How to Plant, Grow, and Care for Swamp Milkweed

The fluffy silks of various milkweeds floating around in the fall breeze are a sure sign of the imminent season change. I always wonder where the seeds landed after parachuting around and how many will successfully pop up across our town and beyond next season. Even after snow arrives, I find something lovely about the brown and desiccated pods on a winter landscape. 

Sometimes thought of as an obnoxious weed, milkweed gets a bad rap. Cultivating milkweed is a great way to attract native bees, birds, and butterflies, adding a creative, wild look to gardens. Let’s discuss how to plant, grow, and care for swamp milkweed.

What Is It?

Swamp milkweed is a flowering, herbaceous perennial herb known for attracting native Monarch butterflies, bees, and birds and its ability to grow in marshy, wet regions. 

Brief History

Milkweeds love moisture and swampy marshlands, so their name is a nod to this love and the milky sap that comes out when broken open. The genus name Asclepias is for the Greek god of medicine, Asklepios. 

Botanists always knew the milkweed and dogbane families were related, and recently, milkweed was shifted into the dogbane family as a unique, flowering subfamily. 

Importance in Nature

Swamp milkweed is a valuable food source for native bee species, bumblebees, honeybees, and the Monarch and Queen Monarch caterpillars. Adult Monarch butterflies sip the nectar from flowers and lay tiny eggs on the underside of leaves so the caterpillars have immediate access to food when hatched. If you see eggs on a milkweed leaf, leave it be. Monarchs can’t survive without it!

Other butterflies, hummingbirds, milkweed bugs, and tussock moths flock to the flowers for their delicious nectar. It does not seem to be poisonous to deer and rabbits, who like to nibble on it. In the fall, silks emerge from the mature pod to spread its seeds. Native birds use the silks for building soft, cozy nests. 

Did you know? Milkweed sap contains cardiac glycosides that are toxic to some creatures, but Monarch butterflies have evolved to be unaffected by them. These compounds disrupt molecular functions and can be sensed by natural predators, making the larvae unpalatable. Nature is amazing!

Appearance

One of the ways to distinguish this milkweed from others is by its flower positioning. Their mauve to bright pink flower clusters, or umbels, sit atop the stalks rather than shooting out from the sides of the main stem and are much more compact.

The lightly vanilla-fragranced flowers comprise five little cup-shaped petals surrounding a central horn-like stigma, with tiny stamens surrounding it bedecked with pollen. Each time a pollinator feeds on the nectar, it inevitably slips on the stigma, coating it with pollen from the stamens. That pollen then makes its way to the stigma as the pollinator moves, fertilizing the flower.

The smooth, dark green leaves are much narrower and stiffer than other milkweeds and grow in pairs from the tall, sturdy main stem. The leaves may turn purple in late summer or fall, which is normal. They’ll return to green in the spring. Like common milkweed, a milky sap oozes when cut or broken open.

Native Area/Habitat

Swamp milkweed is native to eastern Canada and the United States, except those along the Pacific coastline. 

Check out the Xerces Society milkweed finder to determine if it naturally occurs in your area and see how you can be a part of protecting the Monarchs. 

How to Grow

This plant grows in bogs, marshes, swamps, and in moist, bottomland areas, including in ditches or along ponds and banks. They thrive in marshy, wet soil in areas with lots of sun. 

Light 

While they will tolerate partial shade, swamp milkweed plants prefer an area with full sun. Young seedlings especially need this light to develop. Without enough light, seedlings will etiolate, stretching towards the nearest light source and weakening. Ideally, these plants require 6-8 hours of sunlight per day for best growth.

Water

When grown in marshy wetland areas, supplemental watering is not needed. Swamp milkweed is a great option if your growing area is particularly wet. Interestingly, these plants become drought-resistant once established. 

Soil

Moderately moist clay soil is preferred. Your standard soil mix can be used if grown in a backyard garden. Don’t let it dry out; give it extra TLC in the first year or two as it establishes. Soil should be neutral to slightly acidic (6.0-7.0). 

Temperature and Humidity 

The temperature for germination should be below 85°. Any higher, and rates decrease. Plants prefer temperatures to be between 65-75° when growing. High humidity welcomes fungal diseases. 

Fertilizing 

No fertilizer is required, but an annual spring feeding of compost or a slow-release general fertilizer doesn’t hurt. Mix this into the soil when establishing new plants and top or side-dress established plants. 

Pruning

Pruning during the growing season is not necessary. Cut back the stalks to about 6 inches above the ground in the late fall or winter. Do so after the plant has produced mature seed pods and has had a chance to spread. Swamp milkweed may slowly emerge in the spring, so use tags or markers to indicate where they are. 

