How to Grow and Care for Swamp Milkweed

Your Contribution to Saving the Monarch Butterfly

Swamp milkweed with small pink flower clumps and buds with monarch butterfly on top

The Spruce / Adrienne Legault

Swamp milkweed is a member of the milkweed family. Its flowers are beloved by bees and butterflies. Like other milkweed species, it serves as critical host for monarch butterflies.

The blooms of swamp milkweed 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, swamp milkweed is 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.

Milkweed plants can be planted in spring or fall

Milkweed is toxic to humans, and toxic to pets.

Common Names Swamp milkweed, rose milkweed, pleurisy Root, white Indian hemp 
Botanical Name Asclepias incarnata
Family Apocynaceae
Plant Type  Perennial, herbaceous
Mature Size  3-5 ft. tall, 2-3 ft. wide
Sun Exposure  Full
Soil Type  Clay, loam
Soil pH Acidic, neutral, alkaline
Bloom Time Summer
Flower Color  Pink, white
Hardiness Zones  3 -6 (USDA)
Native Area  North America
Toxicity  Toxic to humans, toxic to animals

Swamp Milkweed Care

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. It is best to plant it in a permanent location because the deep taproots should be left undisturbed once the plant is established.

Milkweed is slow to emerge from its dormancy so don't worry if it lags behind other perennials. As the soil warms up, it will send up new shoots. The same applies to bare-root milkweed, which is how some nurseries ship it in the spring. Plant it promptly upon arrival when the soil can be worked and keep it moist but not overly wet.

If you plant swam milkweed in the fall, give it enough time to get established before the cold weather sets in.

Swamp milkweed plant with small pink flowers clustered on edge of stems with orange butterfly in middle

The Spruce / Adrienne Legault

Swamp milkweed plant with small pink flowers and buds clustered at end of stems

The Spruce / Adrienne Legault

Swamp milkweed plant with small pink flowers clustered at end of stem with long thin leaf blades

The Spruce / Adrienne Legault

Light

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

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.

Varieties of Swamp Milkweed

While the straight species (Asclepias incarnata) is the best to plant if you want to attract monarch butterflies, you can find milkweed cultivars that have been bred of landscape use. Popular ones include:

  • 'Ice Ballet' has vanilla scented, pillowy clusters of snow-white flowers.
  • 'Cinderella' with vanilla-scented flowers that are deep pink with white centers.

Pruning

Swamp milkweed does not need any pruning during the growing season. After the plant has died back in the winter, you can remove the dead stalk, just make sure to mark the location, as swamp milkweed is slow to emerge in the spring.

Propagating Swamp Milkweed

Swamp milkweed spreads through rhizomes. Dividing established plants in late spring can be done but the long taproot does not like to be transplanted. For a natural spread, allow the wind-borne seeds to find their way, or start it from seeds.

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. Discard any seeds that show signs of damage by weevil larvae.

  1. 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.
  2. Sow the seeds four to eight weeks before the last frost in seedling trays with cells, which are preferably two inches wide and four inches deep. Fill with a seedling soil mixture and moisten the soil. Press three seeds in each cell gently. Cover with a thin layer of soil. Spray or mist.
  3. Keep the seeds moist and give them light and warmth between 65 and 75 degrees Fahrenheit. Germination rates decrease significantly at higher than 85 degrees Fahrenheit.
  4. Transplant seedlings when they have one set, or more, of true leaves and the danger of frost has passed.

Potting and Repotting

You don't need a back yard to grow swamp milkweed, you can also grow it in containers; in fact, because milkweed tends to spread, some gardeners even prefer to grow it in pots. The container should be at least 14 inches in diameter and have good drainage holes. Terra cotta is ideal because it is heavier than plastic so the tall milkweed is less likely to topple over. Fill the container with well-draining potting mix and keep it evenly moist at all times.

Repot the plant when to roots fill the container and start to grow out of the drainage holes.

Overwintering

Swamp milkweed that has been planted in the ground is winter hardy to USDA zone 3 and does not need any protection. When grown in containers, on the other hand, the roots are vulnerable to cold damage in freezing temperatures. Place the container in an insulating silo, or cover it with burlap and bubble wrap.

Common Pests/Diseases

The main pest that bothers swamp milkweed is the colorful orange and black milkweed leaf beetle that feeds on the juices inside the leaves. A few beetles won't harm the plant but you want to stop them before the infestation gets heavy. Usually the beetles can be simply washed off with a garden hose.

How to Get Milkweed to Bloom

If your swamp milkweed does not bloom, it could be that the plant is too young. Milkweed may not flower in the first year because it is putting its energy into developing roots. Excess nitrogen in the soil, such as runoff from a fertilized lawn, can also lead to failure to bloom. And finally, plants that are drought- or heat-stressed will also not bloom.

FAQ
  • What's the difference between swamp milkweed and common milkweed?

    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.

  • Is swamp milkweed invasive?

    When planted in its native range in the eastern U.S. and Canada, swamp milkweed is not invasive. It does spread though so if this is a concern, plant it in a container or a large planter.

  • Should you deadhead swamp milkweed?

    Removing spent flowers is not necessary but can be done for. neater appearance and it can prolong the bloom.

Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Toxic Plants. University of California.

  2. Milkweed. U.S. Department of Agriculture, Agricultural Research Service.