Milkweed (Asclepias) is the only genus of plant that Monarch butterfly caterpillars can consume, making it an absolute requirement for the species’ life cycle across North America. This dependence has led to widespread planting efforts by gardeners aiming to support the declining butterfly population. However, this enthusiasm has sometimes resulted in the planting of non-native varieties, leading to unintended consequences for Monarch health and migration patterns. Selecting milkweed species that naturally align with the Monarch’s biology and the local environment is crucial.
Why Certain Milkweeds Cause Concern
The primary concern about certain milkweed varieties centers on their perennial nature in climates where they should naturally die back. The most common species causing issues is Tropical Milkweed (Asclepias curassavica), which is native to Central and South America. In parts of the southern United States and California that do not experience hard winter freezes, this species can persist year-round, unlike native milkweeds that go dormant.
This year-round availability of foliage disrupts the Monarch’s natural migratory and reproductive cues. Instead of continuing their journey south, Monarchs can be encouraged to breed continuously through the winter, creating non-migratory populations in the southern US. The persistent presence of the plant also allows for a dangerous buildup of the protozoan parasite, Ophryocystis elektroscirrha (OE). Since native milkweeds die back annually, the OE spores on the leaves are eliminated. However, the constant growth of the tropical variety allows successive generations of caterpillars to ingest high concentrations of the debilitating spores, often resulting in adults with deformed wings, reduced lifespan, or impaired flight ability.
Recommended Native Milkweed Varieties
Switching to milkweed varieties native to North America ensures the plants are synchronized with the Monarch’s natural biology. These native species naturally go dormant in the fall, which effectively breaks the OE parasite cycle and encourages the Monarchs to continue their migration.
One of the most widespread and recognizable native options is Common Milkweed (Asclepias syriaca), which thrives in well-drained soils across much of the eastern and midwestern United States. This species is known for its large, fragrant, pinkish-purple flower clusters and its ability to spread vigorously via rhizomes, making it a good choice for larger restoration areas.
For areas with moist conditions, Swamp Milkweed (Asclepias incarnata) is an excellent selection, despite its misleading name. It features clusters of pink or sometimes white flowers and is more clumping than Common Milkweed, growing 3 to 4 feet tall and making it suitable for rain gardens or average garden soil.
A popular choice is Butterfly Weed (Asclepias tuberosa), valued for its vibrant, bright orange flowers and tidy, clump-forming habit. This species is highly drought-tolerant due to a deep taproot and prefers dry, well-drained soils. Unlike other milkweeds, it lacks the characteristic milky sap, but it remains a host plant for Monarch caterpillars.
Matching Milkweed to Your Ecoregion
The most effective way to support Monarchs is by planting milkweed that is not only native but also geographically appropriate for your specific location. The term “native” is regional, meaning a species native to the Southeast may not be suitable for the Midwest.
Consulting local resources, such as state extension offices or native plant societies, is the first step in determining appropriate species. These organizations often provide county-level lists that specify which milkweeds naturally occur in your immediate area. While the USDA Plant Hardiness Zone map helps determine which perennial plants can survive winter temperatures, it does not account for ecological factors like soil type or rainfall.
A more precise approach involves referencing ecoregion maps, which focus on plant communities, soil, and climate to define distinct ecological areas. Selecting plants that originated from local ecotype seed ensures the best genetic match for local conditions and the highest value for wildlife. Finally, match the specific needs of the chosen native milkweed—such as its preference for dry, average, or wet soil and sun exposure—to the conditions present in your garden.