Swamp milkweed

Sun

Full sun is best; tolerates light partial shade, but may flower less and lean.

Water

Keep the root zone consistently moist, especially during establishment and hot weather. Established plants tolerate brief dry spells, but prolonged drought causes leaf loss.

Soil

Moist to wet, humus-rich soil, including clay. Tolerates periodic flooding, but is not a plant for permanent deep water.

Height

3-5 ft (0.9-1.5 m)

Spread

2-3 ft (0.6-0.9 m)

Flowers

Summer into early autumn; clusters of pink to mauve-pink flowers, occasionally white.

Foliage

Opposite, narrow, lance-shaped green leaves on upright stems; dies back for winter and can emerge late in spring.

Growth

Moderate; usually forms a substantial flowering clump within two or three growing seasons.

Care needed

Low

• Water deeply during dry spells rather than letting the soil repeatedly dry out.
• Apply a thin layer of compost in spring; avoid heavy nitrogen feeding, which encourages floppy growth.
• Mulch with 2-3 in (5-8 cm) of organic material, keeping it clear of emerging shoots.
• Use discreet supports early if stems lean in exposed sites or partial shade.
• Leave established clumps undisturbed unless division is necessary; their deep roots make moving them difficult.

Pruning. Leave dead stems standing over winter, then cut them to 4-6 in (10-15 cm) above ground in late winter or early spring before new shoots develop. To limit self-seeding, remove seed pods before they split, checking first for caterpillars.

Late winter

Cut back old stems, leaving short stubs, and keep removed stems loosely stacked outdoors where practical for overwintering insects.

Spring

Plant container-grown plants once the soil is workable; water thoroughly and mulch.

Late spring

Check for emerging shoots before disturbing apparently empty ground; add supports if needed.

Summer

Water during dry weather and check growing tips for aphids without disturbing butterfly eggs or caterpillars.

Early autumn

Collect ripe seed or remove pods before they open if you do not want seedlings.

Late autumn

Sow fresh seed outdoors in a protected pot or seedbed so winter cold can help break dormancy.

• Plant in groups along the moist edge of a rain garden, above any permanently submerged area.
• Combine with Joe-Pye weed and cardinal flower for a tall, summer-flowering border in damp soil.
• Thread through moisture-retentive meadow planting with switchgrass for upright structure and soft pink colour.
• Use near a pond margin where its flowers can be watched for visiting insects without putting its crown under deep water.

Garden styles it suits: Native meadow, Wildlife garden, Pollinator garden, Bog garden, New perennial (prairie) and Cottage garden.

Plant it with: Joe-Pye weed, Cardinal flower, Great blue lobelia, Turtlehead, Switchgrass and Blue flag iris.

Wildlife: Flowers provide nectar for bees, butterflies and other insects. It is a monarch butterfly host plant in North America and can also support monarch caterpillars where the butterfly is established elsewhere. Caterpillar feeding may strip leaves; avoid insecticides if you are growing it as a host plant.

• Oleander aphids can crowd soft shoots. Tolerate small colonies or dislodge them with water after checking for butterfly eggs and caterpillars; avoid broad-spectrum insecticides.
• Rust and fungal leaf spots may mark foliage in humid conditions. Improve spacing, water at soil level and remove badly affected fallen leaves.
• Prolonged dry soil causes wilting, yellowing and early leaf drop. Water deeply and mulch, or move the plant to a more moisture-retentive site while dormant.
• Too much shade or nitrogen can produce weak, leaning stems. Provide more sun, reduce feeding and support stems if necessary.

Toxic to cats, dogs, horses and people if eaten. All parts, including leaves, stems, roots and seeds, contain cardiac glycosides; ingestion can cause digestive upset, abnormal heart rhythms and potentially fatal poisoning. The milky sap can irritate skin and eyes. Keep it away from grazing animals, do not include it in hay, and seek urgent medical or veterinary advice after suspected ingestion.

• Common milkweed (Asclepias syriaca)

  Has much broader, thicker leaves and usually rounded, drooping flower clusters; spreads strongly by underground shoots, unlike the more clump-forming swamp milkweed.

• Butterfly weed (Asclepias tuberosa)

  Usually has orange flowers, alternate rather than opposite leaves, and no obvious milky sap; prefers drier, well-drained soil.

• Tropical milkweed (Asclepias curassavica)

  Typically has red-and-yellow or all-yellow flowers rather than pink clusters and is frost-tender, remaining leafy in warm climates.

Asclepias honours Asclepius, the Greek god of medicine, while incarnata means flesh-coloured, referring to the pink flowers, and swamp milkweed describes its moist habitat and milky sap.

Propagation. Raise from seed, using fresh autumn-sown seed outdoors or about 30-60 days of moist chilling in a refrigerator before spring sowing. Seedlings may take more than one growing season to flower. Established plants can be divided carefully in early spring, but their deep roots make seed propagation less disruptive.

Accepted botanical name

Asclepias incarnata

In the US and Canada

Native in Canada and the lower 48 states.

Native range

Alabama, Arkansas, Colorado, Connecticut, Delaware, District of Columbia, Florida, Georgia, Idaho, Illinois, Indiana and Iowa.

Measured

Soil pH 5 to 8. Average height 95 cm.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.