Ornamental · Apocynaceae
Milkweed
Asclepias syriaca
Grow it for the monarchs, give it sun and lean soil, and a native milkweed lends a garden its quiet wild grace.
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 it carries that purpose into any garden that makes room for it. Its caterpillars feed on nothing else, and the cardiac glycosides they draw from the leaves make them unpalatable to birds, which is why gardeners set it at the heart of a pollinator planting. Through summer, its clusters of pink to mauve flowers keep bees and other pollinators moving across the space.
The name covers several native species, and the two most often planted suit very different corners of a garden. Common milkweed (A. syriaca) travels underground on rhizomes and builds wide colonies, a fine gesture in a meadow or a wild margin but a lot to manage in a small bed. Swamp milkweed (A. incarnata) keeps to a tidy clump and prefers damper ground, so it settles into a mixed border and a designed scheme without overrunning its neighbors.
These are tough, hardy perennials that ask little of a daily routine once they have taken hold. The single point of care 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 from the planting.
Care
The short version: full sun and ordinary, well-drained soil are all milkweed asks of a garden, and once established it wants almost no extra water or feeding. Match the species to the setting, allow it room, and handle the sap with gloves.
Light
Milkweed wants full sun and flowers most freely where the light reaches it all day. It will accept a little partial shade, but in a shadier corner the plants grow leggier and give fewer blooms.
Water
Once it has settled in, common milkweed is genuinely drought tolerant and lives on rainfall in dry to medium soil, which suits a low-maintenance garden. Water new plantings until the roots take hold, and then ease off. Swamp milkweed is the exception, asking for steadily moist to wet ground.
Soil
Almost any well-drained clay, loam, or sandy soil serves milkweed well, and common milkweed even thrives on the poor, dry sites that discourage fussier plants. Swamp milkweed is the one that prefers moist or wet ground, so choose the species to suit the setting rather than the other way around.
Temperature
These are cold-hardy perennials that carry through winters down to -40°F (-40°C) and take summer heat up to around 86°F (30°C) in stride. That wide range is what lets milkweed find a place in gardens across so much of the country.
Humidity
Outdoor humidity is no concern for milkweed, so there is no figure to meet. The plants take whatever the local air offers and carry on.
Feeding
Leave the fertilizer aside. These natives are made for lean soil, and feeding them adds nothing while it can force soft, floppy growth that spoils their poise, so let them be.
Pruning
Cut the spent stems to the ground in late fall or early spring to keep the clump trim. To slow the self-seeding, remove the pods before they split and scatter their seed. Keep gloves on for the task, since the sap is an irritant.
Size & spacing
Plants stand 2 to 5 feet (60 to 150 cm) tall and spread from 9 to 36 inches (23 to 91 cm) depending on the species. Give running common milkweed generous room in the scheme, and set the clump-forming kinds closer where a tidy stand suits the design.
When to plant
Milkweed keeps time with the garden's seasons, and its seed needs a cold rest before it will wake. Sow outdoors in fall and let winter do the chilling, or stratify the seed in the refrigerator and start it indoors in late winter, then look for its flowers from early summer onward, most often 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 arises with milkweed concerns where it travels in a bed and who comes to feed on it, not disease. Many of those visitors belong on the plant, so identify before you act. Work through the entries below, and give the sap itself the small measure of care it deserves.
New shoots arriving well beyond the planting you intended
This restless habit belongs to common milkweed (A. syriaca), which extends along shallow rhizomes and sows itself once the pods break open. In a designed border, lift the wandering shoots and clip the pods before they release to keep the composition intact. For a bed that should hold its shape, the clump-forming swamp milkweed is the more disciplined choice, since it stays where it is placed.
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 massing along the stems and buds
These are oleander aphids, a familiar visitor to milkweed that settles into dense yellow-orange colonies and leaves a sticky film of honeydew across the planting. A firm jet of water clears them, and wiping the colonies away by hand handles the rest without disrupting the display. Set the insecticides aside, since the same treatment removes the monarch eggs and caterpillars the plant was chosen to shelter.
