Does it need a lot of water?

Yes, it prefers consistently moist to wet soil.

Will it spread aggressively?

No, it grows in clumps and does not spread by underground runners.

Plant guide

Swamp milkweed.

Asclepias incarnata

Pronounced as-KLEE-pee-us in-kar-NAY-tuh

Also known as Pleurisy Root, Swamp Milkweed

Swamp milkweed is a lovely native perennial that brings a splash of color to wet garden spots. It is a favorite for butterflies, especially monarchs, who rely on it for their life cycle. You will find it is quite hardy and easy to keep happy in the right conditions.

Another photo of Swamp milkweed — Photo by Mompkin (CC BY) via iNaturalist

At a glance

Care level: Easy
Plant type: Perennial
Light: Full sun
Indoor light: Direct sun
Water: Every 1 to 3 days
Mature size: 3 to 5 feet
Growth rate: Medium
Hardiness: USDA 3 to 9
Bloom time: Mid to late summer

Care

Keep the soil consistently moist, as this plant naturally thrives in damp environments. Watch for aphids on the flower buds, which are common but usually harmless. Ensure it gets plenty of sun to keep the stems strong and upright.

Watering: Keep the soil evenly moist at all times.
Feeding: A light application of compost in spring is usually plenty.
Pruning: Cut the stems back to the ground in late winter or early spring.
Repotting: Divide the root clumps every few years to keep the plant vigorous.
Soil: Moist to wet soil
Soil pH: 6.0 to 7.0
Temperature: -30 to 35°C
Humidity: 40 to 80%
Drought tolerance: Low
Lifespan: Perennial, typically 5 to 10 years
Fragrance: Strong
Deer and rabbits: High

Signs it needs repotting

Roots growing out of the drainage holes.
Water draining through the pot too quickly.
Stunted growth during the active season.

Safety

People: All parts of the plant are toxic if ingested due to the presence of cardiac glycosides.
Pets: This plant is toxic to cats and dogs if ingested.
Children: Keep children away from the plant as the milky sap is an irritant and the foliage is toxic if eaten.
Contact allergens: The milky sap can cause skin irritation in sensitive individuals.
Airborne allergens: The pollen is not considered a major allergen.
Edibility: The plant contains cardiac glycosides and should not be ingested by humans or pets.

How to identify it

Clusters of fragrant pink flowers
Lance shaped opposite leaves
Upright branching stems
Milky white sap in stems
Seed pods split open when ripe

Height: 0.9 to 1.5m
Spread: 0.3 to 0.6m
Leaves: green, deciduous
Flowers: pink, mauve, white, 1 to 1.5cm
Fruit: brown, Autumn

Problems to watch for

AphidsCommon
Milkweed BugsCommon
Leaf SpotUncommon
RustUncommon

By symptom

Plant problems by symptom

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Growing it

Propagation: seed, division
When to plant: Spring, Autumn
Overwintering: Cut the stems back to the ground in late autumn or early winter after the foliage has died back. No special protection is needed as it is very cold hardy.
In containers: It grows well in large pots that can hold moisture, but ensure the pot is at least 30 cm deep.

Seed

1. Collect seeds in autumn.
2. Store seeds in a cold, dry place.
3. Sow seeds in pots in early spring.
4. Keep soil moist until germination.

Division

1. Dig up the plant in early spring.
2. Carefully separate the root clump.
3. Replant divisions immediately.
4. Water thoroughly.

Plant in groups of three or more to create a strong visual impact and better support pollinators.

Grows well with

Joe Pye Weed · They share similar moisture needs and provide a great height contrast.
Blue Flag Iris · Both thrive in the wet conditions of a rain garden.

Similar species

Common Milkweed · It is a related species but spreads aggressively via rhizomes.
Butterfly Weed · It is a related species that prefers much drier soil.

Science

Family: Apocynaceae
Genus: Asclepias
Order: Gentianales
Class: Magnoliopsida
Native range: North America
Conservation: This species is secure throughout most of its native range.

It grows naturally in marshes and along stream banks where it provides essential food for monarch larvae.

Value to wildlife: larval host plant, nectar source
Pollinators: bees, butterflies, moths, hummingbirds
Uses: pollinator garden, rain garden, meadow planting, cut flowers

Through the year

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Flowering

Fruiting

Foliage

Ecological indicators

Salinity: 2/9
Soil reaction: 6/9
Light: 7/9
Temperature: 5/9
Moisture: 8/9
Nutrients: 5/9

Taxonomy resolved against GBIF.

Questions people ask

Does it need a lot of water?: Yes, it prefers consistently moist to wet soil.
Will it spread aggressively?: No, it grows in clumps and does not spread by underground runners.

Where the name comes from

The genus name honors Asclepius, the Greek god of medicine, while the species name means flesh colored.

Good to know

It is also known as swamp milkweed because it thrives in wet soil.
The flowers have a lovely vanilla scent.
It is much better behaved in garden beds than common milkweed.
The seed pods split open to release silky seeds carried by the wind.

Also in

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This guide was generated with AI on September 26, 2026 and checked against GBIF taxonomy. It is general information, not horticultural or veterinary advice. Read about how we build these guides.