Ornamental · Apocynaceae
Milkweed
Asclepias syriaca
Grow it for the monarchs, hand it sun and lean soil, and a native milkweed will happily look after itself.
Full sun
Every 7–14 days
USDA 3–9
2–5 ft · 0.6–1.5 m
Overview
Milkweed is the one plant a monarch butterfly truly cannot do without, and that makes it a joy to grow. Its caterpillars eat nothing else, and the cardiac glycosides they draw from the leaves make them taste awful to birds, which is exactly why so many gardeners fall for it. All summer, the clusters of pink to mauve flowers keep bees and other pollinators happily coming back.
The common name covers several native species, and the two you are most likely to meet make very different neighbors, so do not worry about telling them apart. Common milkweed (A. syriaca) roams underground on rhizomes and builds wide colonies, which is wonderful in a meadow and a bit much in a small bed. Swamp milkweed (A. incarnata) holds a tidy clump and enjoys damper ground, so it eases into a border without ever taking over.
These are tough, forgiving perennials that ask for very little once they settle in, so a first try is unlikely to go wrong. The one thing to mind is the milky sap, which carries the very toxins that protect the monarchs and can irritate skin and eyes, so keep your gloves on and keep grazing animals well away.
Care
The short version: full sun and ordinary, well-drained soil are all milkweed really asks, and once it settles in it wants almost no extra water or feeding. Match the species to your spot, give it room to stretch, and handle the sap with gloves, and you will do just fine.
Light
Milkweed loves full sun and blooms its best under all-day light, so pick your sunniest spot. It will put up with a little partial shade if it must, but the plants grow leggier and flower less when the light runs short.
Water
Once it is settled, common milkweed is truly drought tolerant and gets along on rain alone in dry to medium soil, so you can relax about it. Water new plantings until the roots take hold, and only then ease right off. Swamp milkweed is the one exception, since it likes steadily moist to wet ground.
Soil
Almost any well-drained clay, loam, or sandy soil will make milkweed happy, and common milkweed even thrives on the poor, dry sites that defeat fussier plants. Swamp milkweed is the one that prefers moist or wet ground, so pick the species to fit the spot and let the plant do the rest.
Temperature
These are wonderfully cold-hardy perennials that shrug off winters down to -40°F (-40°C) and take summer heat up to around 86°F (30°C) in stride. That wide range is exactly what lets milkweed feel at home across so much of the country.
Humidity
Outdoor humidity is nothing to fuss over with milkweed, so there is no number to chase. The plants simply take whatever the local air gives them and carry right on.
Feeding
You can happily skip the fertilizer. These natives are made for lean soil, and feeding them does no good while it can push soft, floppy growth, so leave them be and let them thrive.
Pruning
Cut the spent stems to the ground in late fall or early spring to tidy the clump, and it is a forgiving job. If you would like to slow the self-seeding, remove the pods before they split and scatter their seed. Do keep gloves on, since the sap is an irritant.
Size & spacing
Plants reach 2 to 5 feet (60 to 150 cm) tall and spread anywhere from 9 to 36 inches (23 to 91 cm) depending on the species, so plan a little. Give the running common milkweed plenty of room, and set the clump-forming kinds closer where you want a neat, tidy stand.
When to plant
Milkweed keeps time with the seasons and needs a cold rest to wake its seed, so a little patience pays off. Sow outdoors in fall and let winter do the chilling, or stratify the seed in the fridge and start it indoors in late winter, then watch for blooms from early summer onward, mostly in the plant's second year.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Start seed indoors 6 to 8 weeks before last frost after 30 days of cold-moist stratification in the refrigerator
- Plant: Transplant after last frost; direct-sow seed in fall or early winter so natural cold stratification occurs over winter
- Harvest: Blooms June to August (common milkweed); swamp milkweed July to September
Hardiness
USDA hardiness zones: 3–9
Temperature: down to -40°C
Troubleshooting
Most milkweed worries come down to where it spreads and who arrives to feed on it, not disease, so relax a little. Plenty of the insects you find are meant to be there, so take a moment to identify before you act. Match what you are seeing below, and remember the sap alone is worth a gentle bit of care.
Fresh shoots turning up well beyond the clump you planted
Take heart, because this is just common milkweed (A. syriaca) being itself, creeping along shallow rhizomes underground and dropping seed from pods that split open. You can gently dig out the stray shoots and snip the pods before they open, and the plant will forgive you. If you would rather not chase it, the well-behaved swamp milkweed stays right where you set it.
Fix:
- Remove unwanted rhizome shoots by digging them out
- Cut and remove seed pods before they split to limit self-seeding
- Site common milkweed where spread is acceptable, or choose clump-forming swamp milkweed for small beds
A crust of bright orange aphids gathering on stems and buds
Do not panic when you spot them, because these are oleander aphids, a familiar milkweed visitor that clumps into dense yellow-orange colonies and leaves a sticky trail of honeydew. A firm blast of water will send them flying, or you can simply wipe the colonies off by hand. Please leave the sprays in the shed, since the very same chemicals would harm the monarch eggs and caterpillars you planted this milkweed to welcome.
Fix:
- Dislodge aphids with a strong spray of water
- Wipe off or hand-remove colonies
- Avoid insecticides, which kill monarch eggs and caterpillars
Nibbled leaves and bare stems with brightly colored bugs busy at work
Milkweed is a table set for many guests, so take a friendly look at who is dining before you worry. The black-and-orange milkweed bugs and beetles are harmless, and you can pick them off by hand if their nibbling gets carried away. Those yellow, white, and black striped feeders, though, are monarch caterpillars, the reason you grew the plant at all, so let them feast and never reach for a spray on a host plant.
