Tropical Milkweed

The milkweed nurseries actually sell, and the one to stop buying. Its red-and-yellow flowers pull monarchs in, but where winters are mild it never dies back, so parasite spores accumulate on the leaves and the butterflies break off migrating to breed through winter. Plant a milkweed native to your region instead.

How to recognize it on a nursery bench or in your own border, and what to buy instead. Red-and-yellow flowers, glossy opposite leaves and milky sap; the Silky Gold and Silky Deep Red strains are the same species under a friendlier name.

This entry is here so you can identify tropical milkweed and replace it, not so you can grow it. Start with the plant.

It is an upright, slightly shrubby perennial to about 3 feet, with smooth green stems and glossy, lance-shaped leaves in opposite pairs, bleeding white latex when cut. The flowers give it away: flat clusters at the stem tips of small five-petalled blooms with brilliant orange-red reflexed petals and a contrasting yellow-orange crown. No milkweed native to the United States has that two-tone red-and-yellow flower. It blooms almost continuously in warm weather and produces slender 3-inch pods that split to release silk-tufted seed.

Watch the names. It is sold as bloodflower, scarlet milkweed and Mexican butterfly weed, and the popular all-yellow and deep-red strains — Silky Gold and Silky Deep Red — are selections of the same species with the same problem. “Butterfly weed” on a label is ambiguous and worth checking: the native Asclepias tuberosa is a low clumping plant with flat orange umbels, hairy stems and clear rather than milky sap.

What to buy instead depends on where you garden. East and Midwest: swamp milkweed for ordinary or damp soil, butterfly weed for dry sun, common milkweed where a running patch is welcome. California: narrowleaf milkweed. Interior West and Plains: showy milkweed. All of them host monarchs, all of them die back for winter, and all are perennial in the ground rather than something to replace each spring.

The management protocol if you already have it: cut every stem to 6 inches once a month from October through February in frost-free regions, or better, take it out over winter and put a regional native in the hole. Near California's overwintering groves, remove it outright.

If tropical milkweed is already in your garden, there are two honest options and one of them is much better.

The cutback protocol. Where frost does not kill it, cut every stem back to about 6 inches from the ground, once a month, from October through February, and bag the cuttings for the trash. Done properly this strips the accumulated OE spores off the foliage and removes the lush green cue that tells arriving monarchs to settle and breed. Done sporadically it achieves very little, which is the honest weakness of the approach: it depends entirely on someone remembering, every month, for five months, every year.

Removal, which is better. Dig the crown out in winter, when there are no caterpillars to displace, bag the top growth and any pods, and plant a milkweed native to your region in the same spot. That is one afternoon's work against five reminders a year forever, and the replacement is a longer-lived plant that does the job properly.

Either way, some rules hold. Before any cutting during the growing season, check the plant over and move caterpillars onto a native milkweed. Deadhead spent flowers and cut green pods before they split, because the seed travels and this species naturalizes readily in the warm South. Never spray it or anything near it. And if you garden within roughly five miles of the California coastal overwintering groves, the advice is not to cut back but to remove milkweed altogether, this species first.

It grows from seed almost too easily - fast, flowering in its first year, and self-sowing on the wind. That is exactly why it dominates the nursery trade and why it escapes gardens in the warm South.

Understanding how readily this plant propagates explains both why it is everywhere and why it needs containing.

Seed germinates fast and without the cold-stratification that native milkweeds insist on, seedlings grow quickly, and a plant raised in February is flowering by early summer. For a nursery that is an ideal crop: one season from seed to a saleable plant in bloom, versus two or three years for most native milkweeds, which is very largely why tropical milkweed came to dominate the bench.

In the garden the same vigour becomes self-sowing. It flowers and sets pods more or less continuously in warm weather, and the silk-tufted seed carries on the wind into neighbouring beds, gravel, fence lines and, in Florida, the Gulf states, south Texas and parts of California, out into ditches, field margins and natural areas, where it is now naturalized. It also strikes from cuttings without much trouble.

The practical consequence is that removing the parent plant is not quite the end of the job. Cut and bag green pods before they split, and check the surrounding ground for volunteers for a season or two afterwards. They pull out easily while small.

Not a food plant - it is toxic. The ecological accounting is the point: it is a genuine monarch host that nonetheless produces sicker butterflies and disrupted migration wherever it stays evergreen, which is why more milkweed is only helpful when it is the right milkweed.

Nothing here is eaten by people; the plant contains cardiac glycosides throughout and is toxic to pets and livestock. The accounting that matters is ecological, and it is worth setting out plainly, because the honest answer is not simply that tropical milkweed is bad.

