By Mary Free, Extension Master Gardener
Part 4. Residential Land Use: Invasive Species
Natural habitat loss results not only from human incursion but by the introduction—intentionally or accidentally—of invasive1 species. A 2019 United Nations (UN) report found that since 1970 there has been a 70% increase in the numbers of invasive species across 21 countries with detailed records.
What is the difference between a native, non-native, and invasive plant?
Native plants occur naturally in, and have adapted over time to, a given ecosystem or ecoregion, coevolving with other species in that system. Native plants support native pollinators and vice-versa.
Non-native plants occur outside of their native range. They may have been introduced to a region intentionally, accidentally, or by a shift in their hardiness zone ranges because of global warming. They, along with cultivar and hybrid plant species that have been manipulated by humans to enhance vigor or appearance, may affect the abundance of native plant species and the pollinators that rely on them. Although some non-native plants provide the quality pollen and nectar that pollinators need to thrive and reproduce, others can disrupt the ecosystem.
Some native plants can be aggressive, spreading quickly and squeezing out some preferred species, but that is the fault of gardeners not planting these natives in the right place or of disruptions to the ecosystem in which they naturally grow. Such aggressiveness is different from being invasive, a word applied to non-native organisms (also called aliens or exotic aliens) whose introduction causes or is likely to cause harm to the economy, environment, or human, animal or plant health. Invasive plants lack the natural controls of native plants, so they can outperform native plants by establishing easily, spreading rapidly, emerging earlier and lasting longer through the growing season, shading out competitors, producing bio-chemicals that negatively affect neighboring plants, or sometimes adapting more readily to changing climates. Sometimes the threat invasives pose is not readily apparent, as they “can stay dormant for decades or even centuries after they have been introduced into an environment before rapidly expanding and wreaking ecological havoc” (Dooley, 2024). Not all non-native plants are invasive—only about 6% of alien plants worldwide—but all invasive plants are non-native.
DID YOU KNOW?
You can volunteer to track changes in the timing of plant and animal seasonal activity through the Nature’s Notebook program. Observations by Nature’s Notebook citizen-scientists have led to important discoveries. For example, a study of invasive shrubs in eastern U.S. forests found that the leaves of invasive shrubs in the south opened earlier and lasted 77 days longer than surrounding native shrubs and tree canopy giving the invasives a competitive advantage to the detriment of native flora and fauna (Maynard-Bean et al, 2020).
Additionally, invasive plants are sometimes the preferred host species of invasive insects—invasive buddleia attracts the invasive brown marmorated stink bug and invasive tree-of-heaven is the most preferred host plant of invasive spotted lanternfly. In fact, spotted lanternflies, which sequester defensive chemicals when they feed on tree-of-heaven, were avoided more frequently by predators than the lanternflies that fed on maple, walnut, or other plants (Johnson et al., 2023).
Left to right: Non-native Pachysandra procumbens, native to the Allegheny Mountains, is an alternative to invasive Pachysandra terminalis, commonly planted in residential landscapes. Native Clematis virginiana spreads aggressively but it is non-native Clematis terniflora that is considered invasive.
DID YOU KNOW?
Some states, like Delaware, Indiana, Maine, Massachusetts, and Ohio, now ban the sale of some invasive plants, but in most states, many nurseries and big-box stores continue to sell invasive plants. Before you purchase plants for your property, check the list of invasive plants for your locality (or for the City of Alexandria or for Arlington County, Virginia).
Though these options have passed for 2024, you can plan to purchase native plants at the MGNV Spring Celebration and Plant Sale at the Glencarlyn Library Community Garden and at the BIG Plant Sale at Green Spring Gardens, both advertised on mgnv.org in the springtime. NOVA Natives also lists periodic native plant sales in Northern Virginia.
Purchase native wildflowers that have been propagated in reputable nurseries and not harvested from the wild. Look not only for a “native” label but for the species’ full scientific name to determine if it is native to your region or a cultivar or hybrid. Native plants whose seeds were collected in your eco-region are more likely to survive transplantation and thrive in your garden. Visit MGNV’s Tried and True Native Plant Selections for the Mid-Atlantic for fact sheets on native plants.
