Milkweed questions and answers

Most species do. Thirty days of cold-moist stratification at about 4°C raises germination from roughly 20 percent to 80 percent or more. The exceptions are warm-climate species such as tropical, aquatic and pinewoods milkweed, which germinate readily from fresh seed with no chilling.

Seven to 21 days after stratification, at 20–24°C with light. Southern species sown fresh can emerge in as little as four days.

Yes, if you scatter it in the fall onto bare soil so winter provides the stratification, and press it in rather than burying it. Scattering onto established grass in spring almost never works.

The three usual causes are missing or insufficient cold stratification, sowing too deep so light cannot reach the seed, and compost that was either bone dry or waterlogged. Old seed stored warm also loses viability quickly.

November in most of the United States. Fall sowing lets winter supply the cold stratification the seed needs, and germination happens naturally in early spring. If you have missed fall, stratify in the fridge for 30 days and sow indoors six to eight weeks before your last frost.

Potted plants can go in during early summer if you water them consistently, but midsummer is the worst time for transplanting, especially for taprooted species like butterfly weed. Sowing seed in summer usually fails because the seed has had no cold period.

Seed can be sown outdoors any time before the ground freezes, and in mild regions right through winter. Potted plants should go in at least six weeks before your first hard frost so the roots can settle.

Native plant society sales and specialist native plant nurseries are the best sources, offering regionally appropriate species, no neonicotinoid treatment and knowledgeable staff. Mainstream garden centers usually stock tropical milkweed and often cannot tell you whether it has been treated.

Ask the seller directly. There is no way to tell by looking, and the residues persist in leaf tissue for months. If nobody can answer the question, assume it has been treated and buy from a native plant nursery instead.

Far cheaper. A few dollars of seed can plant an area that would cost hundreds in plugs. The trade-off is time, because seed-grown plants usually flower in their second or third year, and most species need cold stratification first.

Monarch caterpillars eat only milkweed and closely related plants in the Asclepiadoideae. Adult butterflies drink nectar from a wide range of flowers, but eggs are laid only on milkweed and larvae will starve rather than eat anything else.

At least ten stems in one place. A single caterpillar eats roughly 20 to 25 large leaves, and females locate dense patches far more easily than scattered plants. Two or three species with staggered bloom times is better than ten of one.

Usually yes, if you are within the breeding range and the patch is big enough to be found. It commonly takes a season or two, because plants need to reach a decent size before females start finding them reliably.

Generally no, or only a few. Indoor rearing at scale spreads the OE parasite and there is evidence it impairs migratory navigation. Growing more milkweed and leaving eggs where they were laid helps more.

In frost-free regions, yes. Because it never dies back it encourages monarchs to breed through winter instead of migrating, and OE parasite spores accumulate on leaves that are never shed. In cold-winter regions where frost kills it annually, the evidence of harm is far weaker.

Cut it to within six inches of the ground in October and keep it cut through February if you garden in USDA zone 9 or warmer. That reproduces the winter dieback of native milkweeds and reduces parasite build-up.

Not nationally. Some California counties and municipalities restrict its sale, and many native plant nurseries have stopped stocking it voluntarily, but there is no federal ban.

Ophryocystis elektroscirrha, a protozoan parasite. Spores on milkweed leaves are eaten by caterpillars, and infected adults emerge weakened, often with deformed wings and shortened lives. Infection rates are far higher in non-migratory populations breeding on year-round milkweed.

A garden or habitat patch that provides milkweed for monarch caterpillars, nectar plants for adults, sun and shelter, and is managed without insecticides. Monarch Watch certifies waystations and issues numbered signs, but the certification is optional. The habitat is the point.

About 100 square feet works well, with at least ten milkweed stems and four nectar species. Smaller gardens and even container groups still help, particularly in cities where any habitat is scarce.

Late-season nectar above all: goldenrod, New England aster, ironweed, blazing star and Joe Pye weed, which fuel the fall migration. Add spring and summer nectar such as golden alexanders, bee balm and mountain mint so something flowers continuously.

Look for an umbel of flowers where each flower has five petals bent sharply backward and five hoods forming a crown above them. Then break a leaf: almost every milkweed bleeds thick white latex. That combination is unique to the genus.

Both have milky sap and opposite leaves, but dogbane has small simple bell-shaped flowers rather than the two-tiered milkweed flower, reddish repeatedly branching stems, and long thin paired pods instead of one fat follicle.

Butterfly weed, Asclepias tuberosa. Its sap is clear or only faintly cloudy, and it also has alternate rather than opposite leaves, giving two exceptions in one plant.

Yes. Female monarchs lay eggs on black and pale swallowwort because the chemical cues resemble milkweed, but the larvae cannot survive on it. It functions as an ecological trap and should be removed wherever it is found.

No. Hemp dogbane is a related plant with milky sap and opposite leaves, but its flowers are small simple bells rather than the two-tiered milkweed flower, and its pods are long, thin and paired. Monarchs cannot use it.

That is normal. Only about 1 to 2 percent of milkweed flowers set fruit, because pollen travels in packets that must be clipped onto and off insect legs precisely. Milkweeds are also largely self-incompatible, so a patch that spread by rhizome from one plant cannot pollinate itself. Add a second, seed-grown plant.

