Black Swallow-Wort (EwA Guide to Common Invasive Species in Middlesex County (MA, US) )

Black Swallow-Wort

Vincetoxicum nigrum

Summary ⁷

Vincetoxicum nigrum, a species in the family Apocynaceae, also known as black swallow-wort, Louise's swallow-wort, or black dog-strangling vine, is a species of plant that is native to Europe and is found primarily in Italy, France, Portugal, and Spain. It is an invasive plant species in the northeastern United States, parts of the Midwest, southeastern Canada, and California. In 2020, wild plants were found in Timaru, New Zealand.

Short Description ⁸

An herbaceous perennial plant that has opposite, oval, and smooth leaves. The leaves are dark green and shiny, with pointed tips and short petioles. When they bloom, the flowers are dark purple, comprised of 5 triangular petals (star shape) covered with fine white hairs. The fruits are thin pods that turn from green to light brown or golden when mature. The seeds are flat and attached to fine filaments that are dispersed by the wind.

Source: EwA Invasive Pocket Fieldguide | © Earthwise Aware

Description ⁹

Vincetoxicum nigrum is a perennial, herbaceous vine bearing ovate leaves with pointed tips. The leaves are 3–4 inches (7.6–10.2 cm) long, and 2–3 inches (5.1–7.6 cm) wide, occurring in pairs on the stem. The flowers have five petals, and are star-shaped with white hairs. The flowers range in color from dark purple to black. The fruit of Vincetoxicum nigrum is a slender, tapered follicle that ranges in color from green through light brown and is tightly packed with seeds, each bearing a fluffy pappus to allow distribution by the wind.

Habitat

Vincetoxicum nigrum tends to grow in upland areas and is tolerant to variable light, salt, and moisture levels. In the United States, the vine is often found in abandoned fields, hedgerows, brushy areas, woodlands, river banks, transportation corridors, quarries, agricultural fields, and gardens. In gardens, the plant is seen as a fast-growing weed.

Reproduction

Vincetoxicum nigrum emerges from an underground stem in the spring, and flowers during June and July. Vincetoxicum nigrum is self-pollinating, and follicles form throughout the summer. The number of follicles formed is directly linked to the amount of light the plant receives. If there is a lower level of light, then there are fewer follicles compared to a plant exposed to a higher level of light. The seeds begin to be released by mid-August, and continue to be dispersed by air currents into early October.

Each seed is polyembryonic and contains about one to four embryos per seed, increasing Vincetoxicum nigrum's proliferation. Seeds have delicate fibrous "parachutes" used in wind dispersal over long distances. In addition to seeds for reproduction, Vincetoxicum nigrum also uses rhizomes as a method of propagation, meaning that the plant clones itself underground and produces new plants. After seed dispersal, the plant dies to the ground in the winter, reappearing in the spring.

Invasiveness ⁹

The first sighting of Vincetoxicum nigrum 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". Vincetoxicum nigrum escaped from a garden in the Cambridge area of Massachusetts and naturalized in the surrounding states and is still spreading today. Massachusetts and New York classify the plant as an invasive species. Beyond the northeastern US, the plant has been reported in Wisconsin and California.

Ecological Implications

In the United States and Canada, Vincetoxicum nigrum is a threat to native species because it crowds them out. For example, it can completely replace a field of native goldenrod. Crowding out other species results in reduced habitat for wildlife, which may become endangered because they can no longer find their optimal habitat.

Vincetoxicum nigrum threatens the rare limestone pavement barren ecosystems by crowding out plants that the native wildlife needs to survive. It may also decrease bird presence in grasslands, which may in turn, cause certain insect species populations to increase.

In Vermont, Vincetoxicum nigrum crowds out the endangered species Jesup's milk vetch (Astragalus robbinsii var. jesupii). In Rhode Island, Vincetoxicum nigrum has been reported as reducing the effectiveness of electric fences, which may allow livestock to be put into danger or lost.

