Ornamental · Apocynaceae
Milkweed
Asclepias syriaca
Plant it for the monarchs, give it sun and lean soil, and a native milkweed mostly takes care of 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. Its caterpillars eat nothing else, and the cardiac glycosides they pull from the leaves make them taste foul to birds, which is the whole reason gardeners grow it. The clusters of pink to mauve flowers draw bees and other pollinators all summer long.
The common name covers several native species, and the two you will meet most are very different neighbors. Common milkweed (A. syriaca) travels underground on rhizomes and builds wide colonies, which is fine in a meadow but a handful in a small bed. Swamp milkweed (A. incarnata) stays in a tidy clump and likes damper ground, so it slots into a border without taking over.
These are tough, hardy perennials that ask for little once they settle in. The catch is the milky sap, which carries the same toxins that protect the monarchs and irritates 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 it is established it wants almost no extra water or feeding. Match the species to your spot, give it room to grow, and handle the sap with gloves.
Light
Milkweed wants full sun and flowers best with all-day light. It will put up with a little partial shade, but the plants grow leggier and bloom less when the sun is short.
Water
Once it is established, common milkweed is genuinely drought tolerant and gets by on rain alone in dry to medium soil. Water new plantings until the roots take hold, and only then ease off. Swamp milkweed is the exception, since it wants steadily moist to wet ground.
Soil
Almost any well-drained clay, loam, or sandy soil works, and common milkweed even thrives on 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 rather than the other way around.
Temperature
These are cold-hardy perennials that shrug off winters down to -40°F (-40°C) and handle summer heat up to around 86°F (30°C). The wide range is what lets milkweed grow across so much of the country.
Humidity
Outdoor humidity is no concern for milkweed, so there is no number to hit. The plants take whatever the local air gives them and keep going.
Feeding
Skip the fertilizer. These natives are built for lean soil, and feeding them does nothing useful while it can push 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. If you want to slow the self-seeding, remove the pods before they split and scatter their seed. Wear gloves for the job, since the sap is an irritant.
Size & spacing
Plants stand 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. Give the running common milkweed plenty of room, and place the clump-forming kinds closer where you want a tidy stand.
When to plant
Milkweed runs on the seasons and needs a cold spell to wake its seed. Sow outdoors in fall so winter does the chilling, or stratify the seed in the fridge and start it indoors in late winter, then look 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 problems are about where it spreads and who comes to feed on it, not disease. Many of the insects you find are meant to be there, so identify before you act. Match what you see below, and remember the sap itself is worth a little caution.
New shoots popping up far from the original clump
This is common milkweed (A. syriaca) doing what it does best, traveling underground on shallow rhizomes and seeding itself from split pods. Dig out the wandering shoots and cut the pods before they open if you want to hold it in check. For a tidy bed, plant clump-forming swamp milkweed instead, since it stays where you put 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
Bright orange aphids crusting the stems and buds
These are oleander aphids, a milkweed regular that gathers in dense yellow-orange colonies and leaves sticky honeydew behind. Knock them off with a hard jet of water or wipe the colonies away by hand. Skip the insecticides, since the same spray kills the monarch eggs and caterpillars you planted the milkweed for.
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 with bold-colored bugs at work
Milkweed feeds a whole cast of specialists, so check who is eating before you reach for anything. Black-and-orange milkweed bugs and beetles are harmless and can be hand-picked if the damage gets heavy, but the yellow, white, and black striped feeders are monarch caterpillars, which is the entire point of the plant. Leave them alone 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
Skin rash or stinging eyes after the sap touches you
The milky latex sap carries cardiac glycosides and resinoids that irritate skin and eyes on contact. Wash the affected skin with soap and water, and flush your eyes well if any sap gets in, calling a doctor if the sting won't settle. Wear gloves whenever you prune and keep your hands away from your face.
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. Every part holds cardiac glycosides in the milky sap, 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 the 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 nibbles the leaves or sap tends to drool, throw up, go off its food, and turn weak and dull. A bigger mouthful can push things further, into seizures, labored breathing, a faint pulse, and dilated pupils.
Dogs Moderate
Watch for vomiting, diarrhea, a heavy low mood, and weakness once a dog has chewed any part of the plant. Larger amounts can bring on seizures, trouble breathing, and a fast, thready pulse that needs a vet right away.
Horses Severe
Horses are hit hard, with colic, diarrhea, weakness, and a depressed, listless look. The glycosides throw the heartbeat off rhythm and can lead to seizures, struggling breath, and death.
Rabbits Toxic
The cardiac glycosides are just as dangerous to a rabbit, so none of the plant is safe to offer. Expect digestive upset and weakness, and keep both fresh stems and dried trimmings well out of reach.
Livestock Severe
Cattle and sheep can be down within hours, staggering and weak before seizures and collapse set in. A relatively small amount of green plant has killed grazing animals inside a day or two, so keep stock off it entirely.
Birds Toxic
Birds aren't spared from the cardenolides either, and they shouldn't be allowed to feed on any part. Treat both garden and poultry birds as at risk and fence the plant off from coops and runs.
People Caution
No part of milkweed is meant for 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 group of native species, and the right one depends on whether you want a colony in a wild patch or a single clump in a bed.
Asclepias syriaca (common milkweed)
A tall plant of 3 to 5 feet with broad oval leaves and domed pink-purple flower clusters that spreads hard by rhizomes and forms big colonies in dry to medium soil.
Asclepias incarnata (swamp milkweed)
A well-mannered clump former of 3 to 5 feet with narrow leaves and pink to mauve flowers that wants moist to wet ground, sits on a deep taproot, and never runs by rhizomes.
Asclepias incarnata subsp. pulchra (eastern swamp milkweed)
An eastern form of swamp milkweed with hairier stems and leaves that suits wet meadows and stream banks.
How to propagate
Milkweed is grown mostly from seed, which needs a cold spell before it will sprout, though common milkweed can also be split 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 usual route, but it needs cold and light to wake up. Sow outdoors in fall or early winter and let the season chill it, 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 into the surface rather than burying it, expect sprouts in spring, and be patient, since most plants wait until their second year to bloom.
DivisionModerate
Early spring as new growth emerges · One growing season
You can split or take rhizome and root cuttings from common milkweed in early spring as the new growth pushes up, and it settles in within a single season. Leave swamp milkweed out of this, though, since its deep taproot resents being moved and does best left where it stands.
Myths
All milkweed is a noxious weed that should be wiped out.
Native milkweeds are key pollinator plants and the only food a monarch caterpillar will eat. Common milkweed can run loose in 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) lingers year-round in mild climates, which throws off monarch migration and diapause and lets the OE parasite build up. Stick to species native to your area instead.