The common names “milkweed” and “butterfly weed” often lead to confusion, but they do not refer to the exact same plant. Butterfly weed is a specific species of milkweed, meaning both are related members of the same botanical family. Understanding their scientific classification and distinct physical traits reveals why they are referred to by different names. This relationship is important for supporting local ecosystems, particularly the Monarch butterfly, which relies on these plants.
The Taxonomic Relationship
Butterfly weed is scientifically classified as Asclepias tuberosa. This places it squarely within the genus Asclepias, which is the group of plants collectively known as milkweeds. Therefore, all butterfly weed is technically a type of milkweed. The reverse is not true, as there are over 100 species of milkweed native to North America.
The shared genus name, Asclepias, is derived from the Greek god of medicine, Asklepios, referencing the historical medicinal uses of some of these plants. Common milkweed, for example, is classified as Asclepias syriaca. This shows they are distinct species within the same botanical family, explaining the overlap in their common names.
Distinguishing Physical Characteristics
One of the most notable differences between butterfly weed and most other milkweeds is the sap. When a stem or leaf of most milkweed species is broken, a thick, white, sticky latex sap is exuded. Butterfly weed (Asclepias tuberosa) is an exception, as its stems contain a clear, watery sap instead of the characteristic milky fluid.
The flower color of butterfly weed is another immediate identifier. It features showy clusters of bright orange, yellow, or sometimes reddish blooms. In contrast, the flowers of other widespread native milkweeds, such as common milkweed (A. syriaca) and swamp milkweed (A. incarnata), are typically pink, purple, or white.
Leaf arrangement also provides a helpful distinction for plant identification. Most common milkweed species have broad leaves that grow directly opposite each other on the stem. Butterfly weed leaves, by comparison, are generally narrower and arranged alternately, or sometimes scattered, along the stalk.
Supporting Monarchs and Other Wildlife
All species within the Asclepias genus, including butterfly weed, are the sole host plants for the Monarch butterfly caterpillar. The Monarch is dependent on milkweed leaves for food during its larval stage. The caterpillars ingest toxic compounds, called cardiac glycosides, from the leaves, which makes them and the resulting adult butterflies unpalatable to most predators.
While Monarch caterpillars can feed on butterfly weed, the plant is more renowned as a high-nectar source for adult pollinators. The bright orange flowers attract a wide array of butterflies, bees, and other insects. This makes butterfly weed valuable for fueling adult Monarchs and other butterflies during their migration.
For Monarch conservation, both the larval food source (leaves) and nectar source (flowers) are necessary for the butterfly’s life cycle. Other milkweed species, like common milkweed, often serve as a preferred egg-laying site due to their softer, broader leaves. Butterfly weed, with its dense flower clusters, supplies the energy needed for adult butterflies to reproduce and migrate.