Milkweed Is More Than Just a Common Weed

Sep 11, 2018

Credit Scott Bowe

In this month's installment of Field Notes, Scott Bowe of Kemp Station discusses Milkweed in Wisconsin.

In the past few years, milkweed has taken root at Kemp Station.  If I had to guess, we have more than a few acres of milkweed along our roads and in our open areas.  Milkweed plants are an American genus, Asclepias L., a herbaceous perennial consisting of more than 140 different species, most of which are toxic. Wisconsin has 14 native milkweed species including the common milkweed (Asclepias syriaca), swamp milkweed (Asclepias incarnata), and butterfly milkweed (Asclepias tuberosa) to name a few. Let’s look at the milkweeds and learn why so many people are interested in this common plant.

Milkweed is named for the milky substance containing latex, that exudes from the plant when the leaves or stem is damaged.  Separate from the plant’s sap, this milky substance is a response to wounding, which helps protect the plant from insect and animal attack.  The latex content is about 2% and was examined as a source of natural rubber during WWII without commercial success.  By contrast, rubber tree latex is concentrated at 30 to 40 percent making it much more suitable for natural rubber production.

I mentioned earlier that most milkweed plants are toxic. Milkweed contains cardiac glycoside poisons that inhibit animal cells from maintaining a proper potassium and calcium concentration gradient.  This is the same type of poison that natives of Africa and South America used for poison tipped arrows because it was so effective at stopping the heart.  Milkweed may cause death when animals consume large quantities of the plant.

Milkweed species produce some of the most beautiful flowers in the plant kingdom, with a complexity comparable to orchids. Nectar produced in milkweed flowers are an important food source for many insects including honey bees and butterflies.  I promise, we’ll get back to the butterflies in a minute. The milkweed seeds develop in a pod called a follicle. The seeds are arranged in overlapping rows and are attached to white, silky, filament-like hairs known as the coma. The coma is the cotton-like “fluff” that help distribute the seed far and wide. As the follicles ripen and split open in the fall, the seeds, each carried by its coma, are blown by the wind.

The coma of the milkweed has an interesting history. The filaments are hollow, coated with wax, and have good insulation qualities.  It is grown in small scale commercial applications and harvested for hypoallergenic filling for pillows and as insulation for winter coats.

Milkweed is a perennial, so it lives more than two years.  In fact, it will take three years to flower and produce seed pods.  If you have flowering milkweed, take some time to look below the mature plants for shorter first and second year plants.  These smaller plants are building their root systems to support flowering in the third year.  After germinating from seed the first year, the plat will die back in the fall, then grow from the established root system on the second and third years.

So why is there so much interest in this common plant?  It all comes down to the butterflies, the monarch butterfly to be specific. For monarchs, milkweeds are essential for its lifecycle. Its caterpillar feeds solely on milkweed leaves, and the adult butterflies lay eggs on the plant. In addition, milkweed flowers’ nectar is an important food source for the adult monarch butterflies. Four generations of butterflies live each year including the egg, larva, pupa, and adult.  The monarch butterfly has adapted to tolerate the plant’s toxins and thrive on milkweed.

It’s funny how we choose our “favorite” creatures to support.  We had an outreach program on insects at Kemp Station a few weeks ago.  I asked the group if they would encourage milkweed growth if it attracted another critter, such as the very large fishing spider. The resounding answer was – NO!

So milkweed is more than just a common weed. It is springing up as a flower of choice in gardens across Wisconsin. It’s a fascinating plant with a complex flower, useful fluff, and provides critical habitat to the regal monarch butterfly.

For Field Notes, this is Scott Bowe from the University of Wisconsin-Madison’s Kemp Natural Resources Station.