The red and yellow blooms of tropical milkweed are ubiquitous in North Florida butterfly gardens. This non-native milkweed has exploded in popularity as demand for milkweed grows to support declining monarch butterfly populations. This tropical species, native to Mexico, is very easy to propagate. Growers are able to quickly produce plant material of this species to meet the milkweed demand. It’s also very showy, blooming prolifically all season and regrowing quickly after being decimated by hungry caterpillars. But unlike our native milkweed species, the lush green foliage of tropical milkweed will stay up all winter if not killed back by frost – and that is a problem.
Tropical milkweed has been an invasive species in Central and South Florida for a while now. The fast growth and prolific re-seeding of this species has remained unchecked by warm winters, resulting in large monocultures of tropical milkweed in natural areas. This replaces native plants and disrupts native ecosystems. But it’s not just the invasive quality of the plant that is damaging.
A protozoan parasite that evolved with monarch butterflies, Ophryocystis elektroscirrha (OE) lives on infected monarchs and is deposited on the plants they land on; especially when the butterflies deposit eggs on the plants. The resulting caterpillars hatch and ingest the OE when they begin eating the plant, and the parasite is able to replicate inside them. Those caterpillars will grow into butterflies infected with an increased load of OE.
Many species across the animal kingdom have evolved with their own particular parasites – humans included. Those parasites are often not too harmful unless they receive some sort of increase in advantage; a weakened immune system or way to accumulate in numbers on its host. Monarchs evolved with OE and are able to prosper while still carrying some of the parasite, but high OE levels in adult monarchs can cause them to fail to emerge from their pupal stage because they are too weak and unable to fully expand their wings. Monarchs with even mild OE infections can appear normal (though are usually a little smaller) but they don’t live as long, cannot fly as well, and are not able to migrate successfully.
In contrast to this tropical species, our native milkweeds naturally senesce in the fall and stay leafless and dormant through the winter. This effectively cleans the plant of the parasite. When the leaves die back, the parasite dies along with them so that when the butterflies return each spring and summer, they feed on fresh, parasite-free foliage. Whereas tropical milkweed, remaining evergreen throughout the winter, allows OE levels to accumulate on the plant. The following generations of monarch caterpillars that feed on those plants can be exposed to dangerous levels of OE.
As our winters have become increasingly warmer in North Florida, we are noticing the invasive potential of tropical milkweed is growing. Warmer winters also mean tropical milkweed is less likely to be killed by frost and more likely to accumulate OE on its leaves. Tropical milkweed can also interfere with monarch migration and reproduction. In northern areas it grows later in the season than native species, and just the presence of tropical milkweed may confuse monarchs into breeding at a time when they should be migrating. There is evidence that suggests the chemical composition of tropical milkweed may trigger this disruption of the innate migration cycle of the monarchs that interact with it – they are fooled into thinking they are in the safe wintering grounds of Mexico.
With this mounting evidence of the detrimental effects of tropical milkweed, we have decided to only sell native species of milkweed. Unfortunately our growers are still working hard to build supplies of stock to meet demand. Though supplies are limited of native species, we have decided it is better to be without milkweed at some points than to sell a tropical milkweed plant. While it may feed your caterpillars in the short term, it is harmful to the species as a whole in the long term.
Florida has 21 species of native Asclepias, but many are difficult and slow to grow and only a few are well suited for home gardeners. We will continue to work with our growers to provide an increasing variety of native species, but we have found the following species to be the easiest and fastest growing to provide leaf matter for caterpillars:
PINK SWAMP MILKWEED
Asclepias incarnata
A pink-blooming native milkweed that gets big! It grows 4-6ft tall, providing the most amount of caterpillar food (of the native milkweeds) for monarch and queen butterflies. The plants regenerate leaf matter quickly during the growing season and have a short season of bloom in late summer - providing nectar-rich blooms for adult butterflies and other pollinators. They grow best in full sun with moist to wet soils, rich in organic matter. I have found this species to be adaptable in my yard in part sun with average moisture and clay soil amended with compost.
WHITE SWAMP MILKWEED
Asclepias perennis
This free-flowering native milkweed usually only grows 12-24inches tall, continually sending up stalks topped with white blooms that often have a pink blush. They grow best in full sun with moist to wet soils - even growing directly in water! I've found them to be adaptable to quite a bit of shade and also average moisture and clay soils. The leaves provide substantial larval food for monarch and queen butterflies, and the constant flowers attract lots of adult butterflies as well as many other pollinators.
BUTTERFLYWEED
Asclepias tuberosa
One of the most common and noticeable native milkweed species, the clusters of bright orange blooms are often seen on roadsides and grow as far north as Canada. Like many plants with such a large native range, it is important to find plants that are grown from seed sources in your region - this is called an "eco-type". Our plants are North Florida eco-types, so they are well adapted to our hot and humid climate and our soils. Butterflyweed in N. FL tends to be lower growing than it's northern relatives - only 1-2ft tall. It prefers full to part sun and average to dry soils - it can be quite drought tolerant once it has an established root system. I've found these plants to be pretty tough and low-maintenance, and adaptable to sandy or light clay soils. They usually bloom in spring, and then will often rebloom later in the summer.