Fix:
- Identify the insect before acting; tolerate monarch caterpillars as this is the plant's purpose
- Hand-pick pest milkweed bugs or beetles if damage is excessive
- Do not spray insecticides on host plants
A skin rash or stinging eyes after the sap has touched you
It is easy to fix, so do not fret if it happens: the milky latex sap holds cardiac glycosides and resinoids that can irritate skin and eyes on contact. Wash the affected skin with soap and water, flush your eyes well if any sap reaches them, and call a doctor should the sting refuse to settle. A pair of gloves at pruning time and keeping your hands from your face makes it easy to avoid.
Fix:
- Wash affected skin with soap and water
- Flush eyes with water and seek medical care if irritation persists
Toxicity
Milkweed is a poisonous plant, and that poison is the whole point, since it is what shields the monarchs that lean on it. Every part holds cardiac glycosides in the milky sap, toxic to cats, dogs, horses, rabbits, birds, and livestock, and a genuine cause of fatal poisoning in grazing animals. People should not eat it either, and the sap stings skin and eyes, so keep gloves on and keep children and pets away from the plant, and you will all be fine.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Moderate
A curious cat that samples the leaves or sap usually starts to drool and vomit, loses interest in food, and grows weak and dull. Try to catch it early, because a bigger bite can carry on into seizures, labored breathing, a faint pulse, and dilated pupils.
Dogs Moderate
If a dog chews any part, keep an eye out for vomiting, diarrhea, a heavy low mood, and weakness. A larger helping can lead to seizures, trouble breathing, and a fast, thready pulse, so please get to a vet without delay.
Horses Severe
Horses feel this one hard, showing colic, diarrhea, weakness, and a dull, listless look. The glycosides unsettle the heartbeat and can slide into seizures, struggling breath, and death, so a vet call is urgent.
Rabbits Toxic
The cardiac glycosides threaten a rabbit just as much, so no part of the plant is ever safe to offer. Look for digestive upset and weakness, and please keep fresh stems and dried trimmings alike well out of nibbling range.
Livestock Severe
Cattle and sheep can go down within hours, wobbling and weak before seizures and collapse take hold. A fairly small mouthful of green plant has killed grazing animals inside a day or two, so keep your stock off it completely.
Birds Toxic
Birds have no shield against the cardenolides either, so none should be left to feed on any part. Treat garden and poultry birds alike as at risk, and fence the plant away from coops and runs to be safe.
People Caution
No part of milkweed belongs in the kitchen, and eating much of it brings stomach upset, weakness, and an unsteady heartbeat. Keep children from chewing the pods or stems, and wash the irritating sap off skin and out of eyes.
The milky sap is an eye and skin irritant; wear gloves when handling and keep grazing animals away from plants and trimmings.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Milkweed is not a single plant but a friendly group of native species, and the right one for you simply depends on whether you want a colony in a wild patch or a single tidy clump in a bed.
Asclepias syriaca (common milkweed)
This is the tall one at 3 to 5 feet, easy to know by its broad oval leaves and rounded pink-purple flower heads, though it does run hard on rhizomes and gathers into big colonies in dry to medium soil.
Asclepias incarnata (swamp milkweed)
A gracious, well-mannered clump former of 3 to 5 feet with slim leaves and pink to mauve blooms that loves moist to wet ground, rests on a deep taproot, and happily never runs by rhizomes.
Asclepias incarnata subsp. pulchra (eastern swamp milkweed)
This eastern take on swamp milkweed carries hairier stems and leaves and feels right at home in wet meadows and along stream banks.
How to propagate
Milkweed is grown mostly from seed, which needs a cold rest before it will sprout, and it is easier than it sounds, though common milkweed will also let you split it at the root.
SeedModerate
Sow outdoors in fall or early winter, or stratify cold-moist for about 30 days and sow indoors 6 to 8 weeks before last frost · Seedlings germinate in spring; plants typically flower in their second year
Seed is the friendly, usual route, and it only asks for a little cold and light to rouse it. Sow outdoors in fall or early winter and let the season chill it for you, or give it about 30 days of cold-moist stratification in the fridge and start it indoors 6 to 8 weeks before your last frost. Press the seed onto the surface rather than burying it, watch for sprouts in spring, and be kind to yourself about the wait, since most plants hold off blooming until their second year.
DivisionModerate
Early spring as new growth emerges · One growing season
Come early spring, as the new growth pushes up, you can confidently split common milkweed or take rhizome and root cuttings, and it will settle in within a single season. Leave swamp milkweed out of this one, though, because its deep taproot dislikes being moved and is happiest left right where it stands.
Myths
All milkweed is a noxious weed that should be wiped out.
Rest easy, because native milkweeds are cornerstone pollinator plants and the one food a monarch caterpillar will ever accept. Common milkweed can wander in a garden, but it more than earns its keep, and clump-forming species like swamp milkweed behave themselves in a bed.
Any milkweed helps monarchs, tropical milkweed included.
It is a gentle myth to let go of, since non-native tropical milkweed (A. curassavica) hangs on year-round in mild climates, upsetting monarch migration and diapause and letting the OE parasite build up. Stick to species native to your area and you cannot go wrong.