Black Swallow-wort – from Jeanne Nevard

 This is the first in a series on invasive plants. The Pepperell Garden Club has designated this as “ Invasives Awareness Year”. At each meeting an invasive plant will be the focus. As an extension, I am providing more detailed information for identification and control. Please clip out & keep for your reference.

Invasive plants have been with us for 100’s of years. They were brought here as favorite plants from settlers’ homelands, imported as exotic, horticultural specimens, and even brought in by accident.

The New England Wildflower Society of Framingham, MA states that “about 60% of invasive species were introductions from arboretums, botanic gardens, &  gardeners”. Purple loosestrife was first brought to the U.S. in the hold of a ship via ballast water, then later introduced for horticultural purposes.

Invasive species are described as being non-native, non-indigenous, aggressive plants with little or no predators to keep them in check. They displace our native vegetation, and disrupt the food chain of our native insects, birds, & wildlife. There is also a substantial financial cost as well as an environmental detriment in this battle.

According to the North Carolina Botanical Gardens “Biota of North America” study, “at least 4,000 species of non-native plants occur outside the United States. 79 species cost the U.S. economy more than 97 billion dollars annually in lost crops, failed recovery efforts for endangered species, and control efforts.

Invasive species have contributed to the decline of 42% of U.S. endangered and threatened species.”

For the first invasive, I’ve chosen Black Swallow-wort (Cynanchum louiseae). It’s a non-showy dark green leaved plant in the milkweed family.

The Plant Conservation Alliance, Alien Plant Working Group states that:

        “The first collection of black swallow-wort in North America was (found) in Ipswich, Massachusetts, in 1854 and also “a weed escaping from gardens in the CambridgeMassachusetts area.”

The vines can twine from 3 to 6 feet high, often smothering nearby vegetation. Its leaves are shiny, dark green, oval-shaped from 2 to 5 inches long, with dark purple star-like flowers in the spring. They prefer forests, woodland edges, old fields, fence-rows, and roadsides, persisting in sun or shade. They have an extensive “witches broom” of white roots and any piece remaining in the soil will grow back.

The chili pepper-like seed pods explode into 100’s of fluffy parachutes with a seed attached at each puff. They can fly for  great distances insuring its spread. Sometimes cleverly disguised among the native milkweeds, they become bushier & begin to twine their long tendrils.

The urgency at this time of year is to be able to ID it & intercept the seed pods that are about to split open, “Thick infestations in full sun can produce 2,000 seeds per square meter”, states The Plant Conservation Alliance (PCA).

The reason this plant is so insidious is the fact that it IS a milkweed a part of the family of plants and the sole host food for the Monarch butterfly larvae. The unwitting Monarch lays its eggs on the plant, the eggs hatch into caterpillars; they feed on the wrong host plant & die. The swallow-wort leaves are toxic, unlike the native milkweeds. The Monarchs cannot differentiate between the native & non-native. And that butterfly’s progeny is gone, ending that part of its relay journey to the South.

When a monarch ingests the native milkweed sap it is not poisonous to it. But if a bird attempts to eat the butterfly it tastes terrible & learns to avoid them. A genetic mimicry takes place as the Viceroy butterfly looks like the Monarch’s coloration, thereby protecting it from predators.

Below are suggestions for its control, sourced from The N. E. Wildflower Society.

TREATMENT: Black Swallow-wort (Cynanchum louiseae) and pale swallow-wort (Cynanchum rossicum)

  • Small infestations can be      repeatedly removed by digging out roots for several years – each small      piece of root will produce a new plant.
  • Can cut in mid July to      prevent spread of seeds but cutting will not kill plant – plants cut      during the flowering period will recover, flower, and produce seeds.
  • Best option is spraying      foliage using 1% Triclopyr at flowering in June – Can also cut stems and      apply Glyphosate to cut stems, but stems are very small, and this      treatment may not be as effective as foliar applications.

When using herbicides to treat invasive plants, certain precautions must be emphasized:

  • Follow all precautions & directions on the label
  • Wetland conditions require special herbicides
  • Only apply at the appropriate time of year to be effective, thereby reducing the impact of unnecessary chemicals into the environment
  • Be extremely cautious when using these chemicals, & only apply on your own property
  • If in doubt, always consult an expert such as the NEWS for detailed info., and wetland considerations

The best strategy is early identification & intervention. I have battled this plant for years in my own garden. They are extremely difficult to dig up. My suggestion is to cut off all seed pods. But you must be vigilant & keep at it. It is well worth the effort to save your gardens & local landscapes from this formidable foe.

For more information go to:

Leave a Reply

Your email address will not be published. Required fields are marked *