All About Milkweed

August 8, 2022
BY LEAH MCFARLAND

There are 72 species of milkweed (Asclepias spp.) native to North America. Here, in New England, we have only 7 species native to our area, while in places like Arizona and Texas have 29 and 37 species, respectively! With all this diversity, it comes to no surprise that milkweed plants have a variety of flower colors, seed pod shapes, and leaf size.

The Bring Back the Monarchs campaign urged the community to plant milkweed to help the decreasing monarch butterfly population. Monarch caterpillars rely solely on milkweed plants for food. Monarch butterflies will drink the tantalizing nectar; however, they are poor pollinators for milkweed.

Milkweed has a unique mechanism for pollination compared to other flowering plants. In most other flowers, pollen is freely available to attach to pollinators. In milkweed, pollen is kept in a small, wavy sac called Pollinia. A pair of Pollinia are attached inside the five vertical grooves (stigmatic slit) of the flower. Milkweed flowers attract pollinators by its abundantly, high-quality nectar.

When a pollinator goes to drink the nectar, it must slip into the stigmatic slit to access the nectar. A gland releases the Pollinia and it attaches itself to the pollinator. The Pollinia then must be deposited into another stigmatic slit of another milkweed flower. Since Pollinia are made up of hundreds of pollen grains, only large pollinators are successful in carrying. The best pollinators for milkweed are large bees, wasps, and butterflies.

Monarch caterpillars have evolved the ability to consume the milky sap, which thwart other predators, parasites, and pathogens. The milky sap contains latex and a complex chemical that discourages predation. This complex chemical, cardenolides, can be found in almost all milkweed plants at difference concentrations. Butterfly milkweed is the only species that does not produce the milky sap. A process called, cardenolides fingerprints, was used to determine which milkweed species monarch caterpillars preferred to eat. It was determined that 85-92% of monarchs consume common milkweed.

Besides being a source of nectar for pollinators, milkweed has other benefits. Its long root system, help make this a drought tolerant species. In one year, the common milkweed will have roots 6 feet long. At maturity, they reach 12 feet!

Floss, the white fluffy hairs attached to the seeds are soft, lightweight, buoyant, and lustrous. During World War II, floss was collected to make life jackets because of its buoyant properties that allows it to support as much as 30 times its own weight. During the war over 25 million pounds of pods were collected, resulting in 2 million pounds of floss to be used to make floatation devices!

Next time you see a milkweed plant, take a moment to marvel at this complex, beautiful plant.

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