Wild Side: Butterfly milkweed

This native plant has a key role in the Island ecosystem.


If there is a particular plant worthy of being named the Vineyard’s National Wildflower, it is surely butterfly milkweed, Asclepias tuberosa. It’s a plant everyone on the Vineyard has seen, and it’s one that sits smack in the middle of the Island’s web of ecological relationships.

While not really rare in mainland Massachusetts, this colorful plant is still scarce enough to seem special to folks in America. Some years back, when the state maintained a quasi-official “Watch List” of declining species, butterfly milkweed was on it.

But on the Vineyard? The stuff is all over the place. A. tuberosa is a perfect example of the way in which certain wildlife species find exactly the conditions they prefer on Martha’s Vineyard. We have entire fields covered with it. And if you are interested in growing native plants around your home, you couldn’t pick a better species to start with: butterfly milkweed is easy to grow, stunningly beautiful when in bloom, and an ecological powerhouse that supports a host of other species.

Butterfly milkweed is most easily noticed and recognized during its early summer bloom period, currently winding to a close. Each mature plant, typically from one to two feet tall, is topped with a broad cluster of intensely orange flowers. The flowers will give way as the season progresses to elongated seed pods, filled with gossamer fluffs that can transport their attached seeds for miles on a favorable wind. But even when no flowers or pods are present, this is an easy wildflower to recognize: a ladder of lance-shaped, smooth-edge, slightly fuzzy leaves climb the stem.

As one of the Vineyard’s six native species of milkweed, A. tuberosa may be best known for its role in supporting the monarch butterfly. Famously, monarch caterpillars feed exclusively on members of the milkweed family, ingesting chemicals called cardenolides as they eat, which makes the striped caterpillars and then the orange-and-black adult monarch mildly poisonous. Of the Vineyard’s milkweeds, butterfly milkweed may be the most favored species for this butterfly to lay its eggs on.

Less well known is the fact that many other insects also feed on butterfly leaves or seeds. There’s a milkweed tussock moth, Euchaetes egle, with caterpillars as devoted to milkweeds as those of the monarch. The collared cycnia moth, Cycnia collaris, is a Threatened species in Massachusetts; butterfly milkweed is this moth’s preferred larval food, and the moth’s bright orange caterpillars appear to have evolved their coloration with an eye toward staying hidden on butterfly milkweed flowers. There’s a milkweed longhorn beetle, Tetraopes tetrophthalmus, that sometimes turns up on butterfly milkweed (though this beetle’s true love is really common milkweed, A. syriaca). And the small milkweed bug, Lygaeus kalmia, happily feeds on the sap and seeds of A. tuberosa.

But it is as a resource for pollinators that butterfly milkweed truly shines. The flowers of this plant produce copious amounts of nectar and pollen. The broad panicle of flowers on a butterfly milkweed presents an inviting landing pad and offers ample room for multiple insects to feed simultaneously. And the individual flowers are shaped and sized so that the goodies they contain are accessible to a wide variety of insect mouth parts.

Bees, wasps, flies, butterflies, you name it: If it likes to visit flowers and is active in early summer, odds are high that you can find it on butterfly milkweed. It’s not unusual to find individuals of a half-dozen or more species feeding at once on a single butterfly milkweed flower head. All those insects, of course, go on to play other ecological roles: Pollinating other plant species, preying on other arthropods, serving as prey for birds, bats, and larger insects. The energy and nutritional resources of A. tuberosa flowers, this is to say, play a huge role in keeping our ecosystem running smoothly.

Butterfly milkweed flourishes here in part, I expect, because it is well adapted to sandy soils like the Vineyard’s. In our loose, granular soils, A. tuberosa can easily drive its taproot deep into the ground; such a deep root system gives this plant access to water even during extreme drought, so butterfly milkweed shrugs off the worst the Vineyard has to offer in terms of summer weather. And the wealth of insects that feed on butterfly milkweed flowers return the favor, ensuring ample pollen transport for good fertility. In every respect, this is a plant that was meant to flourish here.

For the gardener, A. tuberosa is a great choice for a first foray into gardening with native plants. Butterfly milkweed is widely available from commercial growers, and this plant is so spectacular even in its wild form that plant breeders have felt little need to “improve” it through selective breeding. Better still, our wild population adapts easily to life in a garden, its seeds germinating well after cold stratification and maturing into a long-lived perennial that will look great — and support a host of wildlife — for many years to come. Sometimes nature just gets it right.