Deadheading

Deadhead after the first flush of flowers to encourage a second flush. Only do this once, though, if you want your plant to reproduce. Use gloves as the sap can be irritating to human skin. 

Mulching

Milkweed is just fine without mulch, but if you live in a particularly dry region, mulching may help retain the moisture in the soil, which your milkweed will appreciate. 

Propagation 

Swamp milkweed can be started easily from seed or propagated by division, rhizome spreading, or stem cuttings. It’s slower to spread than its cousin, common milkweed, and typically doesn’t become invasive when grown in its native areas. 

Growing From Seed

Collecting seeds in late fall, around October and November, can be a fun annual activity. Give the pods a quick squeeze. If they pop and the innards are exposed, peek inside and confirm the seeds are medium brown. If they’re any lighter, leave them for a bit longer to mature.

Once you have collected mature pods and opened them, release the seeds from the follicle using your fingernail as a rake, going with the grain. Collect them into a paper bag or box to avoid mold. Let them dry out before storing them in paper envelopes. 

Try winter sowing the seeds to provide them with the natural cold stratification cycle needed for successful germination, which can take 10-14 days. Sow them in milk jugs or pots and keep them outdoors for the winter.

Protect them from critters and harsh winds. They’ll germinate when the snow melts and spring signals it’s time to wake up. Alternatively, you can store the seeds in your refrigerator until the spring and sow as you would normally. 

Growing From Cuttings

Taking stem cuttings is a way to start new plants without the hassle of seed-starting or digging up rhizomes. When the stem is at least ⅓ inch in diameter, cut the stem using clean, sharp shears. Dip the bottom in rooting hormone and then pot it up into fresh, moist soil. Keep it moist. 

You can transplant this new plant in six to ten weeks or allow it to grow in a pot for a season or two as it establishes, stepping up the size container as needed. Don’t let the soil dry out; protect the pot from freezing. 

Rhizome Division

The root system is slow-growing and rhizomatous, which lends itself well to division every few years. This will prevent it from getting out of control. 

You can divide this plant. However, since it develops a strong, deep taproot and does not like root disturbance, dividing with hopes of starting a new plant is not always a successful venture. 

To divide, use a sharp spade or shovel, dig down around the base of the plant, and locate the clump of rhizomes. Divide the clump of rhizomes with your tool to include one or two buds each. This should be done when the plant is dormant, around October. Replant them right away so they can establish roots before winter.

Ample irrigation for the first year will give them their best shot at surviving. Remember, they won’t bloom for one to two years while they work to establish a healthy root system. 

Natural Seed Spread 

Seeds easily blow out from the seed pods in the fall, allowing them to spread naturally. This is a fun route if you don’t mind where the seeds land. 

Planting 

Swamp milkweed is sensitive to root disturbance, so transplanting takes patience. Get the timing right to avoid transplant shock. 

Spacing 

Swamp milkweed will form a rhizomatous clump about 36” wide and naturally spread. Plant them close and plan to thin each year or so to give them at least 36” each. 

Transplanting

To prevent transplant shock, transplant seedlings when they’re about three inches tall and have at least one set of true leaves. They could lose their leaves from the inevitable shock, but it’s ok! The plant just focuses its energy under the soil surface on the root system. When it’s ready, it will produce new leaves. 

Many growers suggest starting seeds in peat pots to avoid this issue altogether. Milkweed is extremely sensitive to root disturbance, so placing a peat pot into the garden will allow the seedling to settle in without much trauma. If you decide to start in regular pots, use ones that are large enough that will not allow the plant to become rootbound. 

Transplant in the spring after the risk of frost has passed and when the soil temperature is around 60° or above or in late fall. Dig a hole about the same depth as the root system and double the width. Place your plant in the hole and backfill the native soil, tamping down as you go. You may choose to fertilize lightly at this time, but swamp milkweed performs just fine in poor soil. 

Location

Soggy meadows, rain gardens, and lowlands are ideal locations. You can grow it in backyard garden soil. Just don’t let it dry out, especially in the first year. Plant it in a place that’s guaranteed full sun. It will spread, so don’t plant it somewhere it will interrupt your other garden perennials’ growth. 

Blooming

During its first year, while building roots and getting established, swamp milkweed won’t bloom. It may take another season or two before you see blooms. But remember, the strong root system will help it survive our ever-fluctuating climate, making it more resilient to drought, flooding, and extended cold and hot weather periods. 

Popular Varieties 

Swamp milkweed can be grown if you’re looking to attract native moths, butterflies, and Monarchs. However, many have been bred for landscaping and offer slightly different fragrances. 