Although invasive plants can have a devastating impact on the native plant populations, their impact on native pollinators—visitations, nutritional value, harm—is not widely understood. Specialist pollinators that depend on one or limited plant taxa may be more affected if invasive plants displace or otherwise harm those native species. Research has found some examples (listed in the box below) where non-natives, cultivars, and invasive plants can adversely affect native pollinators. Invasive plants can have a devastating effect on other wildlife. For example, when invasive plants displace native fruits on which migratory birds rely, they will fill up on invasive fruits that may not provide the nutrition needed to complete their voyages south, so that they plummet to their death or get stranded well short of their destinations and die.
Left to right: Unlike the double-flowered Echinacea cultivar, which attracts few pollinators, Echinacea purpurea ‘Magnus’ ranked 9 out of 75 in pollinator visits in a Mt. Cuba Center trial. When a monarch butterfly confuses invasive Cynanchum rossicum for its host plant Asclepias incarnata, its caterpillars do not survive. Likewise, if a West Virginia white butterfly lays its eggs on invasive garlic mustard rather than one of its host plants, two-leaf toothwort, its caterpillars die.
DID YOU KNOW?
• For herbaceous plants, “the more manipulated the cultivars became, the less attractive they became to pollinators” (White 2013/2016). For example, double flowered cultivars have replaced the reproductive parts with petals so they no longer benefit pollinators.
• Plants “selected to change green leaves to red, blue, or purple, either throughout the season or during leaf senescence in the fall, significantly reduce insect herbivory three- to five-fold” (Baisden et al. 2018).
• In the Northeast, some monarch butterflies confuse two invasive swallow-worts in the milkweed family (Cynanchum louiseae and C. rossicum aka Vincetoxicum rossicum) as host plants and lay their eggs on the leaves. Their caterpillars die soon after hatching. (Fargione, 2015)
• Besides outcompeting native toothworts, host plants of the West Virginia white butterfly (Pieris virginiensis), the chemical makeup of invasive garlic mustard (Alliaria petiolata) is responsible for the failure of larvae hatched from P. virginiensis eggs regularly deposited on its leaves (Davis 2015).
Invasive species also include wildlife
Invasive insects, which often hitch a ride on imports, have had a devastating impact on native flora and fauna in the United States, where they lack natural predators, outcompete native species for food, and adapt better to climate change. Examples are the spongy (formerly gypsy) moth, which has defoliated more than 83 million acres of trees and shrubs in the U.S. since 1970 (it was discovered in Virginia in 1980); the emerald ash borer, spread primarily by the movement of infested firewood, which has killed tens of millions of native ash trees in the Mid-Atlantic and eastern Midwest (it also attacks native white fringetrees); the brown marmorated stink bug, which feeds on more than 170 plant species, including valuable agricultural crops, mostly concentrated in the Mid-Atlantic Region; and the spotted lanternfly, which damages fruit, ornamental, and hardwood trees and woody vines including grapes.
Left to right:: a female spongy moth and its caterpillar; brown marmorated stink bug eggs and 1st instars and a 3rd instar all pictured on invasive buddleia leaves; an adult emerald ash borer and its larvae tunneling in an ash tree; spotted lantern flies and their egg masses.
Pollinators seem to be more affected by invasive animals than invasive plants (Montero-Castaño & Vilà, 2012). For example, alien bees can compete with native bees over the same food resources exploiting those sources more quickly. They also can display opportunistic nesting behavior, taking over the nests built (and sometimes occupied) by native pollinators. Wasps and hornets, which are predatory insects, can feed on native pollinators, precipitating declines in some populations.
DID YOU KNOW?
• The social European hornet (Vespa crabro), introduced to New York from Eurasia in the mid-1800s, is now found in the eastern half of the United States. Mostly carnivorous, it eats bees, yellow jackets, flies, and butterflies, as well as fallen fruit and plant sap. They can damage the plants whose bark they remove to make their nests, which are found in concealed hollows (in trees, walls, attics, etc.).
