Leaves of asclepias subverticillata
are relatively long and very narrow, less than 0.2 inches across. They grow in whorls (usually 3 or 4) at closely-spaced intervals all along the stem, from the base to just below the flowers at the top. Leaf tips taper to a point. Leaf margins may be folded up slightly along the axis. Stems contain milky sap, and may be branched or unbranched.
Flowers form in small clusters, less than 2 inches in diameter, from the upper leaf nodes (one or two umbels per node), held on stalks up to 2 inches long. Individual flowers consist of five greenish-white sepals, or calyx lobes, below five larger, similarly-colored petals, or corolla lobes. At the center are five white or pale yellow hoods, much shorter than the petals, each containing a short horn that projects out above the white stigma at the middle. Sometimes the corolla is greyish-purple.