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 stripped stems where colorful insects have gathered
Milkweed keeps company with a whole cast of specialist insects, so identify the diners before intervening. The black-and-orange milkweed bugs and beetles are harmless guests that can be lifted off by hand when the feeding grows heavy, while the yellow, white, and black striped feeders are monarch caterpillars, the very reason the plant belongs in the garden. Let those stay, and keep any spray away from 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
The milky latex sap holds cardiac glycosides and resinoids that unsettle skin and eyes on contact, a detail worth respecting in any daily garden routine. Wash the affected skin with soap and water, flush the eyes should any sap reach them, and consult a doctor if the sting lingers. A pair of gloves at pruning time, kept well away from the face, prevents the trouble before it starts.
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 precisely its purpose, 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 genuine 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 planting.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Moderate
A cat that samples the leaves or sap generally begins to drool, vomit, refuse food, and turn weak and dull. A larger mouthful carries the picture further, into seizures, labored breathing, a faint pulse, and dilated pupils.
Dogs Moderate
A dog that chews any part tends to show vomiting, diarrhea, a heavy low mood, and weakness. Larger amounts can bring on seizures, trouble breathing, and a fast, thready pulse that calls for a vet at once.
Horses Severe
Horses are affected severely, presenting colic, diarrhea, weakness, and a depressed, listless bearing. The glycosides throw the heartbeat off rhythm and can lead on to seizures, struggling breath, and death.
Rabbits Toxic
The cardiac glycosides are every bit as dangerous to a rabbit, so no part of the plant is safe to offer. Digestive upset and weakness are the likely signs, and both fresh stems and dried trimmings belong well out of reach.
Livestock Severe
Cattle and sheep can be down within hours, staggering and weak before seizures and collapse take hold. 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 are not spared from the cardenolides either, and none should be allowed to feed 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 is meant for the table, 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 a family of native species rather than one plant, and the right choice depends on whether a garden calls for a spreading colony in a wild patch or a single clump held within a bed.
Asclepias syriaca (common milkweed)
At 3 to 5 feet with broad oval leaves and domed pink-purple flower heads, this is the expansive species that runs by rhizome and settles into large colonies across dry to medium soil.
Asclepias incarnata (swamp milkweed)
This refined 3 to 5 foot clump former carries narrow leaves and pink to mauve flowers, favors moist to wet ground, rests on a deep taproot, and stays put rather than running by rhizome.
Asclepias incarnata subsp. pulchra (eastern swamp milkweed)
An eastern form of swamp milkweed marked by hairier stems and leaves, at home in wet meadows and along stream banks.
How to propagate
Milkweed is raised mostly from seed, which needs a cold rest before it will sprout, though common milkweed can also be divided 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 customary route, though the seed insists on cold and light before it will stir. Sow outdoors in fall or early winter and let the season supply the chill, or give it about 30 days of cold-moist stratification in the refrigerator and start it indoors 6 to 8 weeks before last frost. Press the seed onto the surface rather than burying it, look for emergence in spring, and expect the reward of bloom to arrive in the plant's second year.
DivisionModerate
Early spring as new growth emerges · One growing season
Common milkweed takes well to division or to rhizome and root cuttings lifted in early spring as the new growth rises, and it establishes within a single season. Swamp milkweed is best excused from this, since its deep taproot resents being moved and the plant fares better left where it stands.
Myths
All milkweed is a noxious weed that should be wiped out.
Native milkweeds are cornerstone pollinator plants and the single food a monarch caterpillar will accept. Common milkweed can range beyond its bounds in a garden, yet it more than earns its place, 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) persists year-round in mild climates, which unsettles monarch migration and diapause and lets the OE parasite accumulate. The considered choice is to stick to species native to your area instead.