Ornamental · Apocynaceae
Milkweed
Asclepias syriaca
Grow it for the monarchs: broad leaves, domed pink flower heads, and milky-white sap on lean, sunny ground.
Full sun
Every 7–14 days
USDA 3–9
2–5 ft · 0.6–1.5 m
Overview
Milkweed is the plant a monarch butterfly cannot live without, and up close it shows why the caterpillars keep coming back. They eat nothing else, and the cardiac glycosides they pull from the thick, leathery leaves make them taste foul to birds. Through summer the stems carry rounded domes of small pink to mauve flowers, each one crowded with waxy petals that bees and other pollinators work over.
The name covers several native species, and the two you will handle most look plainly different in leaf and flower. Common milkweed (A. syriaca) carries broad, oval leaves and packed pink-purple flower heads on stout stems that run underground and rise again nearby. Swamp milkweed (A. incarnata) shows slimmer, narrow leaves and flatter heads of pink to mauve bloom, staying to a single tight clump.
These are tough, hardy perennials, coarse to the touch and quick to grow. Snap any part and it bleeds a thick, milky-white sap that carries the same toxins that protect the monarchs and stings skin and eyes, so keep gloves on and keep grazing animals well away.
Care
The short version: full sun and ordinary, well-drained soil suit milkweed fine, and once its roots take hold it wants almost no extra water or feeding. Match the species to your spot, give the coarse stems room, and handle the white sap with gloves.
Light
Milkweed wants full sun, and the broad leaves and flower domes fill out best under all-day light. It will take a little partial shade, but in it the stems stretch leggy and thin and carry fewer heads.
Water
Once the roots take hold, common milkweed is genuinely drought tolerant and lives on rain alone in dry to medium soil. Water new plantings until they root, then ease off. Swamp milkweed is the exception, since its slimmer stems want steadily moist to wet ground.
Soil
Almost any well-drained clay, loam, or sandy soil works, and common milkweed even thrives on the poor, dry, gritty sites that defeat fussier plants. Swamp milkweed is the one that prefers moist or wet ground, so match the species to the spot rather than the other way around.
Temperature
These are cold-hardy perennials whose coarse stems shrug off winters down to -40°F (-40°C) and take summer heat up to around 86°F (30°C). That wide range is what lets milkweed grow across so much of the country.
Humidity
Outdoor humidity means nothing to milkweed, so there is no number to hit. The leathery leaves take whatever the local air gives them and keep on.
Feeding
Skip the fertilizer. These natives are built for lean soil, and feeding does nothing useful while it pushes soft, floppy stems that flop under the heavy flower heads.
Pruning
Cut the spent, hollow stems to the ground in late fall or early spring to tidy the clump. To slow the self-seeding, remove the gray-green pods before they split and spill their floss. Wear gloves for the job, since the cut ends bleed irritant sap.
Size & spacing
Plants stand 2 to 5 feet (60 to 150 cm) tall on upright stems and spread from 9 to 36 inches (23 to 91 cm) depending on the species. Give the running common milkweed plenty of room, and set the tight clump-forming kinds closer for a solid stand.
When to plant
Milkweed runs on the seasons, and the flat brown seed will not stir without a cold spell first. Scatter the floss-tailed seed outdoors in fall so winter does the chilling, or stratify it in the fridge and start it indoors in late winter, then watch the domed flower heads open 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 of what shows on milkweed is about where its rhizomes travel and which bright insects come to feed, not disease. Many of those bugs, from striped caterpillars to black-and-orange beetles, are meant to be there, so look closely before you act. Match what you see below, and give the milky sap a little caution.
Pale new shoots pushing up far from the parent clump
Those slim spears breaking the soil are common milkweed (A. syriaca) running along pale, shallow rhizomes and casting seed from pods that have split down the seam. Trace the wandering shoots back and dig them free, and cut the fat gray-green pods before they crack open. For a stand that stays tight and unbroken, the clump-forming swamp milkweed holds its ground instead.
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 glossy orange aphids coating the stems and buds
Those beads of bright yellow-orange packed tight along the stem are oleander aphids, a milkweed regular that builds dense, shining colonies and coats the leaves below in sticky, glistening honeydew. A hard jet of water sends the whole crust flying, or the colonies smear away under a wiping finger. Leave the sprays alone, since the same chemical erases the pinhead monarch eggs and the striped caterpillars the plant was set out to carry.
Fix:
- Dislodge aphids with a strong spray of water
- Wipe off or hand-remove colonies
- Avoid insecticides, which kill monarch eggs and caterpillars
Ragged holes in the leaves and bare stems where bright insects feed
Milkweed carries a whole cast of specialists on its broad leaves, so look closely at the bodies before doing anything. The milkweed bugs and beetles wear a flat black-and-orange pattern, cause no real harm, and lift off between finger and thumb when the chewing runs deep. The plump feeders banded in yellow, white, and black are monarch caterpillars, the whole reason for the plant, and a host is never sprayed.
