Its flowers are very flagrant and when in season, they fill the woods with their sweet exhalations and make it agreeable to travel in them, especially in the evening.
~Peter Kalm, Swedish scientist, on Common Milkweed, 1750’s
I don’t really remember how long ago it was that I spread some Common Milkweed (Asclepias syriaca) seeds around the yard. I think I did it a couple of years in a row, and finally, one spring, there were some shoots that popped up. Not where I had put the seeds, of course (and that should have prepared me for how this plant “behaves” in garden), but I was excited nevertheless. Years went by, and the milkweed would appear and send up new shoots well beyond where I had intended it to grow. But no flowers…I figured it was due to our wooded setting and these normally field habitat plants just could not get enough sunlight to send energy into flower production. Then, a few years back, the first flowers appeared. At first, on only a couple of the taller stems. But last year and this one have been spectacular showings of the sweet-scented floral globes.
The patch of twenty or so milkweeds has been abuzz with activity since the flowers opened with honeybees, bumblebees, and large butterflies being the most frequent visitors.
Besides their wonderful fragrance and obvious benefit to pollinators, milkweed flowers have something else going for them…their unusual structure. In addition to a whorl of sepals and petals (the petals are reflexed downward), milkweed flowers have a third whorl of five hoods (seen here as the upward facing openings) each of which encloses a horn (the modified filaments of the anthers).
Instead of the loose, powdery pollen grains most of us are familiar with, milkweed pollen grains are packed into two connected sacs (pollinia). Together they are called a pollinarium and are shaped like a tiny saddlebag.
When an insect lands on the flower, its leg slips into the crevice between the hoods and can pick up a pollinarium. When it visits another flower, the waxy pollen sacs can be deposited if the leg slips into a vacant crevice. These slits can occasionally capture some insects if their leg gets stuck.
I have seen a few bumblebees that have several of these pollen structures stuck to their legs so I think large (and strong) insects like bees and butterflies make the best milkweed pollinators.