• The European paper wasp (Polistes dominula), accidentally introduced to Boston from Eurasia in 1978, has spread across the U.S. outcompeting and causing the decline of native Polistes in some locations. Adults feed on floral nectar and other sugary liquids and are well adapted to urban areas where they make their open-comb nests near human structures. To feed their developing larvae, they prey not only on pests like the destructive cabbage white but on native caterpillars like monarchs, perhaps aiding the decline of butterfly populations. According to entomologist Whitney Cranshaw, the European paper wasp “is devastating to essentially all species of yard and garden Lepidoptera” (Lewis, 2016).
• The solitary giant resin bee (Megachile sculpturalis) from Asia was first reported in North Carolina in 1994. The extent of its ecological impact remains unclear. It pollinates invasive species like butterfly bush, golden raintree, kudzu, and purple loosestrife, which may facilitate their spread. Observers report damage to some native plants on which it has foraged as well as the appropriation of carpenter bee nests, both unoccupied and occupied.
Individual property owners have the most flexibility to remove invasive plant species or replace them with native plants and to monitor for invasive insects. Those, who live in condominiums or in areas with homeowner associations, can work with their governing bodies to deal with invasives. Apartment dwellers with access to outside spaces, such as patios and balconies, may not be confronted with problem invasives, but still can take actions to help pollinators by container gardening with keystones, which, of course, are also suitable for beds and borders. Remember that pollinators can find mass plantings easier than one of a kind, so if you are pressed for space, choose fewer species with three pots of each.
WHAT ARE KEYSTONES?
Keystone species are animals that have an outsized effect on their ecosystem. Research into food webs found that within ecoregions, a small percentage of plant genera fed the vast majority of lepidopteran larvae, which in turn feed birds, other insects, frogs, lizards, and rodents. Researchers postulate that an ecoregion’s entire food web would collapse without the presence of one or more of these plant genera, hence they are considered “keystones” (Narango et al., 2020). Most of the keystone plants identified are trees and shrubs, but two perennial/annual genera rank among the top twenty: Solidago (goldenrod) and Symphyotrichum (aster), each of which supports about 100 lepidopteran species.
Further refining the concept, Homegrown National Park™ identifies keystone plant species that are the most productive and commercially available across an ecoregion and that also are suitable for container gardening. The following species are recommended for Northern Virginia (NoVA), which falls in its Southeastern USA Plains ecoregion: Helianthus angustifolius (narrow-leaved or swamp sunflower), Oenothera biennis (common evening-primrose), Rudbeckia hirta (black-eyed Susan), Solidago caesia (blue-stemmed or wreath goldenrod), Solidago nemoralis (gray or old field goldenrod), Solidago rugosa (rough-stemmed or wrinkle-leaf goldenrod), Symphyotrichum lateriflorum (calico aster), and Coreopsis lanceolata (long-stalk or lanceleaf coreopsis)—because this species is believed to be introduced rather than native to Virginia, substitute native Coreopsis verticillata (whorled tickseed).
To find your ecoregion, visit Container Gardening with Keystones.
What You Can Do
- Do not plant invasive species on your property.
- Find the best way to manage invasive plants already on your property at: To Cut or Not to Cut: Alternative Management Tools for Invasive Plants.
- Consult your local Cooperative Extension Office if you have questions about invasive species you find on your land. In Alexandria and Arlington, you can also contact the MGNV Help Desk at mgarlalex@gmail.com or 703-228-6414.
- Dispose of invasive plants properly. Do not compost them.
- Use natives when replacing invasive plants. Include some keystone plant species. Take care in choosing non-native plants, cultivars, and hybrids.
- Keep a boot brush in your vehicle and use it to brush off the treads of your shoes (and your pets’ paws) before you walk in natural areas or public parks and gardens so that you don’t track in invasive seeds or insects.
- Join community-sponsored invasive removal parties to help remove invasive plants from public lands.
DID YOU KNOW?
You can find invasive plant removal events on the Arlington County government website Resources on Invasive Plants page. See the events listed in the column on the righthand-side of the page. Alexandria may also advertise invasive plant removal events from time to time.
In summary, this series has focused on four of the threats that pollinators face—climate change and habitat loss due to impervious surfaces, lawns, and invasive species—and followed up by outlining specific actions that we can take to try to mitigate these threats. No matter where we live, our education, our profession, our expertise, we do not have to be scientists or government officials to take action to confront the environmental and ecological problems to which we all contribute. To quote entomologist Doug Tallamy: “We all share the responsibility of good Earth stewardship because we all depend on the biodiversity that good Earth stewardship protects.”
