Black Swallow-wort

Vincetoxicum nigrum

Description and biology 8

  • Plant: herbaceous perennial twining vine; stems pubescent with the hairs often in longitudinal bands; sap is not milky.
  • Leaves: opposite; oblong or ovate, rounded or subcordate at base, acute to acuminate at tip; smooth margins; 2-5 in. long by 1-2½ in. wide, largest leaves in the middle of the stem;petioles about ½ in. long; lightly pubescent on margins and major veins on underside of leaf.
  • Flowers, fruits and seeds: flower buds globose with a rounded apex, unopened petals not twisted; flowers purple to almost black with fine hairs on inner surface, 5-parted, about ¼ in. across; corona is a fleshy lobed cup; fleshy petals ovate to broadly deltoid or triangular, about 1/10 in. long; flower stalks usually curved and hairy and less than 1 in. long; fruit is slender to plump, smooth hairless seedpod (follicle) about 1½ in. long; seeds are about ¼ in. long, dark brown, ovoid to obovoid, flattened with narrow membranous wing on margin and silky filament at tip.
  • Spreads: by wind-dispersed seed.
  • Look-alikes: honeyvine (C. laeve), native to the mid-Atlantic and Southeast, has heart-shaped leaves and white flowers; pale swallow-wort (C. rossicum) non-native and invasive, has flowers that are pink to maroon in color and strap-shaped petals.

Ecological threat in the united states 9

Black swallow-wort can form extensive patches that crowd out the native vegetation. Old field habitats of goldenrod and grasses can be replaced almost exclusively by swallow-wort, completely changing their physical structure. Impacts of the change in physical structure and insect communities on grassland and old field nesting birds is not yet known. A preliminary study of impacts on grassland birds in New York suggests that as coverage by a sister species of swallow-wort (pale swallow-wort, Cynanchum rossicum) increases, grassland bird presence declines. The globally rare alvar ecosystems (limestone pavement barrens supporting unique plant communities), are threatened by many rapidly expanding swallow-wort populations in Jefferson County, New York. Black swallow-wort threatens to displace Jessop’s milk vetch, a federally listed plant, on the banks of the Connecticut River valley in central Vermont. Investigations into impacts on the monarch butterfly, which requires milkweeds for reproduction, indicate that the butterfly will cue into black swallow-wort and lay eggs, but the larvae do not survive.

Impacts and control 10

More info for the terms: alvar, competition, cover, density, fern, fire management, invasive species, natural, phenology, prescribed fire, root crown, shrubs

Impacts: Information regarding the impacts of black or pale swallow-wort on invaded communities includes evidence that swallow-worts may displace existing vegetation [41,42,43], threaten rare plants, and negatively impact at least one rare ecosystem. Swallow-worts are of great management concern in the globally rare alvar ecosystem of the Lower Great Lakes Basin [16,41,42,43,56,60]. Reviews note the potential replacement of the federally endangered Jessop's milkvetch (Astragalus robinsii) by black swallow-wort and Hart's-tongue fern (Asplenium scolopendrium) by pale swallow-wort [16,41,42,56,60]. Displacement of native plant species may lead to detrimental effects on host-dependant species such as the monarch butterfly [10,11,17,29,50]. Pale swallow-wort may also have a negative effect on some grassland bird species 16. It has altered soil microbial communities [25] and insect communities 19.

A stewardship report documents 2 areas in New York where pale swallow-wort populations increased from occasional patches to large stands in 50 to 100 years; one stand now covers 12 acres (5 ha) and the other 74 acres (30 ha) [43]. Similar information was not available for black swallow-wort populations as of 2009.

A fact sheet states that the sap of the pale swallow-wort causes an allergic reaction in some people [28].   European swallow-wort infestation.

Control: Control of swallow-worts is complicated by some of their life history attributes. For example, swallow-worts have very effective seed dispersal [43]; one author speculates that prevailing winds off the Great Lakes have facilitated pale swallow-wort spread in the region [60]. Land management activities may also contribute to seed dispersal for both species; mature plants and seeds may be harvested during haymaking operations and moved to new sites when the hay is transported [43] and potentially when it is consumed. Furthermore, the ability of swallow-worts to regenerate vegetatively makes control efforts difficult (see Vegetative regeneration).

