Milkweed Plants & Monarch Butterflies

Populations of Monarch butterflies have declined by 90% in the past 20 years. We can help increase their numbers simply by planting particular beautiful flowers, Milkweed varieties (Asclepias) in our gardens and landscapes.

Many immature butterflies (caterpillars) have specific food requirements, often limiting their diet to a single genus. This can threaten their numbers when that plant becomes difficult to find. An excellent example of this phenomenon is the Monarch butterfly. This butterfly relies solely on the genus Asclepias (“Milkweed”) as a food source in its larval stage. In the days prior to herbicide-resistant agricultural crops, milkweed grew in abundance in our farm fields. It is now rare to see milkweed growing wild. The resultant inability of Monarchs to find food sources for their young has contributed to the dramatic decrease in Monarch butterfly populations in the past decade or so.

Distinctive seedheads and seed pods of Asclepias.

Asclepias foliage contains cardiac glycoside, a poison that causes heart muscle disturbance in mammals and birds. Monarch caterpillars are not affected by the toxin and ingesting it, in fact, provides them with protection by rendering them unpalatable to predators. This has led to the evolutionary mimicry of the Viceroy butterfly, which displays a near duplicate of the adult Monarch’s distinctive markings and thereby enjoys the same protection. Viceroys can be distinguished from Monarchs by the black bar across the bottom of their wing pattern, their slightly smaller size and their more erratic flight pattern. Interestingly, there is another, much lesser known, caterpillar called the Milkweed Tiger Moth that also feeds exclusively on Asclepias foliage. These hairy orange and black caterpillars are also provided protection by the toxins in the foliage they ingest. They mature into small, tan moths.

Asclepias syriaca

While too aggressive for most gardens, the common milkweed can make a wonderful addition to a naturalized area. It bears clusters of extremely fragrant lavender-colored flowers in summer, followed by decorative seed pods that crack open to release fluffy airborne seeds. Milkweed increases rapidly by rhizomes and can quickly take over a cultivated garden so is best suited to areas where it will encounter competition that will curtail its spread.

Asclepias tuberosa

Butterfly Weed is a Midwestern dry prairie native that thrives in full sun and well drained soil. It produces showy bright orange flowers in mid-summer, followed by the distinctive “milkweed” seedpods. Its carrot-like taproot allows it to survive periods of drought. Butterfly Weed is intolerant of wet soil so will not survive in gardens with heavy clay soil. This plant spreads by seed, which requires a cold period to induce germination. If growing from seed either sow outdoors in fall or start seeds indoors after a 30-day moist cold treatment.

Asclepias incarnata

This is perhaps the Asclepias best suited to our southeastern Wisconsin gardens. While it prefers stream-side and wet meadow sites in the wild, it will happily grow in ordinary garden soil. It is a vigorous grower that forms a robust clump to 36” in height. It blooms for many weeks in summer with pink flowers that are very attractive to butterflies and other pollinators. It is a short-lived perennial but does reliably self-sow.