native invasive plant, instead of bowing to peer pressure, do the right thing and, “Just say no!”

By refusing a targeted weed and choosing a good native alternative, nurseries see the market trend for fewer exotic plants and more natives. Furthermore, you will help create a healthier environment for your children and generations to come.

Both of these images portray woodlands during a season when the dominant plant species are evident. The late spring and summer find native woodlands fully leafed (A). This botanical diversity includes lycopodium, sassafras, grape, mapleleaf and arrowwood viburnums, trefoil, red oak and hickory saplings, red maple seedlings, and northern highbush blueberry. In late autumn, many non-native invasive plants still cling to their leaves, making them apparent (B) when most natives are dormant. This image shows low biodiversity in an area overrun by autumn olive, Amur honeysuckle, and winter creeper groundcover.

Q: What can I do to help the natural areas?

A: Join an Invasive Management Area (IMA) workday, where volunteers eradicate non-native invasive weeds from parkland and restore the habitat. Check out the IMA workday calendar. Removing any target weeds from your home lot prevents them from spreading. When leaving the leaves is not possible, properly dispose of yard debris through curbside recycling or composting on your property. Invasive plant seeds or vines capable of surviving the mulching process should be thrown in the trash destined for the landfill or incinerator. Never dump yard debris or otherwise encroach onto natural areas outside of your property. Consider adding native plants to your yard, and encourage others to do the same, including neighbors, churches, schools, and HOAs. Before doing any work, always seek the landowner’s permission first. Please help park volunteers by picking up after your dog each time, every time, and toss the waste in proper trash receptacles.

Q: After these IMA workdays, the area looks devastated. Bushes where birds nested and privacy screens are gone. How can this be better than simply leaving the invasive plants?

A: The level of disturbance depends on factors such as the target species and its intensity. For example, garlic mustard pulls have little effect on the untargeted surroundings. Like surgeons operating on a patient to remove a tumor, IMA volunteers only hit specific, non-native invasive weeds, leaving native plants to help an area heal and recover. If the work results in pulling every plant on a plot, then the infestation was dense.

There may be more going on than you see. Invasive bushes may hold bird nests but not host the insects that the chicks need to grow. By removing the weeds today, we both prevent them from imminent dispersal and foster the native plants conducive to future bird populations.  People interested in all-season screens might consider planting fast-growing red cedars (Juniperus virginiana) on their side of the fence line. These junipers (not true cedars) are tall, bushy, and evergreen. Other excellent screening species are American holly (Ilex opaca), mountain laurel (Kalmia latifolia), sheep laurel (K. angustifolia), and American rhododendrons (Rhododendron maximum), although these species grow slower than red cedars.

Q: Hostas and crepe myrtles are from Asia, but I don’t see them spreading. Are they OK to keep in my yard?

A: Many exotic species do not escape cultivation and are considered safe to grow. Some species, like hybrid tea roses (Rosa x odorata), struggle to survive in the Mid-Atlantic and southeastern climates. A handful of non-native species, such as Callery pears and Japanese honeysuckle (Lonicera japonica), were initially thought to be infertile or slow to reproduce, but decades later these plants proved to be invasive. Ecologically, hostas (Hosta sp.) and crepe myrtles (Lagerstroemia indica) are like plastic plants because they contribute little to the ecosystem, but they can help with erosion control, provide shelter, and supply added benefits, such as air purification. Hostas, for example, directly benefit only a handful of animals, such as deer, bumblebees, slugs (most of which are themselves European invaders), and the occasional hummingbird.

Q: Slugs are invasive? I thought that they were natural garden pests.

A: Some slugs are invasive. The most obvious example is the giant leopard slug (Limax maximus), which originated from Europe and North Africa and reaches four inches or more. Many of the exotic slugs have short mantles ( the shield starting on top of its head and stretching down the body) extending less than halfway down the specimen. Native slug mantles, such as those belonging to the Philomycidae family, cover the whole body. Philomycid slugs are native in other parts of the world, but those species are not a problem in Northern Virginia.

Q: Are some species native to Fairfax County invasive in other parts of the world?

A: Yes. Goldenrod (Solidago sp.) escaped cultivation in Europe and Asia. In China alone, Canadian goldenrod (S. canadensis) is blamed for dozens of native plant extinctions. In America, black cherry trees (Prunus serotina) are kept in check by potent strains of soil-borne, pathogenic Pythium fungi that cause root rot and seedling mortality. These trees are weeds in Europe due to the weaker Pythium varieties and favorable growing conditions there. The large, aggressive American bullfrog (Lithobates catesbeianus) devours just about any animal that fits in its mouth. From its native range, the eastern half of North America, bullfrogs have spread to the Pacific coast and to parts of Europe, Asia, and South America. Extending into the Chesapeake Bay, ship ballast water carried comb jellies (including Mnemiopsis leidyi) to the Black Sea and other marine bodies. Unchallenged in their new home, comb jellies eat newly hatched fish and planktonic fish food, resulting in significant fish population reductions. None of these species are “bad” or “evil” but are organisms that cause problems when out of place.

Author Greg Sykes is a biologist whose academic and professional careers include evolution, the environment and genetics. His current work involves DNA-based species identification and deducing how closely related organisms are based on genetic similarities. Sykes is a volunteer site leader with Fairfax County Park Authority’s IMA program.