Posted by Aaron Barton on Mar 4th 2023
Asclepias (as-KLE-pee-us), or milkweed, belong to the dogbane family, Apocynaceae, and are largely native to North America including Minnesota. Masters of evolutionary adaptation, Asclepias are perfect for adding native vibrance to your home landscape, unbothered by deer or rabbits because of their namesake white latex sap.
Swamp milkweed, A. incarnata, is an upright species reaching 48-60” tall with lance-like opposite leaves and bright pink blooms, thriving in wetland areas and other consistently moist or wet full sun sites. Butterfly milkweed or butterfly weed, A. tuberosa, is a bushy, drought tolerant, full sun perennial reaching 24” round with vibrant orange blooms, and is a phenomenal native plant for low sunny borders and pollinator gardens. Showy milkweed, A. speciosa, is less vigorous than common milkweed, A. syriaca, with a similar appearance and drought tolerance better suited for the home garden, reaching 36” high with large, fragrant pink and white blooms in full sun.
Milkweeds bloom from mid-summer to fall with complex flowering umbels, their five-petaled blooms strongly recurved to expose the gynostemium, a reproductive structure fusing both male and female components into a single organ, surrounded by a five-membrane corona composed of a five-paired hood-and-horn structure where nectar is produced. Pollinariums, located in antheral gaps between the hood structures, are specialized pollen structures composed of a pair of pollen sacs, or pollinia, instead of more traditional pollen grains.
Pollinators land on the center of the slippery gynostegium, which then, by design, slip partially into gaps formed by adjacent anthers, where they rub against glands with pollinariums that then attach to the insect, meant to be pulled away when they fly away, though sometimes instead trapping insects too weak to remove the pollinia.
Pollination occurs when properly oriented pollinia are deposited into a stigmatic slit, possible only after pollinia are rotated 90 degrees, an evolutionary adaptation to slow the deposition process in favor of cross-pollinating the blooms of another plant instead of another bloom of the same cluster.
One or two pollinated flowers develop into mature seed-bearing follicles, milkweeds aborting the rest to allocate more resources to the formation of viable seed, produced in pods, called follicles, and overlapping in rows with filamentous white hairs (coma) which aid in wind dispersal after ripened pods split.