Cynanchum laeve (honey-vine climbing milkweed)

Cynanchum spp.

swallowwort

Apocynaceae Africa, Asia, and the Americas

Attracts Pollinators Deer Resistant Drought Tolerant

Native to North America

SunFull Sun – Part Shade

WaterLow · drought-tolerant

HardinessZones 4–8

Overview

Cynanchum spp. is a large genus of twining and climbing perennial herbs, vines, and subshrubs in the dogbane family (Apocynaceae), with somewhere between two and three hundred species spread through tropical and temperate regions worldwide. Most species twine anticlockwise to 3-10 feet (1-3 m), climbing over other vegetation, and exude a milky latex when cut. The opposite leaves are usually heart-shaped to lance-shaped, 1-5 inches (2.5-13 cm) long. Small five-lobed star-shaped flowers 0.1-0.4 inch (3-10 mm) across are borne in stalked clusters from the leaf axils and range from white and greenish to pink, maroon, and near-black depending on species. These are followed by slender paired follicles that split to release flat seeds, each carried on a tuft of silky hairs for wind dispersal. Several species, including the black swallowwort C. louiseae and pale swallowwort C. rossicum, have become aggressive weeds outside their native range, smothering vegetation and spreading by both seed and root. The milky sap of many species contains toxic compounds that deter grazing and can poison livestock. Because of this weedy and toxic reputation the genus is seldom cultivated, though a few species are grown in specialist collections.

Native Range

Cynanchum spp. occurs naturally across the warmer parts of every inhabited continent, with major centres of diversity in Africa, Asia, and the Americas. Individual species are native to habitats ranging from tropical forest margins to temperate grassland and rocky slopes. Several Old World species have naturalised as invasive vines in North America.

Suggested Uses

Most Cynanchum spp. species are not planted deliberately and are managed as weeds, particularly the invasive swallowworts of North America. A few non-aggressive species are grown in specialist and botanical collections for their dark, star-shaped flowers. The flowers attract a range of small pollinating insects during summer.

How to Identify

Recognise Cynanchum spp. as a twining perennial vine with opposite heart- to lance-shaped leaves and milky sap that bleeds from cut stems. Small five-pointed star-shaped flowers 0.1-0.4 inch (3-10 mm) across appear in axillary clusters, white through pink to dark maroon. Slender paired pods follow and split to release seeds tufted with silky hairs. The twining habit, paired follicles, and latex together distinguish the genus.

Appearance

Size & Dimensions

Height3' - 10'

Width/Spread3' - 6'

Colors

Flower Colors

Foliage Colors

Bloom Information

Bloom Period

~10 weeks

Spring Summer

Cynanchum spp. flowers chiefly from late spring through summer, with many temperate species blooming June to August. The small starry flowers open in successive clusters over several weeks. Paired seed pods develop through late summer and split in autumn to release plumed seeds.

Detailed Descriptions

Flower Description

white, pink, to dark maroon

Foliage Description

green

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade

Requires 4-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.5 - 7.5(Neutral)

357912

Soil Types

loam sand clay

Drainagewell drained

Water & Climate

Water Needs

Low

Frost Tolerance

hardy

Time to Maturity

2-3 years

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Cynanchum spp. species grow in full sun to part shade on a wide range of soils, tolerating dry, poor, and disturbed ground, and twine readily over supports or neighbouring plants. The vigorous climbing species spread by seed and creeping roots and can overrun gardens and natural areas, so the weedy types are managed by removal rather than planting. Most species need little water once established and tolerate drought through deep roots. The milky sap contains toxic compounds, and contact may irritate skin in sensitive people. Where invasive species occur, repeated cutting and root removal limit their spread. Few pests trouble the genus because the latex deters most herbivores.

Pruning

Pruning amounts to cutting back the twining stems to control spread, ideally before the seed pods ripen and disperse. Invasive species are cut repeatedly through the season and the roots dug out to weaken regrowth. Cut stems exude milky latex, which can stain and irritate skin.