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Invasive swallow-warts difficult to irradicate

Black swallow-wort and roots with blooms just starting to form

Black swallow-wort and roots with blooms just starting to form.

Question: I have a vine growing along the edges of my property that has small dark purple flowers. It is taking over the area. Its leaves are dark green. Can you tell me what it is and how to control it?

Answer: The vine you have is a black swallow-wort (Vincetoxicum nigrum; syn. Cynanchum louiseae), or dog-strangling vine. It is characterized by dark green, shiny oval or heart shaped leaves with dark purple flowers that have five petals and a star shape. The blooms generally grow in clusters of six to 10. Similarly, the pale swallow-wort has light pink flowers. These vines are natives of Southwestern and Northern Europe and were likely brought here over a century ago as an ornamental vine.

Black swallow-wort is an invasive perennial vine that is fast growing and has the potential to overgrow and choke out native vegetation in New Jersey. The shoots of swallow-wort start to emerge from the soil in the spring. They can grow seven to eight feet and intertwine on themselves and other nearby vegetation. They are hardy in a variety of soils including woodlands, agricultural and natural fields and even rocky shores. Black swallow-wort prefers full sun but can also thrive in semi-shaded areas. Their counter-part, the pale swallow-wort, prefers shaded areas. Both plants flower in the early summer and produce long green seed pods filled with wind-distributed seeds towards late July and early August. They have a deep-fibrous root system that makes them very difficult to dig up.

Swallow-worts were originally placed in the milkweed family, Asclepiadaceae, but in recent years the Asclepiadaceae has been recombined with the periwinkle family, Apocynaceae. Monarch butterflies often lay their eggs on swallow-wort seed pods, but unlike common milkweed, the black swallow-wort is poisonous to monarchs. The larvae will die either when they feed or they will starve to death.

They are often able to blend with other ornamental vines, largely unnoticed, until infestation has occurred. Controlling them without the use of chemicals can be very difficult. You can hand pull these vines when they are in the very early stages, but in later stages digging them up is more effective. Whether you hand pull or dig them up, make sure you get as much of the root ball as possible, collect the seed pods and destroy them. Any roots left behind may quickly sprout and continue to spread. Close monitoring over several months to be sure roots left behind are not producing new plants is critical. Simply collecting new seed pods, even in smaller areas and destroying them is likely not enough to be effective and can be very time consuming over the course of the growing season. Be sure to avoid opening the seed pods as the seed will spread very easily. While mowing these vines is generally not effective as a long-term solution, you can mow them when the seed pods have started to form, but before they have viable seed, to control the spread of the vines.

For more information on the removal of black swallow-wort from your property, you can contact your local extension office. Atlantic County residents can contact the Master Gardener Helpline at 609-625-0056. Cape May County Residents can call 609-465-5115, ext.3607.

Events: Atlantic County Master Gardeners will be available to answer gardening questions and take samples for plant identification or diagnoses throughout the county this spring and summer. You can find us at the Ventnor City Farmers Market on June 7, July 19, and Aug. 8; the Galloway Green Market on June 27 and July 25; the Brigantine Farmers Market on June 15 and July 13; and Brigantine Green Fest on Aug. 24.

Do you have a gardening related question you would like answered here? Please forward your questions to Belinda Chester, Master Gardener Program Coordinator, Rutgers Cooperative Extension Office, 6260 Old Harding Highway, Mays Landing, NJ 08330. You can also submit questions at Rutgers-atlantic.org/garden or email them to currents@catamaran.com; please include “garden question” in the subject line.

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