How to Plant & Grow Swamp Milkweed (Asclepias incarnata)


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Swamp milkweed's pink flowers
Swamp milkweed has complex, pink blooms that attract a wealth of pollinators. Agnieszka Kwiecień, Nova, CC BY-SA 4.0, via Wikimedia Commons

Asclepias incarnata is an herbaceous perennial that is known for its showy features. This wildflower is commonly referred to as swamp milkweed, white Indian hemp, and rose milkflower. It is classified under the Apocynaceae or dogbane family, which includes several temperate to tropical taxa that are pharmacologically significant due to their toxic contents. Asclepias is no exception as many members of this genus produce potent substances. Complex flowers, with features comparable to those of orchids, set this genus apart.

Native to central and eastern USA, swamp milkweed is distinguished by its stunning umbel-shaped clusters of bright summer blooms. Its species epithet, incarnata, is based on the Latin word for flesh. This alludes to the colors of its flowers, which tend to be mauve-pink or dusty rose.

To fully appreciate the complexity of each bloom, you’ll have to peer closely at the compact inflorescences. Composed of 5 nectar cups crowning over 5 delicate and seemingly upturned petals, this species is undeniably invested in attracting a wealth of pollinators.  The clusters are borne on upright shoots (growing to 150 cm or 59 inches) arising from a bed of fleshy roots. The lance-shaped leaves are deep green and occur opposite one another.

Facts, Benefits & Uses of Swamp Milkweed

A monarch butterfly on a swamp milkweed flower
Swamp milkweed is adored by monarch butterflies; they even lay their eggs on the plant! Don Faulkner, CC BY-SA 2.0, via Wikimedia Commons

A definite must-have in butterfly gardens, swamp milkweed is irresistible to monarch butterflies (Danaus plexippus). Monarch Watch, a nonprofit organization dedicated to tracking the movement of monarchs, recommends cultivating this species in areas that can serve as migratory waystations to support local butterfly populations. Monarchs don’t simply pollinate milkweed blooms; they also lay their eggs on the plant.

Apart from attracting butterflies, swamp milkweed nectar is a nutritious food source for bees, flies, moths, beetles, and wasps. The fragrance emitted by the flowers is likened to that of cinnamon! Perfect for rearing alongside other moisture-loving plants, dense stands can be situated close to a water feature to create a truly stunning summer garden. When conditions are optimal, the flowering period can even occur twice in a single growing season.

Non-ornamental uses of milkweed include twine and rope production with its tough stem fibers, the use of seed down to stuff pillows, and the infusion of roots for medicinal purposes. Considered deer-resistant due to its toxicity profile, A. incarnata can be planted next to susceptible species. Despite its slow spread and long lifespan, it is unfortunately underutilized in private gardens. If you’ve been looking for a pond edge plant to provide seasonal interest year after year, definitely consider this one!

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Swamp Milkweed Fact Sheet:
PLANT TYPE
Herbaceous perennial
HARDINESS ZONES
USDA 3 – 9
LIGHT REQUIREMENTS
Full sun to partial shade
BLOOM COLOR
Pink, white
BLOOM PERIOD
Summer
MAXIMUM GROWTH
5 feet (1.5 m)
PLANTING DEPTH
Seeds on soil surface, seedlings with crown just above soil line
WATER QUALITY
pH 6 – 8

Swamp Milkweed Growth, Hardiness & Climate

Swamp milkweed plants by a pond
Swamp milkweed thrives in wetland conditions and prefers slightly acidic soil. Andrey Zharkikh from Salt Lake City, USA, CC BY 2.0, via Wikimedia Commons

A. incarnata grows best in areas with wetland conditions. In the wild, dense stands can be found close to swamps, bogs, and marshes or along the banks of ditches and streams. The fleshy taproot system, which tends to remain shallow but can occasionally reach a depth of more than 10 inches (25 cm), favors loamy soil with a high organic matter content. The soil should be kept moderately moist as this species is averse to extended periods of dryness. It can survive in prairies, roadsides, and fields, but will grow best close to the shore of a water source.

Full to partial sun conditions, with direct sun exposure lasting for up to 6 hours a day, is required for maximum growth. Slightly acidic soil is preferred, though the plant can tolerate a pH of up to 8.0 as long as other ambient conditions are optimized. Hardy to USDA zones 3 – 9, swamp milkweed favors temperatures that range from 65 – 75˚F. It relies on insect pollination for fertilization and disperses its seeds via air.


