For many who grew up in the rural Midwest, Asclepias syriaca was part of childhood: its interesting purplish flowers, the white milk that is exuded when leaves are broken, the parental cautions not to eat it, the large spiky seed pods, and the fantastic little seeds themselves, with their cottony tails, floating through the clear fall air. We learned about the common milkweed's role as host for larvae and caterpillars of the monarch butterfly (Danaus plexippus)—and this was, for some of us, our introduction to ecology and conservation.

Milkweed populations in the Midwest have declined precipitously—primarily in croplands, as a result of glyphosate weed killers (Roundup and relatives). The good news, such as it is, is that a recent study by David Zaya and collaborators (2017) at the Illinois Natural History Survey determined that milkweed populations in natural areas of the Midwest have not fared as poorly. Although recapturing all of the milkweed population lost to agriculture is probably not plausible, the authors conclude that "augmentation of natural areas and the management of existing grasslands, such as less frequent mowing and woody- and exotic-species control, may replace some of the milkweed that has been lost from croplands."

Many interesting insects—aside from monarchs and their caterpillars—are often seen on common milkweed, and some of these are found exclusively with milkweeds; see the photos and descriptions below.

Common milkweed is poisonous, according to all of our sources (list below).

Asclepias syriaca can be fairly easily distinguished from similar milkweed species with careful observation. The mature plant is tall—usually in the 3–5 foot range, but occasionally shorter or taller. Several to many flower clusters develop near or at the top of the plant; the purplish flowers are "typical" milkweed flowers with "hoods" and "horns" (see illustration) set on bent-back, slightly fuzzy base-petals (corollas) atop relatively long flower-stems. The large leaves are arranged opposite each other on the stem. Each leaf has a prominent central vein, and the base of the leaf is narrowed to a very short leaf-stem. The undersides of the leaves are slightly to moderately fuzzy. After the flowers mature, distinctive seedpods develop in their place; these pods get quite large by late summer and are covered with soft prickles.

Asclepias purpurascens (purple milkweed) is similar but features darker purple flowers with bald base-petals (corollas) that are less prominent; it also usually features fewer flower clusters. Asclepias sullivantii (prairie milkweed) is also similar; it has leaves with bald undersides and rounded bases and is usually shorter, with fewer flower clusters. Neither of these look-alikes features spiny seedpods.