How to Grow Swamp Milkweed

One Way to Save the Monarch Butterflies

Swamp milkweed (Asclepias incarnata) beginning to bloom.

MichellePatrickPhotographyLLC / Getty Images Plus

Swamp milkweed (Asclepias incarnate) is a member of the milkweed family. Their flowers are beloved by bees and butterflies. All milkweed species serve as critical hosts for monarch butterflies.

Swamp milkweed grows as a herbaceous perennial, hardy in USDA Zones 3 through 6. Blooms come in shades of soft mauve to pink to reddish-violet or occasionally white. Five nectar cups form a crown on five small petals, ideal for intricate pollination. By autumn, narrow pods reveal flat brown seeds attached to the white tufts characteristic of all milkweeds.

Like most milkweed species, they are clump-forming. Sturdy, upright clumps grow on stems four to five feet tall and two to three feet wide. Leaves grow in pairs, stiff, taper-pointed, and lance-shaped. Sometimes the edges of a leaf turn inward and upward.

Common milkweed (Asclepias syriaca) is a single-stemmed variety and looks a little bit different, bearing large pink clusters on especially thick stems that are four to five feet tall. Swamp milkweed is a lesser-known cousin, native to the sunny openings of swamps, marshes, bogs, fens and other moist areas of North America.

Botanical Name Asclepias incarnata
Common Name  Swamp milkweed, Rose milkweed, Pleurisy Root, White Indian hemp 
Plant Type  Perennial
Mature Size  3 to 5 ft. tall, 2 to 3 ft. wide
Sun Exposure  Full sun
Soil Type  Clay, loam
Soil pH  Acid, alkaline, neutral
Bloom Time  Mid to late summer
Flower Color  Soft mauve to pink to reddish-violet, white
Hardiness Zones  3 - 6, USA
Native Area  North America
Toxicity  Toxic

Swamp Milkweed Care

The swamp milkweed is a fantastic addition to a native garden. This low maintenance perennial is ideal for sunny borders, cottage gardens, pollinator gardens, and along the edges of ponds and streams. The deep taproots should be left undisturbed once the plant is established.

Light

Full sun is best, but plants will tolerate part shade.

Soil

While common milkweed is able to grow in average well-drained soil, swamp milkweed thrives in moist, medium to wet clay soil. The plant can do well in average garden soil if it does not dry out. Wet meadows or rain gardens offer ideal conditions.

Water

Keep this plant moist. If given a proper wetland environment, the swamp milkweed will likely not need watering throughout the growing season.

Is Swamp Milkweed Toxic?

Flowers can be cut, but stems will leek a toxic, bitter, milky sap. The toxins are highest in the sap, while the least toxic part of the plant is the roots. Cardiac glycosides, contained in all milkweed species within the Asclepias genus, are poisonous to humans and animals.

Still, humans can consume milkweed. Its toxicity depending on the species, age, preparation, and amount. The Chippewa and Iroquois are known to strengthen the body and heal the navels of newborns with an infusion of swamp milkweed root. The Iroquois and Meskwaki have made decoctions of the roots and other parts of the plant as an emetic, diuretic, and anthelmintic. One other name for swamp milkweed is pleurisy root, inspired by the plant's once-common use to treat lung issues.

Be aware that it is toxic when consumed in large doses and should be prepared only with proper guidance. Avoid planting where children play and where livestock and poultry graze.

Symptoms of Poisoning

Consuming large amounts of any milkweed may induce any of the following symptoms:

  • abdominal bloating
  • respiratory issues
  • pupil dilation
  • muscle cramps
  • high temperatures

Toxicity and Monarch Butterflies

Monarch butterflies almost solely feed on milkweed and so collect the toxins in their bodies. Because birds poisoned by eating monarchs have learned to avoid them, the toxic properties of milkweed actually protect these rare butterflies during their migration.

Varieties of Swamp Milkweed

This species has several cultivars that are grown ornamentally. "Ice Ballet" and "Cinderella" are especially popular.

Propagating Swamp Milkweed

Swamp milkweed lives a long life of slowly spreading through rhizomes. It is possible to divide established plants in late spring if desired. For natural spread, allow the wind-borne seeds to find their way.

Growing Swamp Milkweed From Seeds

To propagate this plant by seeds, collect them in the fall after the brown pods have dried and started to split. Open each pod fully and let the seeds dry for one or two weeks in paper bags, being vigilant for signs of damage by weevil larvae.

When the seeds are dry, put them in plastic bags of moist perlite or vermiculite. Store for four to 12 weeks (this process is known as stratification) in a cool place that stays about 35 to 38 degrees Fahrenheit.

Without stratification, seeds will likely still germinate. Soak seeds in hot water for 12 hours. Repeat twice to increase seed germination rate. Store in a cool place for up to three years. Sow four to eight weeks before the last frost in germination trays with cells, which are preferably two inches wide and four inches deep. Fill with a seedling soil mixture or a mix of sphagnum peat moss and vermiculite. Moisten the soil. Press three seeds in each cell gently. Cover with a thin layer of soil. Spray or mist until the seeds germinate.

Give seeds light and warmth between 65 and 75 degrees Fahrenheit. Germination rates decrease significantly at higher than 85 degrees Fahrenheit. Transplant seedlings when they have one set, or more, of true leaves and the danger of frost has passed.

Common Pests/Diseases

Generally, swamp milkweed is pest free. But it does sometimes attract the orange milkweed aphid. If plants look unhealthy, spray with a soap solution or blasts of water.