How to Plant Swamp Milkweed

Opened swamp milkweed seed pods
You can collect swamp milkweed seeds from their seed pods once they have dried out. Photo by and (c)2009 Derek Ramsey (Ram-Man), GFDL 1.2, via Wikimedia Commons

This species is often planted via seed and purchased from plant nurseries as plug plants. Seed pods should be collected once they have dried out and are ready to be cracked open. Keep the seeds in a paper bag for about 2 weeks prior to storing them in a cool area for up to 12 weeks. For rapid germination the following spring, keep the storage temperature at a range of 35 – 38˚F (1 – 3˚C). Prepare a shallow germination tray with about 4 inches (10 cm) of moist substrate. A store-bought seedling mix or a sterilized peat moss-based mix should suffice.

Once the stratification period is over, spread the seeds on the soil surface, making sure they are lightly spaced apart. Gently push them into the soil prior to coating them in a fine layer. If the substrate moisture levels and ambient temperature are maintained, seedlings should take 4 – 8 weeks to develop their first set of true leaves. Once they are large enough to handle, they can be transplanted into individual pots.

If planting established seedlings or store-bought plug plants outdoors, choose an area with minimal weed growth and ample soil moisture. Plants should be spaced at least 18 inches (46 cm) apart to allow them to reach their maximum spread. Make sure to lightly water the seedlings right after planting. Until they are about 3 – 5 inches (7.6 – 12.7 cm) tall, their roots may struggle to survive in excessively damp soil.


How to Care for Swamp Milkweed

Swamp milkweed in bloom
Try to make sure that swamp milkweed receives full sunlight for optimal growth. USFWS Mountain-Prairie, Public domain, via Wikimedia Commons

This low maintenance plant should not be grown next to highly competitive vegetation. As plant density increases, swamp milkweed shoots may struggle to compete as it is a slow grower and seldom produces flowers within the first year of growth. Make sure to periodically remove weeds that may smother the crown of the plant.

Keep the soil moist to encourage optimal growth and flowering each year. As much as possible, ensure that the shoots receive full sunlight. Regularly monitor the leaves for aphids, which inevitably make their way to milkweeds as they favor the sap. Manually remove these or neutralize them using a homemade detergent spray. If aphid colonies spread, the plant may begin to look sick. Ideally, as the plant attracts a fair number of pollinators, aphid populations should naturally be controlled by their predators.


How to Winter Swamp Milkweed

As perennials that are naturally found in temperate zones of the US, milkweed plants will generally have minor requirements during the cold months. The plant may become dormant after the first few frosts, causing its shoots to die back. It is advisable to cut down the shoot to a height of about 6 inches (15 cm) above the soil level to prevent winter decay.

Pruning, with the use of sterilized, sharp garden shears, can be done in either fall or spring. Some gardeners opt to wait until early spring as birds may benefit from the fibers of the shoots and fluff of seed pods, which they can use for their nests. The roots and crown of the plant may be protected by a generous layer of mulch.


Is Swamp Milkweed Invasive or Toxic?

A. incarnata is non-invasive and is known for being quite weak as a competitor. It can be reared next to other slow growers that share similar requirements. Gardeners cultivating this species must be aware of its toxic profile. All plant organs contain varying concentrations of resinoids and cardiac glycosides. When consumed in large amounts, these can cause diarrhea, weakness, and stupor.

Livestock are generally averse to swamp milkweed leaves as they are said to be bitter. Sheep, horses, and cattle are especially sensitive to the plant’s toxic compounds, which can result in fatalities. Some Native American communities have managed to find ethnobotanical uses for these. Infusions and decoctions of the roots and some other selected parts can supposedly help strengthen the body when consumed in minimal doses. The plant and other Asclepias species, known by some tribes as Pleurisy root, were even used to treat lung problems once.


Is Swamp Milkweed Edible? Do Animals Eat it?

Due to its toxic characteristics, swamp milkweed is not considered an edible plant. Some insects, such as the larvae of monarch butterflies and aphids, are able to consume the plant without harm. Deer, rabbits, and other herbivorous animals will likely stay away.

If you have pets that regularly roam around your garden, make sure they don’t attempt to eat this plant. They may unfortunately be attracted by the strong fragrance of the flowers. Always keep a watchful eye on them as they roam the garden or fence off areas with milkweed stands.


Where to Buy Swamp Milkweed & Seeds? (UK & US)

Asclepias incarnata can be purchased as seeds, potted plants, and plug plants from plant nurseries and garden centers across the US. If located elsewhere, you are more likely to obtain this species as seeds from online plant portals. Be on the lookout for the ‘Cinderella’ and ‘Ice Ballet’ cultivars as these are quite popular among native plant gardeners.

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