It genuinely is a monarch host. Caterpillars eat it, adults nectar at it, and in a garden with a hard winter frost it behaves much like an annual milkweed and does comparatively little harm. Where it fails is in the warm South and the mild-winter West, and it fails in two measurable ways. Monarchs from year-round breeding populations on evergreen tropical milkweed carry far higher OE parasite loads than migratory monarchs — the difference is stark rather than marginal — and heavily infected butterflies emerge undersized, sometimes with crumpled wings, and fly less far and live less long. And the presence of green milkweed in late autumn measurably increases the proportion of monarchs that stop migrating and breed in place. There is also evidence that under warmer temperatures the plant's cardenolide chemistry shifts in ways that reduce caterpillar survival outright.

The useful conclusion is not that milkweed is risky. It is that more milkweed helps only when it is milkweed native to the region, on the seasonal rhythm the monarch's life cycle evolved with: green in summer, gone in winter.

A tropical American plant that colonial trade carried around the world, then the nursery industry made ubiquitous because it flowers in one season from seed. The conservation reversal came in the 2010s, when the parasite and migration research landed.

Asclepias curassavica is native to tropical Central and South America and the Caribbean — the species name points to Curaçao. Colonial-era trade carried it across the tropics as an ornamental and a folk medicine, and it is now naturalized across tropical Asia, Africa, the Pacific and Australia, as well as in Florida, the Gulf states, south Texas and Hawaii.

Its American garden career took off with the monarch conservation movement itself. When gardeners were urged to plant milkweed, tropical milkweed was what the trade could supply: it flowers in its first season from seed, it blooms for months, it is showy on a bench, and it does not require the cold-stratification and two-year wait that native milkweeds do. Through the 2000s it became the default milkweed across the southern United States and in the seed racks nationally.

The reversal came through the 2010s, as researchers tracking OE parasite loads and migration behaviour found year-round breeding populations on evergreen tropical milkweed carrying far heavier infections, and found that late-season green milkweed measurably suppressed migration. Conservation organisations, the Xerces Society and the Monarch Joint Venture among them, moved to recommending regional native milkweeds and, where tropical milkweed is already planted, monthly winter cutbacks or outright removal. Several states and native-plant nurseries have since stopped selling it. The story is a useful one: the intent was right, the plant was wrong, and the fix is straightforward.

Answers to the questions this plant actually raises: whether it matters in a cold climate, what deformed butterflies mean, whether cutting back is enough, and what to do with the caterpillars already on it.

My winters freeze - does this still matter? Much less. Where hard frost kills it to the ground each year, tropical milkweed behaves like an annual and the parasite accumulation largely does not happen. It can still hold monarchs later into the fall than natives would, and it is still a plant you have to buy again every spring, so a regional native is the better choice. But the serious harm is concentrated in zones 9-11 and anywhere else winters are mild.

Is the monthly cutback really enough? It works when it is actually done — every stem, to 6 inches, every month from October through February. The failure mode is human, not botanical. If you are not confident you will keep it up for years, remove the plant instead.

There are caterpillars on it right now and I want it gone. Move them. Transfer larvae onto a native milkweed, leave eggs to hatch first, and do the removal in winter when the plant is empty.

A butterfly emerged small, or with crumpled wings that would not open. That is the classic presentation of a heavy OE infection, and it is the reason this page exists. Remove the tropical milkweed and replace it with a native.

Bright yellow insects all over the stems. Oleander aphids. Do not spray — monarchs are laying on the same leaves. Hose them off or leave them.

Seedlings everywhere. It self-sows heavily. Pull them small and cut green pods off before they split.

Is it illegal? Generally no. This is a conservation recommendation rather than a regulation, though some jurisdictions and many native-plant nurseries have stopped selling it.

The harm is climate-dependent: severe in zones 9-11 where the plant never dies back, real but smaller in zone 8, and mostly moot where hard frost kills it each year. Near California's coastal overwintering groves it should be removed outright.

How much damage tropical milkweed does depends almost entirely on your winter, which is why blanket advice about it can feel contradictory. Here is the honest regional picture.

Zones 9-11 - Florida, the Gulf Coast, south Texas, coastal and southern California, Hawaii: this is where the problem lives. The plant stays green and flowering all year, OE spores accumulate on the foliage, monarchs break off migrating to breed through winter, and it naturalizes out of gardens into wild ground. Remove it, or run the October-to-February monthly cutback without fail.

Zone 8: frost usually knocks it back but does not always kill it, and mild winters let it persist. Treat it as a zone-9 problem and cut it back or remove it.

Zone 7 and colder: hard frost kills it every year, so it functions as an annual and the parasite mechanism largely breaks down. The remaining argument against it is that it holds monarchs late in the fall, and that it costs you a purchase every spring where a native perennial would come back on its own.

Within about five miles of the California coastal overwintering groves — the eucalyptus, cypress and pine stands from Marin to San Diego where monarchs cluster from October to February — the guidance is stronger still: remove milkweed altogether rather than cutting it back, this species first. Milkweed was historically scarce on that immediate coast, and any of it near the groves pulls butterflies out of overwintering. Check the Xerces Society's overwintering-site guidance if you garden there.