In all cases where invasive species are targeted for control, the potential for other invasive species to fill their void must be considered, no matter what method is employed [6]. Information presented in the following sections may not be comprehensive and is not intended to be prescriptive in nature. It is intended to help managers understand the ecology and control of swallow-worts in the context of fire management. For more detailed information on control of black or pale swallow-wort, consult the references cited here or local extension services.

Fire: For information on the use of prescribed fire to control this species, see Fire management considerations.

Prevention: It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" natural communities 48,58. Managing to maintain the integrity of native plant communities and identification of the causal factors enhancing ecosystem invasibility are likely to be more effective than an emphasis on controlling the invader [30].

Prevention of swallow-wort infestations is difficult because of the wide distribution of both species and their ability to easily disperse numerous seeds via wind. Establishment of large monospecific stands may be prevented by manually removing isolated individuals and small patches before they spread, perhaps using some of the mechanical techniques [43] described in Physical and/or mechanical control.

Cultural: There is limited information on the use of cultural practices to control swallow-wort populations. In central New York, glyphosate and triclopyr herbicide application was followed by native rye grass (Elymus spp.) plantings to investigate whether rye grasses could effectively suppress pale swallow-wort. Drought conditions persisted during the study period, and the rye grasses failed to establish after germination. However, the author speculated that using species that provide early season light competition (e.g., broadleaved natives) might be an effective control strategy because swallow-worts accomplish most of their growth early in the growing season [44]. A fact sheet suggests that invasive pale swallow-wort populations might be brought under control by plowing and planting an annual crop that persisted until the pale swallow-wort seed bank was depleted, possibly as long as 5 years [42]. However, this strategy may not be feasible or appropriate in wildlands.

Physical and/or mechanical: Several physical or mechanical measures have been attempted to control swallow-worts, although these strategies have proved problematic due to the regeneration capacity of swallow-worts (see Vegetative regeneration). To date (2009), all of the research in this field comes from attempts to control pale swallow-wort populations and involves either using physical barriers to cover or smother plant populations, cutting and mowing, or digging up plants.

Using a physical barrier to cover or smother populations: On the Fletcher Wildlife Garden near Ottawa, several methods for controlling pale swallow-wort infestations were tested, with variable results. Mulching materials, including thick leaf cover, landscape cloth, or multiple layers of newspapers were used to suppress pale swallow-wort growth around the bases of trees and shrubs. Though some pale swallow-wort plants penetrated through the mulches, they occurred in much lower densities than on sites without mulch [21].

Fletcher Wildlife Garden staff also attempted smothering pale swallow-wort plants with a heavy tarp. Pretreatment density of swallow-wort was not mentioned for this study. After 1 year, some plants had grown through holes in the tarp, but no plants could be seen under intact parts of the tarp. No seedlings were visible, suggesting the tarp had prevented the germination of the previous years' seeds. Upon removal of the tarp after 2 years, only a few plants were found and were manually dug out [21].

A group called Urban Forest Associates also attempted to smother pale swallow-wort plants in an open field near Toronto. Plastic sheeting was applied to block light and appeared to effectively prevent pale swallow-wort growth while intact. However, the sheeting was repeatedly torn open by animals, which ultimately allowed pale swallow-wort to recover to pretreatment levels (Urban Forest Associates, unpublished 1996 study cited in [12]).

Cutting or mowing: Several cutting or mowing strategies have been used to control pale swallow-wort populations. In general, these programs have not succeeded because swallow-worts sprout from stem nodes or from the root crown after damage [21,43,44,45,51]. In many cases, repeated clippings or mowings were necessary, making this type of control long-term and labor-intensive [3,12,21].

In one project in Ottawa, Fletcher Wildlife Garden staff tried cutting pale swallow-wort flowerheads before seed set. However, the plants sprouted rapidly below the cut and sent up new flowerheads. Three or 4 additional cuttings were needed to prevent growth and reproduction. The group decided this treatment was impractical, especially for widespread infestations. Mowing to stop seed production and dispersal was also attempted, though the ideal time for mowing based on plant phenology coincided with bird nesting, and the treatment had to be delayed. Consequently, multiple mowings were needed [21]. Urban Forest Associates also attempted to mow pale swallow-wort plants in an open field near Toronto. Repeated mowing reduced average stem height but did not ultimately reduce cover to a "satisfactory" extent [12]. In New York, experimental plots of pale swallow-wort that were clipped in June and July had greater stem densities than unmanaged plots by September. Repeatedly clipped stems still produced seed occasionally [3]. 

