Black Swallow-wort

Why it’s harmful

Black swallow-wort (also sold historically under the name Vincetoxicum nigrum) is a perennial twining vine in the milkweed family — and that family connection is exactly what makes it dangerous.

• It is an “ecological trap” for monarch butterflies. Because it’s related to milkweed, monarchs sometimes lay eggs on it, but the caterpillars cannot survive — so every egg laid there is lost.
• It forms dense mats that smother native plants, including the true milkweeds monarchs actually need.
• It releases chemicals that suppress neighboring plants (allelopathy) and is toxic to livestock and deer, so nothing grazes it back.
• Once established, it is one of the hardest invasives to eradicate.

How to ID it

• Vine: a slender, twining vine that climbs over other plants and fences, 3–6 feet long.
• Leaves: opposite, shiny dark green, oval with a pointed tip.
• Flowers: small, dark purple to nearly black, star-shaped with five petals, slightly fleshy and faintly hairy — blooming in early summer.
• Seed pods: slender, tapered pods (like skinny milkweed pods) that split open to release seeds carried on white silk.
• Sap: broken stems ooze a milky sap, like other milkweeds.

📸 5 photos to help you recognize it in different settings and seasons.Click any photo to enlarge and browse.

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How to remove it

This one rewards persistence above all.

1. Stop the seeds first. The single most important action is to clip off and bag the seed pods before they ripen and split — this halts the spread even if you can’t kill the plant yet. Trash them; never compost.
2. Small patches: dig out the entire root crown (the dense knot of roots at the base). Any rhizome left behind will resprout.
3. Mowing or cutting alone won’t kill it — it resprouts from the roots — but repeated cutting plus pod removal weakens it over time.
4. Larger infestations: a foliar herbicide (triclopyr or glyphosate) applied to actively growing plants, repeated over multiple seasons, is usually necessary.
5. Be patient: full control often takes several years of follow-up.