A waxy sac containing all the pollen from one milkweed anther. Two pollinia joined by arms to a clip form a pollinarium, which attaches to an insect leg and is carried whole to the next flower. Only orchids and milkweeds package pollen this way.

Large, strong insects: bumble bees, honey bees, big wasps, butterflies including monarchs, and sphinx moths. Small insects cannot pull the pollen packets free, and occasionally get trapped trying.

Knock them off with a jet of water, or squash them by hand after checking for monarch eggs. Do not use insecticidal soap or neem, because both kill monarch eggs and caterpillars. In most gardens, ladybirds and lacewings clear the colony within a couple of weeks if you leave it alone.

Bright orange-yellow ones clustered on stems are oleander aphids. Larger orange-and-black ones on the seed pods are milkweed bugs. Neither seriously harms the plant, and milkweed bugs are native and harmless.

No. They are native and part of the milkweed community. They appear late in the season when monarch use is mostly over, and although they can strip a plant, it returns unharmed the following year.

No. Every effective insecticide, including organic options like neem and insecticidal soap, kills monarch eggs and young caterpillars. Water, fingers and patience are the only safe controls.

Yes, milkweed is toxic to dogs, though poisoning is uncommon because the bitter latex deters them after a mouthful. Symptoms include drooling, vomiting, weakness and, at higher doses, irregular heartbeat and seizures. Call your vet or the ASPCA Animal Poison Control Center on 888-426-4435 if ingestion is suspected.

Yes, and horses are the animal most at risk. The usual route is contaminated hay, since milkweed stays toxic when dried. As little as 0.1 percent of body weight of whorled milkweed can be fatal. Keep milkweed out of hay ground and paddocks.

The sap irritates skin and is genuinely dangerous in the eyes, where it can cause severe pain and temporary vision loss. Wear gloves when cutting stems and wash your hands afterward. Simply brushing past a plant is harmless.

It should not be eaten. Milkweed contains cardiac glycosides, and it is easily confused with dogbane, which is considerably more toxic. Historic food uses involved specific species and careful preparation and do not make casual foraging safe.

When the seam splits under gentle pressure and the seeds inside are brown rather than white. This is usually September or October in the North and October to November in the South, but the squeeze test beats any calendar.

Put the opened pods in a paper bag with a few coins and shake for thirty seconds. The seed strips off and falls to the bottom, the floss stays clumped at the top. It is far quicker than picking seeds off by hand.

One to two years at room temperature, three to five years stored dry in a refrigerator, and longer still if properly dried and frozen. Keep them cool, dry and dark and label them with the collection date.

Yes, provided the pot is at least 12 inches deep, because most milkweeds are taprooted and stall in shallow containers. Swamp milkweed is the easiest, and butterfly weed works well in a deep pot with a gritty mix. Avoid common and showy milkweed, whose rhizomes quickly fill a container.

At least 12 inches deep, ideally 14 to 18. Depth matters far more than width because of the taproot. A tall narrow pot outperforms a wide shallow one.

Only if you protect it. Roots in containers experience temperatures roughly two hardiness zones colder than roots in the ground. Sink the pot in the garden, move it into an unheated garage, or group and insulate pots against a wall.

Yes. Butterfly weed (Asclepias tuberosa) is one of roughly 75 milkweed species in North America. The confusion comes from its clear sap and orange flowers, which look nothing like the pink drooping umbels of common milkweed.

It is famously the last perennial in the bed to break dormancy, often four to six weeks after everything else. In cold regions it may not appear until late May or June. Do not dig, do not water heavily, and do not give up before midsummer.

Rarely with success. The taproot can go two to three feet down and resents disturbance. If you must move it, do so in early spring as growth begins, dig far wider and deeper than feels reasonable, and expect a lost season.

It is native across eastern and central North America, so it is not invasive in the technical sense, but it is genuinely aggressive. Rhizomes spread several feet per year and it will colonize a bed. Site it where it can run.

Plant it inside a buried root barrier at least 18 inches deep, or grow it in a bottomless container sunk in the ground. Cutting shoots at ground level as they appear will weaken a colony over several seasons.

Longitude decides it. East of the Rockies, common milkweed. West of roughly the 100th meridian, showy milkweed (A. speciosa) is the regional equivalent and the better choice.

Yes, with room. It is more ornamental and much less rampant than common milkweed, but it still spreads by rhizome. Give it a bed at least six feet across, or plan on pulling shoots.

Look at the flower. Showy milkweed has long, tapering, sharply pointed hoods that make the cluster look starry; common milkweed’s hoods are short and rounded. Showy milkweed leaves are also greyer and more velvety.

No. It is adapted to wet ground but grows well in average garden soil with normal watering. It simply will not tolerate prolonged drought the way butterfly weed does.

No. It is fibrous-rooted and clump-forming, the least aggressive of the widely grown milkweeds and the right choice for a mixed perennial border.

Mid to late summer, typically July into September, later than common and showy milkweed. That makes it valuable for the final monarch generation of the season.

Yes. The narrow-leaved whorled milkweeds carry higher cardenolide concentrations by weight than broad-leaved species, and poisoning cases in sheep, cattle and horses are documented, usually from contaminated hay. Keep it out of pasture and hay ground.