The Invasive Plant Threat: How Swallow-Wort is Spreading Across America
Swallow-wort is invading U.S. landscapes, harming native habitats and outpacing removal with alarming speed and resilience.
Swallow-Wort: The Unseen Green Invader Overtaking America
Each year, an insidious invasive plant is quietly overtaking American ecosystems, covering ground so rapidly that its annual spread is measured by the equivalent of 50,000 baseball fields . Known as swallow-wort (including both pale and black swallow-wort, genus Cynanchum), this non-native weed is threatening biodiversity, challenging restoration efforts, and unsettling natural landscapes across swathes of the United States.
Key Facts at a Glance
- Invasion Rate: As much as 50,000 baseball fields per year in the U.S. Southeast .
- Growth Rate: Capable of growing a foot per day under optimal conditions.
- Impacts: Displaces native plants, disrupts ecosystems, complicates land management, and resists conventional removal methods.
- Origin: Native to Europe, introduced to North America in the 19th century.
- Range: From New England to the Midwest and beyond—urban gardens, agricultural lands, parks, and wildlands are all affected.
What Is Swallow-Wort?
Swallow-wort describes two closely related species—pale swallow-wort (Cynanchum rossicum) and black swallow-wort (Cynanchum louiseae)—that belong to the milkweed family. Originally brought from Europe as ornamentals, these vines are now among the most aggressive invaders of grasslands, forests, and even urban landscapes across the Northeast, Midwest, and growing parts of the South .
Physical Description
- Twining vines with leathery, shiny, dark green leaves arranged opposite each other.
- Pale swallow-wort has maroon to brownish star-shaped flowers, while black swallow-wort has deep purple-black flowers.
- The plants produce slender, pointed seed pods filled with wind-dispersed seeds.
- Roots form dense mats underground, allowing the plant to resprout and regrow quickly, even after cutting or digging.
Life Cycle and Advantages
- Flowers in early summer, producing pods by mid-summer, then dispersing seeds late summer to fall.
- Extremely resilient; regrows after mowing, cutting, or grazing.
- Outcompetes other plants by creating dense tangles and shading out seedlings.
How Fast Is It Spreading?
Swallow-wort spreads with alarming efficiency. It can expand its territory at a rate close to a foot per day under ideal conditions. Experts estimate its total annual land coverage increase to be equivalent to 50,000 baseball fields—that’s more than 180 square kilometers each year .
Pathways of Spread
- Wind: Seeds are silky and light, carried by wind far from parent plants.
- Human Activity: Seeds hitchhike on mowers, vehicles, boots, and transported soil.
- Landscape Use: Early ornamental planting helped populations escape into the wild.
The Ecological and Economic Toll
Swallow-wort’s seemingly unstoppable spread has profound impacts at every level of the ecosystem, from soil microbiota to top predators.
Effects on Native Species
- Biodiversity threat: Swallow-wort crowds out native wildflowers and grasses, reducing plant diversity.
- Butterflies at risk: The non-native vine resembles milkweed. Monarch butterflies may lay eggs on swallow-wort, but the larvae cannot survive on its leaves, leading to population losses .
- Disrupted food webs: Reduction of native plants affects insects, songbirds, and other wildlife dependent on those species.
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Complications for Restoration and Agriculture
- Restoration challenges: Swallow-wort can dominate prairies and parklands, making native restoration prohibitively expensive and labor-intensive.
- Cropland invasion: Spreads into pastures and fields, reducing yields and increasing control costs.
- Infrastructure impact: Overtakes trails, fencelines, and abandoned lots, complicating maintenance and recreation.
Why Is Swallow-Wort So Hard to Control?
Unlike some weeds that can be managed with routine mowing or herbicides, swallow-wort is particularly tenacious:
- Resprouting: Severed roots start new vines. Hand-pulling can leave fragments that resprout vigorously.
- Seed longevity: Seeds can remain viable in the soil for several years, enabling regrowth and making eradication a multi-year challenge.
- Resistance to herbicide: Traditional herbicides kill above-ground growth, but often don’t reach or destroy the dense root masses.
- Impossible to mow out: Unlike annual weeds, swallow-wort’s perennial root system allows it to rebound repeatedly after mowing.
Case Study: State Parks Overwhelmed
At Robert G. Wehle State Park in New York, pale swallow-wort infestation grew so severe that park managers implemented a comprehensive management plan prioritizing the removal of this species to protect biodiversity and ensure public enjoyment .
- Multiple control methods have been trialed: mowing and hand pulling offer temporary relief but rarely eradicate established plants.