‘Soulmate’

This cultivar has lovely pink petals surrounding a tiny white heart. ‘Soulmate’ matures at just 3-3 ½ feet tall and blooms from mid to late summer. Its sweet vanilla scent lures in hummingbirds and other pollinators. 

‘Ice Ballet’

This unique cultivar features vibrantly white flowers. It is a great backdrop to a showy, colorful garden and highlights the bright wings of Monarchs and bees feeding on their nectar. It will mature to 3-3 ½ feet tall and 2- 2 ½ wide. Plant it somewhere near a window to take advantage of its vanilla perfume. 

‘Cinderella’

More of a Barbie pink, ‘Cinderella’ blooms consistently from midsummer to early fall. Its clusters are full, dense, and vanilla-scented. It typically grows to three to four feet tall but has been known to reach five feet. Try using their silks in dried flower arrangements or wreaths in the fall. 

Common Problems

Few pests or diseases affect swamp milkweed to the extent of demise. Most are more of an annoyance and can be easily avoided and controlled. 

Pests 

Tussock moth caterpillars (Euchaetes egle)

Lovingly nicknamed the tiger milkweed moth, this fluffy striped orange, back, and white caterpillar prefers common milkweed but can be found feeding on different varieties. Natural predators help control their populations and keep them from destroying your milkweed patch, and its small populations are usually harmless. We only mention it because it’s another moth species you will likely see snacking on your milkweed!

Oleander aphids (Aphis nerii)

Aphids are a common pest among milkweed and occur all across North America. They make your plant just unhealthy enough to affect the viability and germination rates of next season’s seeds by sucking out its juices.

They can be controlled by gently rubbing them off between your fingers, hosing them down, introducing natural predators, and planting onions or marigolds nearby to repel aphids.

Thrips (Thysanoptera order)

Thrips, or freckle bugs, nicknamed for the freckling effect they give plants after feeding on them, don’t typically cause an issue. However, if you see them, they can be controlled with a strong jet stream from your garden hose.

Ladybugs and lacewings in a healthy ecosystem should take care of them for you. You can also use insecticidal soap, but as it has risks to your butterfly and moth species, limit it to only when needed.

Slugs

As you may know, slugs come out to feed at night, so catching them in the act is hard. An organic slug and snail bait made from iron phosphate will kill them once ingested but does not affect other wildlife. Sprinkle the pellets at the base of the plant.

Milkweed leaf miners (Liriomyza asclepiadis)

The larvae of milkweed leaf miners feed on the in-between leaf layers, removing all the nourishment Monarch caterpillars need. Remove affected leaves to rid your plant of this problem. 

Milkweed bugs (Oncopeltus fasciatus)

These guys will help milkweed from spreading, so if you don’t want it to reproduce and you see them, leave them be. They eat some, but not all, of the flowers and seeds. They’re also good for your local ecosystem, so removing them if they’re not doing too much damage could be harmful.

However, females can lay up to 2000 eggs per month, so if an infestation overtakes your milkweed patch, hand-pick them into soapy water. Insecticidal spray should be avoided. It may prevent Monarchs from laying their eggs there if residue remains on the plant.

Milkweed beetles (Tetraopes tetrophthalamus)

These beetles are usually harmless and should be left alone unless they are wreaking havoc. These beetles can be controlled by hand-picking or tossing them in soapy water. Their larva hide in the soil and sometimes burrow into the stems, so they’re often missed when gardeners are scouting their plants for pests. 

Diseases

Leaf Spot

More common in areas with long stretches of rainfall and moist environments, leaf spot is caused by an air or waterborne fungus. Spots are brown, black, or red and may coalesce into one larger spot. If the infection is severe enough, leaves may drop off. Trim the affected areas off and dispose of them in an area where spores cannot spread back into your garden.

Root Rot

Root rot is usually caused by overwatering your plants. The roots cannot breathe and are drowning. Although the swamp milkweed prefers swampy areas, when cultivating and growing them at home, do not overwater them. Ensure good soil drainage and aeration to avoid soggy soil.

Verticillium Wilt

This disease is caused by two related fungi, Verticillium dahliae and V. albo-atrum, which are widespread in California and can take out an entire patch of milkweed. Common symptoms are yellowing of leaves, extreme wilting, and death. Once a plant is infected, there’s no way to get rid of the fungus, which can live in the soil for years.

To rid the soil of the fungus, it must be sterilized. Soil solarization may be effective, but this can take a while, so if you encounter these fungal pathogens, plant your milkweed in another area as you remedy the situation. 

Frequently Asked Questions