Swallowwort, Pale

Vincetoxicum rossicum

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Identification

Appearance

Vincetoxicum rossicum is an herbaceous, twining, long-lived perennial vine. The vines can reach 5 feet in length.

Foliage

Leaves are opposite, dark green, oval, shiny, 3-4 inches long and 2-3 inches wide. They are attached to the vine by a petiole about 0.4 inches.

Flowers

Flowering occurs in June to September, when dark pink to deep red, 5-petaled, star-shaped flowers appear in clusters. Flowers are approximately 0.25 inches across and are covered with white hairs.

Fruit

The fruits are similar to milkweed pods. They are slender, 2-3 inches long and split to reveal small seeds with tufts of white hairs. The hairs allow the seeds to be readily dispersed by the wind.

Biology

This species is Quarantined: Class A Noxious Weed

Origin

Pale swallowwort is native to Europe and was likely introduced into the United States for ornamental purposes. It can be found in Connecticut, Massachusetts, Maine and New Hampshire, where it can form large stands in human or naturally disturbed habitats. It is especially competitive in shallow soils over limestone, which makes it prone to coming into contact with rare native plants of limestone habitats.

Habitat

Man-made or disturbed habitats, forest edges, forests, meadows and fields, shores of rivers or lakes.

Threat

Vincetoxicum rossicum invades upland areas and tolerates a wide range of light and moisture conditions. It can grow rapidly over native vegetation to the point of dominating the understory of a woodland. Wind-dispersed seeds allow it to disperse over long distances. When it is cut, this plant resprouts vigorously, making control difficult.

Lifecycle

Pale swallowwort emerges in spring and flowers from June to July. The flowers are self-pollinating and the plant spreads primarily by seed. Fruit pods are formed shortly after flower production. Thick infestations in full sun can produce up to 2,000 seeds per square meter. The seeds are polyembryonic (i.e., having one to four embryos per seed) which greatly increases the potential for establishment. Wind dispersal of seed occurs from late July and continues into the fall. Populations growing under dense wooded canopy may have inadequate resources to produce flowers or seeds. Pale swallowwort dies back to the ground every winter. Pale swallowwort root crown fragments support dormant buds that readily sprout if not destroyed.

Management Options

Chemical

Be careful not to damage or kill nearby native plants when conducting management work.
Always read and follow pesticide label directions. Application of pesticides may require a certification from the Agency of Agriculture, Food and Markets. The Agency website provides information on what applicator certification is needed.

Two systemic herbicides - Garlon® 4 (triclopyr ester) and Roundup Pro® (glyphosate) – have been found to be effective in controlling pale swallowwort. These herbicides should be applied when plants are actively growing, after flowering has begun. Avoid the temptation to spray the plants as soon as they emerge in May. Only when the plants flower will they be large enough to receive enough spray on the exposed leaf surface to deliver a killing dose to the roots. Plants that are sprayed before pods form will probably not produce a viable seed crop that season. Be patient.

Cut stem application

For cut stem applications use a 50 to 100% solution of herbicide concentrate. Roundup Pro® is much more effective than Garlon® 4 for cut stem application. Apply the herbicide solution immediately to cut stem surfaces.

Foliar application

Experience shows that foliar sprays of systemic herbicides (i.e., herbicides absorbed into the plant and carried internally) only kill plants in the upper layers of the infestation, requiring repeated applications to effectively control the entire mass. It is important to treat plants before pods begin to form to ensure that viable seeds are not produced. If that is not possible, plants with pods should be cut or mowed first and then sprayed once they regrow. Regrowth will be rapid in summer. Herbicide application to the new growth should be conducted from August through early September.

Mechanical

Remove pod-bearing plants from the site and destroy them. Eradication on a small scale must be very thorough and requires dedication. The complete root crown must be dug out before the seeds ripen. Plants bearing seeds should be burned or bagged and disposed of in a landfill. Infested land might be brought under control by plowing and planting an annual crop until the seed soil bank is depleted, possibly as long as five years.

Mowing, even several times a year, will not eradicate swallowwort however, it can be employed to prevent a seed crop. Cutting is most effective at preventing a mature seed crop if done in early to mid-July, when there are small, immature pods on the plants. Cutting during the flowering period but before pod formation will allow plants to recover and still produce a viable seed crop. Monitor mowed areas and mow a second time if pods reach mature size in late summer or early fall. Hay cutters can contribute to the spread of swallow-wort if cutting is not timed correctly. Hay crops infested with swallow-wort and then sold elsewhere can be a means of introducing swallow-wort to new areas.