1 The definition of invasive is found in Executive Order 13751 of December 5, 2016, Safeguarding the Nation From the Impacts of Invasive Species,
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MGNV and Other Resources
Uncertain on how to identify or remove invasive species or which native species are the best replacements for specific invasives, then the following resources may help:
- A Management Guide for Invasive Plants of Southern Forests
- Invasive Plants and Better Alternatives
- Invasive Plants in Northern Virginia
- Mistaken Identity? Invasive Plants and their Native Look-Alikes
- Tried and True Native Tree Selections for the Mid-Atlantic fact sheets
References
Campbell J, Frank H, Stanley-Stahr C. Featured Creatures: common name: giant resin bee. UF|IFAS. University of Florida.
Davis SL, Frisch T, Bjarnholt N, Cipollini D. 2015. How Does Garlic Mustard Lure and Kill the West Virginia White Butterfly?. Journal of Chemical Ecology 41: 948–955. doi: 10.1007/s10886-015-0633-3.
Dooley EC. 2024. Invasive Plant Time Bombs: A Hidden Ecological Threat. University of California, Davis.
Drossart M, Michez D, Vanderplanck M. 2017. Invasive plants as potential food resource for native pollinators: A case study with two invasive species and a generalist bumble bee. Scientific Reports. 7, 16242. doi: 10.1038/s41598-017-16054-5.
Fargione MJ. 2015. Invasive species add to monarch butterfly’s woes. Cary Institute of Ecosystems Studies.
Hoadley S. 2020. Echinacea for the Mid-Atlantic Region. Research Report. Mt. Cuba Center.
Janousek WM, Douglas MR, Cannings S, Graves TA. Edited by Inouye BD. 2023. Recent and future declines of a historically widespread pollinator linked to climate, land cover, and pesticides. Proceedings of the National Academy of Sciences (PNAS). 120(5):1–9. doi: 10.1073/pnas.2211223120. [PDF/EPUB]
Johnson AE, Cornell A, Hermann S, Zhu F, Hoover K. 2023. Using community science to identify predators of spotted lanternfly, Lycorma delicatula (Hemiptera: Fulgoridae), in North America. Bulletin of Entomological Research. 113(5):637-644. doi:10.1017/S0007485323000317.
Keystone Native Plants. Plant Virginia Natives. (accessed June 14, 2024).
Laport RG, Minckley RL. 2012. Occupation of active Xylocopa virginica nests by the recently invasive Megachile sculpturalis in upstate New York. Journal of the Kansas Entomological Society, 85(4), 384–386. doi: 10.2317/0022-8567-85.4.384.
Lewis D. 2016. European Paper Wasps Are Widespread. Horticulture and Home Pest News. Iowa State Extension and Outreach.
Media Release: IPBES Invasive Alien Species Assessment. 2023. Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services.
Montero-Castaño A. Vilà M. 2012. Impact of landscape alteration and invasions on pollinators: a meta-analysis. Journal of Ecology. 100(4): 884–893. doi: 10.1111/j.1365-2745.2012.01968.x. [PDF]
Narango DL, Tallamy DW, Shropshire KJ. 2020. Few keystone plant genera support the majority of Lepidoptera species. Nature Communications 11: Article 5751 . doi: 10.1038/s41467-020-19565-4.
Skvarla MJ. Updated 2023. European Paper Wasp. PennState Extension.
Tallamy D. Biodiversity Day. Homegrown National Park™,
Tangren S. Updated 2024. What is a Native Plant? University of Maryland Extension.
Vespa crabro germana. UMass Extension Landscape, Nursery and Urban Forestry Program. Center for Agriculture, Food, and the Environment. University of Massachusetts Amherst.
Wiman N, Hilton R, Kaiser C, Walton V, Suits R, Lowenstein D, Skillman V, Andrews H. 2018. Brown Marmorated Stink Bug (BMSB). OSU Extension Service. Oregon State University.
Wiseman E, Gugercin S, Close D. 2013. Emerald Ash Borer. Virginia Tech. Publication HORT-69NP. Virginia Cooperative Extension.