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
Reddened skin or stinging eyes after the white sap touches you
Break a stem or leaf and it bleeds a thick, milky-white latex sap loaded with cardiac glycosides and resinoids that redden and sting skin and eyes on contact. Wash the marked skin with soap and water, flush the eyes well if any of the milk reaches them, and call a doctor if the burning holds on. Gloves at pruning time, and hands kept clear of the face, keep the sticky sap off you.
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 point, since it is what shields the monarchs that depend on it. Snap any stem and the thick, milky-white sap runs, carrying cardiac glycosides through every part, toxic to cats, dogs, horses, rabbits, birds, and livestock, and a real cause of fatal poisoning in grazing animals. People should not eat it either, and that white sap stings skin and eyes, so wear gloves and keep children and pets away from the plant.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Moderate
A cat that mouths the leaves or the white sap tends to drool, vomit, refuse its food, and turn weak and dull. A heavier bite drives it further, into seizures, labored breathing, a faint pulse, and wide, dark pupils.
Dogs Moderate
A dog that has chewed any part shows vomiting, loose diarrhea, a heavy low mood, and weakness. Larger amounts can bring seizures, gasping breath, and a fast, thread-thin pulse that needs a vet at once.
Horses Severe
Horses are struck hard, with gripping colic, diarrhea, weakness, and a dull, drooping stance. The glycosides throw the heartbeat off its rhythm and can carry on into seizures, struggling breath, and death.
Rabbits Toxic
The cardiac glycosides are just as deadly to a rabbit, so not a leaf or stem is safe to offer. Expect a churning gut and weakness, and keep both the fresh green stems and the dried trimmings well out of reach.
Livestock Severe
Cattle and sheep can go down within hours, staggering on unsteady legs and weak before seizures and collapse set in. A relatively small mouthful of the green plant has killed grazing animals inside a day or two, so keep stock off it entirely.
Birds Toxic
Birds get no shield from the cardenolides either, and none should be left to peck at any part. Treat garden and poultry birds alike as at risk, and fence the plant well away from coops and runs.
People Caution
No part of milkweed belongs on a plate, and eating it in quantity brings a churning stomach, weakness, and an unsteady heartbeat. Keep children from chewing the pods or stems, and wash the stinging white 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 one plant but a group of native species, told apart by leaf and flower, and the right one depends on whether you want a running colony or a single tidy clump.
Asclepias syriaca (common milkweed)
Standing 3 to 5 feet, this one carries broad, leathery oval leaves and rounded domes of packed pink-purple flowers, and it runs by rhizome to raise sprawling colonies across dry to medium soil.
Asclepias incarnata (swamp milkweed)
A neat 3 to 5 foot clump former with slender, narrow leaves and flat heads of pink to mauve bloom, it roots on a deep taproot in moist to wet ground and never runs by rhizome.
Asclepias incarnata subsp. pulchra (eastern swamp milkweed)
An eastern form of swamp milkweed clothed in noticeably hairier stems and leaves, at home in wet meadows and along stream banks.
How to propagate
Milkweed grows mostly from its flat, floss-tailed seed, which needs a cold spell before it sprouts, though common milkweed can also be split at the rhizome.
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
The flat brown seeds, each trailing a tuft of silky white floss, are the usual route, but they need cold and light before they stir. Scatter them outdoors in fall or early winter and let the season chill them, or give them about 30 days of cold-moist stratification in the fridge and start them indoors 6 to 8 weeks before your last frost. Press each seed onto the bare surface rather than burying it, watch for the first green in spring, and wait out the first year, since most plants hold their bloom until the second.
DivisionModerate
Early spring as new growth emerges · One growing season
Common milkweed splits cleanly, or you can slice rhizome and root cuttings from it in early spring as the reddish new shoots break the soil, and it settles in within a single season. Leave swamp milkweed be, though, since its long, deep taproot resents the spade and does best left rooted where it stands.
Myths
All milkweed is a noxious weed that should be wiped out.
Native milkweeds are key pollinator plants and the single food a monarch caterpillar will strip from a leaf. Common milkweed can shoulder its way across a garden, but it earns its keep, and clump-forming species like swamp milkweed behave themselves in a bed.
Any milkweed helps monarchs, tropical milkweed included.
Non-native tropical milkweed (A. curassavica) holds its leaves year-round in mild climates, which throws off monarch migration and diapause and lets the OE parasite gather. Stick to species native to your area instead.