Fact sheets for both species claim that mowing will not eradicate populations but can be used to prevent a seed crop if done in early to mid-July [41,42]. In one study, pale swallow-wort seedlings and mature plants were clipped at different times during the growing season near Ottawa. This study concluded that the best time for controlling population spread with a single cutting treatment was after the first fruits were produced but before fruits fully developed, on or near 26 June in the study region [51].

Digging: Digging up plants may control swallow-wort populations, though it would likely only be practical for small populations or isolated plants [45]. Because broken root crowns left on or in the ground may sprout, care must be taken to remove and dispose of the entire root crown [43,44].

See Integrated Management for projects that combine physical and mechanical techniques with other control methods.

Biological: The potential for biocontrol has not been studied for either black or pale swallow-wort. Both species appear to have few pests, diseases, or other biocontrols in North America [43]. Some reviews suggest that insect and fungal groups from the plants' native range that prey on these or other swallow-wort species should be investigated [16,43]. Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls. Refer to these sources: [69,72] and the Weed Control Methods Handbook [65] for background information and important considerations for developing and implementing biological control programs.

No evidence suggests grazing can effectively control swallow-wort populations (see Palatability and/or nutritional value). Authors anecdotally observed cattle grazing on pale swallow-wort in New York; however, pale swallow-wort quickly reestablished in the pastures within a few years of cattle removal, especially where pale swallow-wort populations were present on adjacent land [16]. Similarly, another author observed that grazing cattle suppressed pale swallow-wort, but plants rebounded once pastures were abandoned. Horses avoid it, which may encourage pale swallow-wort infestation if growth from other plants is reduced [43]. Reviews state that grazing and trampling could stimulate sprouting from stem leaf axils and perennating buds on the root crown [16,43], though no data are given to support this statement.

Chemical: There has been some experimentation on the use of herbicides to control swallow-wort populations. In general, these studies suggest that chemical control alone is not effective, and long-term reapplication is needed to sustain control [12,41,42,44,45]. General guidelines for cut-stem and foliar application of chemicals are available in fact sheets for both black [41] and pale [42] swallow-wort.

In central New York, the effectiveness of foliar spray and cut-stem applications of glyphosate and triclopyr was investigated during the growing season. The chemical treatments lowered pale swallow-wort plant density, cover, and biomass compared to untreated plots, though not by the targeted 90%. Foliar spray applications were more effective than cut-stem applications. However, foliar application of herbicides may be difficult because swallow-worts twine around shrubs and trees. Reapplication would be required for long-lasting control, because a flush of pale swallow-wort seedling growth was observed after treatment. If herbicides are used, application during the reproductive stage is advised; plants treated with glyphosate aborted their flowers and made no further reproductive effort the year of treatment [44,45].

Urban Forest Associates also used herbicides on pale swallow-wort in an open field near Toronto. Repeated applications of 5% glyphosate were needed to strongly limit regrowth the following year, and living buds were found on some roots 1 year after repeated treatments. Single applications did not reduce cover satisfactorily 1 year after treatment (Urban Forest Associates, unpublished 1996 study cited in [12]).

A New York study found that pale swallow-wort plants treated with triclopyr in June did not produce seed within that growing season. After herbicide application, there was a reduction in pale swallow-wort cover and an increase in the cover of other vegetation, though most of these species were nonnatives. The authors speculate that the loss of both adult and seedling swallow-wort plants led to a reduction in light competition and likely stimulated the germination of dormant seeds of other species [3].

See Integrated Management for projects that combine chemical application with other control methods.

Integrated management: A number of sources advocate an integrated management approach to the control of black and pale swallow-wort populations [12,43,51]. However, only a few studies have combined control techniques to investigate the efficacy of integrated management.

