Ornamental · Apocynaceae
Milkweed
Asclepias syriaca
Grow it for the monarchs, hand it sun and lean soil, and a native milkweed practically grows itself.
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 growing it pays off fast. Its caterpillars eat nothing else, and the cardiac glycosides they pull from the leaves turn them foul-tasting to birds, which is exactly why gardeners chase it down. All summer, clusters of pink to mauve flowers pull in bees and a crowd of other pollinators.
The name gathers up several native species, and the two you will run into most make wildly different neighbors. Common milkweed (A. syriaca) roams underground on rhizomes and throws up wide colonies, a treat in a meadow but a wild ride in a small bed. Swamp milkweed (A. incarnata) holds a tidy clump and leans toward damper ground, so it drops into a border without ever muscling out the neighbors.
These are tough, hardy perennials that ask next to nothing once they dig in. The one catch is the milky sap, which packs the same toxins that guard the monarchs and irritates skin and eyes, so keep gloves on and keep grazing animals well away.
Care
The short version: full sun and plain, well-drained soil are all milkweed craves, and once it settles in it barely wants any extra water or feeding. Match the species to your spot, give it room to run, and handle the sap with gloves.
Light
Milkweed craves full sun and blooms hardest under all-day light. It puts up with a little partial shade, but the plants stretch leggier and flower less when the sun runs thin.
Water
Once it is dug in, common milkweed turns genuinely drought tolerant and coasts on rain alone in dry to medium soil. Water new plantings until the roots grab hold, then back right off. Swamp milkweed breaks the rule, since it demands steadily moist to wet ground.
Soil
Almost any well-drained clay, loam, or sandy soil works, and common milkweed even powers through poor, dry sites that stall fussier plants. Swamp milkweed is the one that wants moist or wet ground, so pick the species to fit the spot rather than the other way around.
Temperature
These are iron-tough perennials that shake off winters down to -40°F (-40°C) and take summer heat up to around 86°F (30°C) without blinking. That huge range is what sends milkweed across so much of the country.
Humidity
Outdoor humidity? Milkweed shrugs it off, so there is no number to chase. The plants take whatever the local air throws at them and keep charging.
Feeding
Ditch the fertilizer. These natives run on lean soil, and feeding does nothing useful while it can shove out soft, floppy growth, so leave them to it.
Pruning
Cut the spent stems to the ground in late fall or early spring to tidy the clump. Want to rein in the self-seeding? Remove the pods before they split and scatter their seed. Snap on gloves for the job, since the sap is an irritant.
Size & spacing
Plants shoot up 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. Hand running common milkweed plenty of room, and pack the clump-forming kinds closer where you want a neat stand.
When to plant
Milkweed rides the seasons and needs a cold spell to jolt its seed awake. Sow outdoors in fall and let winter do the chilling, or stratify the seed in the fridge and launch it indoors in late winter, then chase the first 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 puzzles trace back to where it spreads and who shows up to feed, not disease. Plenty of the insects you spot are meant to be there, so name what you see before you act. Match your plant to the entries below, and give that sap a smart bit of caution.
Fresh shoots popping up far from where the clump began
That is common milkweed (A. syriaca) stretching its legs, racing along shallow rhizomes underground and flinging seed the moment its pods burst open. Dig out the runners and snip the pods before they split, and the plant falls right back into line. Craving a bed that holds its shape? Reach for clump-forming swamp milkweed, which stays exactly where it is planted.
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
Bright orange aphids swarming across the stems and buds
Meet the oleander aphids, a milkweed regular that piles into dense yellow-orange colonies and drips sticky honeydew as it goes. Blast them off with a hard jet of water or sweep the colonies away by hand, and the plant bounces back fast. Keep the insecticides on the shelf, since that spray takes out the monarch eggs and caterpillars this milkweed exists to feed.