In addition, Vincetoxicum nigrum crowds out a species of milkweed that monarch butterflies use as their larval food plant. Thus, the spread of this plant threatens populations of monarch butterflies. Overall, Vincetoxicum nigrum reproduces very prolifically, and can easily take over various habitats in a short time. Most of the possible implications of Vincetoxicum nigrum's changing the structure of various ecosystems have not yet been studied.

⭕ Control Methods (EwA Content) ¹⁰

Cutting and manual removal can be effective for small infestation sites, but it is necessary to dig up to remove all the root crowns and rhizomes before the seeds mature (b).

Due to easy dispersal, it is better to remain out of the area once the pods are open, avoiding spreading the seeds even further. – If you are working in an area that has black swallow-wort with pods open, check your clothes and tools for seeds, making sure not to leave any of it in your clothes – The pods can still mature after the plant has been dug up, being necessary to burn it or kill it and bag every vascular part of the plant and send it to a landfill (a, b).

If the plants are in a lawn or pasture, they can be mowed; this will not kill the vine, but if done when pods are still immature, it can reduce the spread, interrupt the reproductive cycle, and prevent seed dispersal in the area; However, this doesn't stop the plants from growing back from rhizomes (a, b, c).

The use of systemic herbicides such as Glyphosate and Triclopyrester^⚠ can be an effective option to control black swallow-wort when applied during the flowering season (early June); however, their use must be carefully analyzed and controlled due to the effect on the surrounding flora and fauna. Many applications may be necessary to achieve control. Herbicides should not be applied while the leaves are still small, as the plant may not absorb enough to kill the roots (b). To reduce the chance of re-infestation, after clearing up the area of swallow-wort, these areas should be quickly replanted with native ground cover species whenever possible (c).

Removal within 30.5 m (100 ft) of wetland resource areas, including vernal pools, or within 61 m (200 ft) of a perennial stream usually requires permits from land managers or approval from competent agencies (a).

⚠ Disclaimers

Disclaimer #1: Harmless chemicals do not exist. Any chemical use is likely to harm non-target species and affect the soil. However, in some circumstances, chemicals might be the only way to succeed in removing an invasive species permanently. In all cases, researching and understanding the latest scientific literature before using a specific chemical or a mix of several is critical to reducing negative impact.

Disclaimer #2: Independently, several European countries recently announced future bans or massive restrictions on the use of glyphosate (e.g., Austria, Germany, France). The EU, at large, is expected to decide on the renewal of the approval of glyphosate Ultimo 2023 (More here).

Sources

[a] Black Swallow-wort. Available in: https://www.concordma.gov/773/Black-Swallow-wort-Cynanchum-Louiseae
[b] Swallow-worts. Available in: https://cceputnamcounty.org/natural-resources/swallow-worts
[c] Invasive in the Spotlight: Swallow-wort. How to control the "dog-strangling vine" taking over your yard. 2018; Available in: https://extension.unh.edu/blog/2018/06/invasive-spotlight-swallow-wort

External links ⁹

Media related to Vincetoxicum nigrum at Wikimedia Commons

Sources and Credits

(c) Claire O'Neill, some rights reserved (CC BY-NC), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/122529379
(c) Claire O'Neill, some rights reserved (CC BY-NC), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/98148571
(c) Bill MacIndewar, some rights reserved (CC BY-NC), uploaded by Bill MacIndewar, https://www.inaturalist.org/observations/126284264
(c) Kate Danziger, some rights reserved (CC BY-NC), uploaded by Kate Danziger, https://www.inaturalist.org/observations/82611391
(c) Bill MacIndewar, some rights reserved (CC BY-NC), uploaded by Bill MacIndewar, https://www.inaturalist.org/observations/26179219
(c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Vincetoxicum_nigrum
(c) Claire O'Neill, some rights reserved (CC BY-SA)
Adapted by Claire O'Neill from a work by (c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Vincetoxicum_nigrum
(c) Esther Meirelles, some rights reserved (CC BY-SA)

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