- Park managers work with university and agriculture agencies for sustainable solutions, including new biocontrol methods and detailed vegetation mapping.
Management and Restoration Efforts
Combating swallow-wort demands a multi-pronged, persistent approach adapted to landscape, infestation size, and surrounding ecosystems.
Control Techniques
| Method | Effectiveness | Drawbacks |
|---|---|---|
| Hand pulling | Good for small patches and early-stage invasions | Labor-intensive, must remove all roots; fragments regrow |
| Mowing | May slow spread if done repeatedly (monthly or more), especially before seeding | Does not kill roots; rapid regrowth |
| Herbicides | Some systemic herbicides show partial success on above-ground growth | Multiple applications needed; risk to non-target plants; root survival |
| Biocontrol | Research into moths and other insects underway; aims for sustainable, specific suppression | Long-term research; complex permitting process |
| Soil solarization | Effective in small garden beds | Impractical for large infestations |
Restoration Best Practices
- After removal, immediately replant with competitive native grasses and flowers to shade the soil and prevent re-invasion.
- Monitor land for new invasions: even isolated seeds can reestablish a population.
- Engage community stewards and volunteers in early-season scouting and rapid response activities.
Prevention: The First Line of Defense
With eradication nearly impossible in deeply infested regions, the most effective management for swallow-wort is prevention:
- Don’t purchase or plant swallow-wort varieties, even as ornamentals.
- Clean boots, equipment, bikes, and vehicles after visiting infested areas.
- Dispose of plant parts in sealed trash—never compost or leave on site.
- Support local and regional invasive plant initiatives and report any new populations to land managers or state agencies.
What the Future Holds
As swallow-wort continues its northward and westward expansion, scientists and land managers are developing new control and monitoring strategies while calling for wider public engagement. Climate change may increase its viable range, and the long-lived seedbank means that today’s lapses could haunt landscapes for decades.
The fight against swallow-wort is emblematic of broader challenges posed by invasive species: no single solution is sufficient. Instead, the collective efforts of scientists, stewards, policymakers, and homeowners are needed to preserve native ecosystems and safeguard recreation, agriculture, and wild beauty for future generations.
Frequently Asked Questions (FAQs)
Q: What does swallow-wort look like, and how do I identify it?
A: Swallow-wort features paired, glossy, dark green leaves and slender seed pods that split open in late summer, releasing silky, wind-blown seeds. Pale swallow-wort has maroon flowers; black swallow-wort has purple to black flowers.
Q: Why is swallow-wort so difficult to control compared to normal weeds?
A: Unlike most weeds, swallow-wort regrows vigorously from root fragments and produces seeds that stay viable for years. Cutting or herbicide may kill the tops, but the roots and buried seeds often persist and resprout.
Q: Can swallow-wort be completely eradicated?
A: In small, new infestations, persistent pulling and herbicide over several years may eradicate populations. In large, established infestations, eradication is rare; long-term management and containment become the practical goals.
Q: How can the public help stop the spread of invasive swallow-wort?
A: The most important steps are identifying and reporting infestations, cleaning gear after visiting wildlands, and choosing only non-invasive species for gardening or landscaping. Participating in local invasive species groups also helps magnify impact.
Q: Are any biological controls available for swallow-wort?
A: Ongoing research is looking for moths and insects that specifically target swallow-wort without harming native species, but biocontrol releases are still in the trial phase and not widely available yet.
Takeaway: A Call for Vigilance and Action
Swallow-wort’s dramatic and ongoing expansion across the United States demands vigilance, rapid response, and sustained collaboration. While the battle is daunting, preventing new introductions and supporting management efforts remain the best ways to protect America’s native landscapes from this invisible—but overwhelming—green invader.
References
- https://www.americanforests.org/article/forest-digest-week-of-july-21/
- https://broomecountyny.gov/sites/default/files/dept/planning/pdfs/Hazard%20Mitigation/2019%20Final%20Plan/Section%205.4.5%20-%20Invasive%20Species%20FINAL.pdf
- https://www.invasive.org/alien/pubs/midatlantic/midatlantic.pdf
- https://parks.ny.gov/documents/inside-our-agency/MasterPlans/RobertWehleStatePark/RobertWehleStateParkAppendixB.pdf
- https://www.ramseycounty.us/residents/environment/soil-water-conservation/invasive-plant-management/invasive-plant-species
- https://dnr.maryland.gov/wildlife/Documents/midatlantic.pdf
- https://www.dnr.state.mn.us/invasives/terrestrialplants/index.html
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