Got Milkweed?

An intimate look at a fascinating native plant.

Most people know how important milkweed is to monarch butterflies — their caterpillars must feed on milkweed in order to develop into adult butterflies, and the plants impart natural toxins that protect both caterpillar and butterfly from predators. Many other insects depend on milkweed as well, including the Queen butterfly, the Clio tiger moth, and the tarantula hawk wasp.

That alone is amazing, but the plant itself has some unique features that are pretty remarkable. Have you ever spent quality time with a milkweed plant and looked closely at its flowers? What looks like one large flower, is actually a cluster of small, individual flowers arranged in what is known as a “spherical umbel” — a rounded cluster of “individual flowers on stems that emerge from a common point” (USDA NRCS) (Fig. 1).

Fig. 1 Desert milkweed (Asclepias erosa) showing flower clusters called spherical umbels.

Each of the 5 petals of the flower surround a waxy, cup-like structure called a hood, each of which contains a curved appendage called a horn. Together these make up the corona, a fitting name for this crown-like structure. (Fig. 2)

Fig. 2 Closeup of an individual desert milkweed flower showing the 5 petals, hoods, and horns.

The male parts (anthers and filaments) and female parts (stigma and styles) of the flower are fused in the center of the flower. The surrounding hoods contain liquid nectar that attract pollinators. Between each hood is the flower’s pollen. (Fig. 3)

Fig. 3 The pollen located between the hoods. (Photo: nrcs.usda.gov)

Now, this is where it gets really interesting….

Instead of producing individual microscopic grains of pollen, the milkweed flower produces something called pollinia — pairs of small waxy sacs that contain the pollen. The two pollen sacs are joined together by a dark gland called the corpusculum. Look closely again at Figure 3 and see if you can spot the pollinia and corpusculum. The photo below, Figure 4, shows the pollinia and corpusculum isolated from the flower.

Fig. 4 Photo showing pollinia and corpusculum on ruler for scale. (Photo: nrcs.usda.gov)

Insects are drawn to milkweed flowers in search of nectar. While visiting the flower, bees and wasps inadvertently insert their legs into a narrow slot between the hoods and the pollinia attaches to them. The insect then flies to another flower, where the pollinia is deposited and pollination and fertilization occur.

Fig. 5 A native bee on desert milkweed (Asclepias erosa) in search of nectar.

Fig. 6 Pollinia attached to the leg of a native bee.

Once the flower is pollinated, the seeds begin to form inside a pod-like fruit. The technical term for this pod is a “follicle”, a dry fruit that splits open along a seam on one side. Milkweed seeds are papery discs attached to a tuft of hairs called floss or pappus. This helps the seeds disperse on the wind when they are released in late summer.

Fig. 7 The fruit containing the seeds of desert milkweed. This type of fruit is called a follicle.

Fig. 8 An open follicle revealing overlapping seeds inside. Note the white silky floss attached to the top of the seeds.

Fig. 9 As the seeds and fruit dry, the floss allows the seed to disperse by wind.

Of the 14 species of milkweed (Asclepias) native to California, more than half of them occur in the Colorado and Mojave Deserts. When selecting plants for your garden, remember: native plants support native biodiversity.

The California deserts are home to some of the most extraordinary landscapes, wildlife, people, and cultures on the planet. The Mojave Desert Land Trust (MDLT) protects this incredibly special place and the natural and cultural resources within it. Since our inception in 2006, MDLT has cultivated a multi-faceted conservation strategy that utilizes land acquisition, research, restoration, stewardship, plant conservation, and outreach to focus on the long-term protection of the California deserts so they may continue to thrive forever.

The Mojave Desert Land Trust’s plant conservation program includes a native plant nursery, a conservation seed bank, and a demonstration garden. We grow native plants in our nursery for desert restoration projects and empower locals to plant native plants at home. MDLT also operates a conservation seed bank that serves as an insurance policy for the desert’s future. Learn more at MDLT.org.