What is black swallow-wort?

Quick, accurate black swallow-wort identification is worth the effort because this vine is still uncommon in much of Pennsylvania, and small patches can be wiped out before they spread. It’s a perennial twining vine from southwestern Europe, brought to North America in the 1800s as an ornamental.

It’s in the same family as milkweed, which is the root of the problem. Monarch butterflies sometimes lay eggs on it, but the caterpillars can’t survive on it. Dense patches also crowd out the native plants in old fields and woods edges.

The Pennsylvania Department of Agriculture lists it as a Class B noxious weed. That means it can’t be sold or moved, and it’s worth reporting or controlling if you find it.

Black swallow-wort identification: key features

From a distance it looks like a tidy, glossy vine. Up close, a few features give it away.

• Growth: stems twine around anything they touch, including each other, forming thick green ropes 3 to 6 feet long. Without support they sprawl across the ground.
• Leaves: opposite pairs, shiny and dark green, oval to heart-shaped with a pointed tip and smooth edges.
• Flowers: tiny, five-petaled, dark purple stars with fine hairs, clustered where the leaves join the stem. Look for them in June.
• Pods: slender, smooth, 1.5 to 3 inches long, shaped like small milkweed pods. They turn from green to brown and split open from mid-August into fall.
• Seeds: flat and brown with tufts of white silk that carry them on the wind.

A close cousin, pale swallow-wort, has pinkish to maroon flowers. It’s just as invasive and is handled the same way.

Where it grows in Pennsylvania

Swallow-wort is much more established in New York and New England than in most of Pennsylvania, which makes every early find here important. It moves into disturbed ground: roadsides, old fields and pastures, fence rows, quarry edges and the sunny edges of woods.

It tolerates almost any soil, dry or damp, and grows in full sun to part shade. Deep shade slows seedlings down, but established plants hang on there too.

Black swallow-wort look-alikes

The most common mix-up is with hemp dogbane, which grows in the same old fields. Dogbane stands upright, has white to greenish flowers and bleeds milky sap. Swallow-wort twines and its flowers are dark purple.

Common milkweed is a stout upright plant with large leaves and pink flower balls. Pennsylvania also has a rare native, smooth swallow-wort, with creamy white flowers and long-stalked heart-shaped leaves, found mainly along the Susquehanna. Hedge bindweed twines too, but has alternate arrow-shaped leaves and big trumpet flowers.

How to get rid of black swallow-wort

This is a multi-year job. The root crown resprouts, and plants that look dead often come back the next spring. Combining methods works best.

• Dig small patches: use a shovel or digging fork to get the entire root crown. Leftover pieces regrow, so check the hole and sift the soil.
• Pull pods: on a few plants, picking green pods before they split stops the seed. Bag them; don’t compost.
• Cut at the right time: cutting after flowering, mid-June to early July, prevents seed but won’t kill the roots. Cutting alone can even make a patch denser.
• Herbicide: extension services report triclopyr or glyphosate foliar sprays work best when timed around flowering and early pod set, with a follow-up later in summer. Cutting first, then treating the regrowth about six weeks later, has given good results.

Read and follow the herbicide label (the label is the law), and confirm it’s labeled for the site, particularly near water. Spraying for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, so larger patches should go to a licensed vegetation-management company.

Bag all root crowns and pods and put them in the trash. Don’t leave them on the ground or in a compost pile.

Keeping it from coming back

Seeds blow in, so check the area every June when the purple flowers make it easy to spot, and again in August before pods open. Pull or treat any new seedlings right away.

Don’t leave the ground bare after removal. Seed it to grass or plant tough natives so there’s competition. Clean mowers and tools before moving from an infested area to a clean one, and never move soil from a swallow-wort patch to another part of your property.

If you think you’ve found it, take clear photos of the leaves, flowers or pods and contact your county Penn State Extension office to confirm the ID. Because it’s still patchy in Pennsylvania, a confirmed sighting helps local land managers watch for it. And if a neighbor has it too, working on both sides of the fence at once saves everyone a lot of repeat work.

Questions we get about black swallow-wort

Similar weeds

Hemp DogbaneApocynum cannabinumOriental BittersweetCelastrus orbiculatusPorcelain BerryAmpelopsis glandulosa