One study in New York investigated pale swallow-wort response to 4 treatments: (1) June herbicide application, (2) June clipping, (3) June and July clipping, and (4) a combination of June herbicide application with July clipping. The study found that one application of triclopyr, regardless of clipping, suppressed pale swallow-wort populations through postapplication growing seasons 1 and 2. On chemically treated compared to untreated plots, cover was reduced by 81%, and stem density was reduced by 86%. Adding clipping to the chemical treatments resulted in no significant difference in swallow-wort cover; chemical treatment alone was as effective as the combination treatments [3]. The combination of fire with other control methods may not improve the success of control efforts (see Use of Fire as a Control Agent).

Cynanchum louiseae 11

Cynanchum louiseae, a species in the milkweed family, is also known as Black swallow-wort, Louise's swallow-wort.,[2] or Black dog-strangling vine,[3]Cynanchum louiseae is a species of plant that is native to Europe and is found primarily in Italy, France, Portugal, and Spain.[4] It is an invasive plant species in the northeastern United States, parts of the Midwest, southeastern Canada, and California.


Cynanchum louiseae is a perennial, herbaceous vine with oval shaped leaves that have pointed tips. The leaves are 3-4 inches long an 2-3 inches wide and often occur in pairs on the stem. The flowers have five petals that are star-shaped with white hairs. The flowers range in color from dark purple to black. The fruit of Cynanchum louiseae is slender, tapered pods that range in color from green to light brown.[4]


Cynanchum louiseae tends to grow in upland areas and is tolerant to variable light, salt, and moisture levels. In the United States, Cynanchum louiseae is often found in abandoned fields, hedgerows, brushy areas, woodlands, river banks, transportation corridors, quarries, agricultural fields, and gardens. In gardens, Cynanchum louiseae is seen as a weed.[4]


Cynanchum louiseae emerges in the spring and flowers during June and July. Cynanchum louiseae is self-pollinating. Seed pods form throughout the summer. The number of pods formed is directly linked to the amount of light the plant is exposed to. If there is a higher level of light, then there are more seed pods. If there is a lower level of light, then there are fewer seed pods compared to a plant exposed to a higher level of light. Its seeds begin to be released by mid-August and continue to be dispersed into early October.

Each seed is polyembryonic and contains about one to four embryos per seed. Polyembryonic seeds increases Cynanchum louiseae's chance of survival. Seeds also use "parachutes" in order to be dispersed by the wind over large distances. In addition to seeds, for reproduction, Cynanchum louiseae also uses rhizomes as a method of reproduction, meaning that the plant clones itself underground and produces new plants. After seed dispersal, the plant dies to the ground in the winter and returns in the spring.[4]


Black swallow-wort invasive species advisory sign, Lake Allen, Cambridge Township, Michigan

The first group of Cynanchum louiseae in North America was recorded in Ipswich, Massachusetts in 1854. In 1864, a plant collector recorded that it was "escaping from the botanical garden where it is a weed promising to be naturalized". Cynanchum louiseae escaped from a garden in the Cambridge area of Massachusetts and naturalized in the surrounding states and is still spreading today.[4] Massachusetts[5] and New York[6] classify the plant as an invasive species.

Ecological implications[edit]

In the United States and Canada, Cynanchum louiseae is a threat to native species because it crowds them out. For example, Cynanchum louiseae can completely replace a field of goldenrod.[citation needed] Crowding out other species results in a reduced habitat for wildlife, which may become endangered because they can no longer find the correct habitat.

Cynanchum louiseae threatens the rare limestone pavement barren ecosystems by crowding out plants that the surrounding wildlife needs. Cynanchum louiseae may also decrease bird presence in grasslands, which may in turn cause insect species populations to increase.

In Vermont, Cynanchum louiseae crowds out the endangered species Jessop's milk vetch. In Rhode Island, Cynanchum louiseae reduces the effectiveness of electric fences, which may cause livestock to be put into danger or lost.[7]

In addition, Cynanchum louiseae crowds out another species of milkweed that monarch butterflies use in order to reproduce. When monarch butterflies try to reproduce using Cynanchum louiseae, the larvae do not survive. Thus, Cynanchum louiseae threatens populations of monarch butterflies. Overall, Cynanchum louiseae reproduces effectively and can easily take over various habitats in a short amount of time. It can easily take dominance over native species' habitats. Most of the possible implications of Cynanchum louiseae changing the physical structure of various ecosystems are yet to be known.[4]


There are four methods of management that can possibly be used for the management of Cynanchum louiseae. These methods are chemical, manual, mechanical, and biological. Only the chemical, manual, mechanical methods are actually used in the United States and Canada. The biological method may be used in the future.[8] Overall, early detection and removal is the best management.


