Milkweed Pollination

There are 73 species of native milkweeds (genus Asclepias) in the United States. Many of them are in the western US and over a dozen occur in California.

Milkweeds have a distinctive and complex method for pollination, involving hoods that hold nectar, pollen sacs joined by a string, stigma slits, and a special structure that holds everything together.

Here’s how it works.

Milkweed flowers have five petals that either point away or open wide from the rest of the flower.

Above the petals, a special gynostemium structure has fused male and female parts. Surrounding the gynostemium are five white hoods that fill with nectar. Many times, as in the picture below, the hoods have curving horns sticking out that bend back to the top of the gynostemium. The hoods and gynostemium top are waxy and slippery, so pollinator legs tend to slip between the hoods.

Narrow Leaf Milkweed - Photo by Wilde Legard

In addition to the exotic white hood and horns, each male stamen grows an anther that splits into two pale-purple pollen sacs, connected by a string and a black sticky structure. This whole unit is called a pollinium.

When pollinators come to gather pollen from the waxy hoods, their legs often slip between the hoods and get tangled in the pair of pollen sacs. They carry the pollen sacs to another milkweed where their legs are likely to slip in again. This time they leave the pollen sacs behind, right next to a slit where the female stigma sits.

Photo by Wilde Legard (detail)

Here’s a photo of a Carpenter bee carrying pollinia from a different kind of milkweed flower. The petal colors and hood shape are different but the pollination system is the same.

Photo by Beatriz Moisset, https://bugguide.net/node/view/13049

Here’s a variation (Antelope Horn Milkweed – A. asperula) where there are no hoods but the nectar is presented in green stripes on the gynostemium. The honeybee picks up and deposits pollinia that you can see on its feet.

Photo from Wikipedia - public domain.

Here’s a close-up of a honey bee taking nectar from a narrow leaf milkweed. Note its tongue in one of the hoods where the nectar is produced. The bee already has many pollinia attached to its legs.

Photo by May Chen

Each pollina has two pollen sacs attached by a thread that tangles on the pollinator’s legs.

Pollinators tend to be fairly large insects, attracted to the milkweed’s large nectar stores and with long enough and strong enough legs to slip into the stigmatic slits and pickup or leave pollina.

Photo © Chris Helzer

Here’s a movie you can download of a honey bee with its right back leg trapped in a milkweed flower. It struggles mightily and manages to free itself with life and limbs intact.

What a surprising and amazing system!

Have fun taking a closer look the next time you see a Milkweed.

Want More?

The USDA has written an excellent article that describes milkweed pollination in more detail: https://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/nvpmctn12764.pdf

Here’s a fun one by Chris Helzer of The Nature conservancy:

https://prairieecologist.com/2021/01/26/milkweed-pollination-a-series-of-fortunate-events/

Here’s a great article from Virginia Tech on milkweed pollinia: https://collection.ento.vt.edu/2016/08/05/milkweed-pollinia-revisited/

Corrections/Comments: bruce@PlantID.net Copyright: https://PlantID.net/Contributors.aspx