Common Milkweed, Asclepias syriaca, is a perennial plant native to a large portion of North America, east of the Rocky Mountains. Once viewed primarily as a weed, its perception has shifted significantly in recent years as gardeners and conservationists recognize its ecological role. Its presence is now encouraged in gardens and natural spaces, reflecting a greater understanding of its value.
Identifying Common Milkweed
Common Milkweed is a conspicuous plant, growing up to six feet tall on a single, stout stalk. Its large, oval-shaped leaves can be 10 to 28 centimeters long. These leaves grow in opposite pairs along the stem, have a velvety texture on their undersides, and display a prominent, light-colored central vein. A break in a leaf or stem reveals a sticky, white latex sap, which is the source of its common name.
The plant’s flowers are also distinctive, forming large, spherical clusters known as umbels at the top of the stems. Each cluster contains numerous small, pinkish-purple flowers that emit a strong, sweet fragrance. Following pollination, these flower heads develop into large, warty, grayish-green seed pods. In the fall, these pods split open, releasing hundreds of seeds attached to a silky white floss that allows the wind to carry them over great distances.
The Monarch Butterfly Connection
The relationship between Common Milkweed and the monarch butterfly is an example of coevolution. Milkweed is the monarch’s only host plant, where they lay their eggs. Once the eggs hatch, monarch caterpillars feed exclusively on the milkweed leaves. A single caterpillar may eat around 20 milkweed leaves before entering its chrysalis stage.
The plant’s milky sap contains chemical compounds called cardiac glycosides, which are toxic to most animals. Monarch caterpillars, however, can ingest these toxins and store them within their bodies. This process, known as sequestration, makes both the caterpillar and the adult butterfly unpalatable to many predators. The nectar-rich flowers also provide a food source for adult monarchs and other pollinators like bees, wasps, and other butterflies.
Habitat and Growth
Common Milkweed thrives in areas that receive full sun. It is frequently found in open fields, along roadsides, and in prairies where it doesn’t have to compete with established vegetation. The plant is well-adapted to a range of soil types but performs best in well-drained soils, though it can tolerate clay and drier conditions.
Common Milkweed’s growth is characterized by its ability to spread aggressively through a network of underground stems called rhizomes. This root system allows the plant to form large, dense colonies over time. For gardeners wishing to cultivate it, this tendency requires management. It is recommended to plant it in a location with ample space to spread or within a contained area to prevent it from overtaking other plants.
Toxicity and Historical Uses
All parts of the Common Milkweed plant contain toxic compounds, making it poisonous if ingested in large quantities by humans, livestock, and pets. The milky sap can also cause skin irritation in some individuals upon contact. Symptoms of ingestion can include vomiting, weakness, and stupor. Despite its toxicity, the plant had various uses in history.
Native Americans developed methods to use the plant for several purposes.
- Its strong bast fibers from the stalk were used to create cordage and textiles.
- The fluffy seed floss served as insulation and as fire-starting tinder.
- During World War II, the floss was used as a substitute for kapok fiber to fill life vests.
- Some parts, like young shoots, were eaten after careful preparation to remove toxic compounds.