The best chemical management over Cynanchum louiseae is through the use of systemic herbicides. Systemic herbicides prevent seeds from being viable and, as a result, the next generation will not exist. Garlon 4 (tridopyr ester) and RoundUp Pro (glyphosate) are the main systemic herbicides that are used to control Cynanchum louiseae. The systemic herbicide is sprayed on the plant after flowering has begun. If the herbicide is used after seed pods have formed, then the herbicide is less effective because viable seeds may form. The most effective treatment using systemic herbicides is through a cut stem application, which is applying the chemical to cut stems.[4]


Manual management is the removal of Cynanchum louiseae from the ground by digging up its roots so that the plant cannot reproduce.[4] The vine has very shallow roots, making it relatively easy to uproot. Seed pods must be disposed of carefully, to avoid inadvertently spreading the seeds to new areas.


Mechanical management is the mowing down of Cynanchum louiseae. This method does not stop growth, but it does stop seed crops. No seed crop means no reproduction and no new generation.[4]


Biological management is the use of Cynanchum louiseae's natural enemies to lower the population of Cynanchum louiseae. In the United States, Cynanchum louiseae has no natural enemies. In its native European regions, Cynanchum louiseae has natural enemies, such as certain caterpillars, beetles, and diseases. Researchers at Cornell University and the USDA are researching into the use Cynanchum louiseae's natural enemies as a way to control Cynanchum louiseae. The use of natural enemies is controversial because the implications of adding more non-native species to threatened areas is unknown. The use of Biological Control is being researched currently and is typically safe and effective.[8]


  1. ^1913 illustration from Britton & Brown, Illustrated flora of the northern states and Canada, PD US, Vincetoxicum nigrum
  2. ^ ab"Classification for Kingdom Plantae Down to Species Cynanchum louiseae Kartesz & Gandhi". USDA Natural Resources Conservation Service. 2008-05-30. Retrieved 2008-05-30. 
  3. ^"Black dog-strangling vine". Center for Invasive Species and Ecosystem Health and USDA APHIS PPQ. 2010-05-24. Retrieved 2011-07-24. 
  4. ^ abcdefghi"Black Swallow-Wort". Plant Conservation Alliance, Alien Plant Working Group. 2006-06-27. Retrieved 2008-05-30. 
  5. ^Sally Kerans (May 31, 2007). "Invasive plants: Old standards get bad name". Danvers Herald. 
  6. ^"Interim List of Invasive Plant Species in New York State". Advisory Invasive Plant List. New York State Department of Environmental Conservation. Retrieved 1 June 2013. 
  7. ^"Swallow-Worts". The Bugwood Network, USDA Forest Service & USDA APHIS PPQ. The University of Georgia - Warnell School of Forest Resources and College of Agricultural and Environmental Sciences - Dept. of Entomology. 2003-11-05. Retrieved 2008-05-30. 
  8. ^ abRamanujan, Krishna (2006-02-01). "Wanted by Cornell and USDA researchers: A natural enemy to curb two invasive, poisonous vines". Cornell Chronicle Online (Ithaca, NY: Cornell Chronicle Online). Retrieved 2008-05-30. 

Sources and Credits

  1. (c) jacinta lluch valero, some rights reserved (CC BY-SA),
  2. (c) Kyle Jones, some rights reserved (CC BY-NC), uploaded by Kyle Jones
  3. no rights reserved, uploaded by Kent McFarland
  4. (c) plantman, some rights reserved (CC BY-NC)
  5. (c) Priyantha Wijesinghe, some rights reserved (CC BY-NC), uploaded by Priyantha Wijesinghe
  6. (c) Rob Curtis, some rights reserved (CC BY-NC-SA), uploaded by Rob Curtis
  7. (c) Susan Elliott, some rights reserved (CC BY-NC), uploaded by Susan Elliott
  8. (c) Unknown, some rights reserved (CC BY-NC-SA),
  9. (c) Unknown, some rights reserved (CC BY-NC-SA),
  10. Public Domain,
  11. (c) Unknown, some rights reserved (CC BY-SA),

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