Fix:
- Dislodge aphids with a strong spray of water
- Wipe off or hand-remove colonies
- Avoid insecticides, which kill monarch eggs and caterpillars
Chewed leaves and stripped stems buzzing with colorful bugs
Milkweed throws a feast for a whole cast of specialists, so scope out who is dining before lifting a finger. The black-and-orange milkweed bugs and beetles cause no harm and pick off by hand when they get greedy, but those yellow, white, and black striped feeders are monarch caterpillars, the whole payoff of growing the plant. Let them chew away, and never spray 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 once the sap lands on you
That milky latex sap packs cardiac glycosides and resinoids that fire up skin and eyes the instant they touch. Scrub the affected skin with soap and water, flush the eyes hard if any sap sneaks in, and ring a doctor if the sting refuses to quit. Snap on gloves before you prune and keep those hands clear of your face, and the whole thing is a non-event.
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 pulls double duty, since it is exactly what guards the monarchs riding on it. Every part loads cardiac glycosides into the milky sap, toxic to cats, dogs, horses, rabbits, birds, and livestock, and a real cause of fatal poisoning in grazing animals. People should skip eating it too, and the sap stings skin and eyes, so pull on gloves and keep children and pets away from the plant.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Moderate
A cat that grabs a bite of the leaves or sap usually starts to drool, throw up, snub its food, and turn weak and dull. A bigger mouthful drives things harder, into seizures, labored breathing, a faint pulse, and dilated pupils.
Dogs Moderate
Once a dog chews any part, keep watch for vomiting, diarrhea, a heavy low mood, and weakness. Larger amounts can trigger seizures, trouble breathing, and a fast, thready pulse, so race to the vet.
Horses Severe
Horses take a hard hit, with colic, diarrhea, weakness, and a dull, listless slump. The glycosides knock the heartbeat off rhythm and can drive on to seizures, struggling breath, and death.
Rabbits Toxic
The cardiac glycosides hit a rabbit just as hard, so not one part of the plant is safe to hand over. Look for digestive upset and weakness, and lock both fresh stems and dried trimmings well out of reach.
Livestock Severe
Cattle and sheep can drop within hours, staggering and weak before seizures and collapse take over. A pretty small amount of green plant has killed grazing animals inside a day or two, so keep stock off it completely.
Birds Toxic
Birds get no free pass from the cardenolides either, so bar them from feeding on any part. Treat garden and poultry birds alike as at risk, and fence the plant off from coops and runs.
People Caution
No part of milkweed heads to the kitchen, and eating it in quantity 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 one plant but a whole lineup of native species, and the right pick comes down to whether you want a colony spreading through a wild patch or a single clump anchoring a bed.
Asclepias syriaca (common milkweed)
This bold 3 to 5 foot species flaunts broad oval leaves and domed pink-purple flower heads, and it charges out by rhizome to raise big colonies across dry to medium soil.
Asclepias incarnata (swamp milkweed)
A tidy 3 to 5 foot clump former with narrow leaves and pink to mauve flowers, it chases moist to wet ground, plants itself on a deep taproot, and never runs by rhizome.
Asclepias incarnata subsp. pulchra (eastern swamp milkweed)
An eastern take on swamp milkweed that sports hairier stems and leaves and thrives in wet meadows and along stream banks.
How to propagate
Milkweed grows mostly from seed, which needs a cold spell before it fires up, 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 go-to route, and it only asks for a cold snap and some light to spark it awake. Sow outdoors in fall or early winter and let the season chill it, or run it through about 30 days of cold-moist stratification in the fridge and launch it indoors 6 to 8 weeks before your last frost. Press the seed onto the surface instead of burying it, cheer on the sprouts in spring, and hold tight, since most plants save their blooms for the second year.
DivisionModerate
Early spring as new growth emerges · One growing season
Split common milkweed or grab rhizome and root cuttings in early spring as the new growth surges up, and it locks in within a single season. Leave swamp milkweed out of the fun, though, since its deep taproot hates being moved and does best planted once and left to run.
Myths
All milkweed is a noxious weed that should be wiped out.
Native milkweeds are powerhouse pollinator plants and the one and only food a monarch caterpillar will touch. Common milkweed can run wild 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.
Not so fast, since non-native tropical milkweed (A. curassavica) sticks around all year in mild climates, scrambling monarch migration and diapause and letting the OE parasite pile up. Stick to species native to your area and you win.