The inflorescence of sphenosciadium capitellatum
is borne on top of lightly branched stems; each branch is topped by a compound umbel of between 4 and 18 (typically 9) umbellets, on bristly stalks (rays) up to two inches long. The umbellets have several dozen, tightly packed flowerheads, purplish when immature and when withered but (usually) pure white otherwise; sometimes pinkish. When mature the stamens protrude well beyond the petals. Between the flowers are many linear, bristly bractlets.
The compact, spherical, button-like flower clusters are about half an inch in diameter, quite well separated from each other, and hence the plant is easy to distinguish from other white-flowered members of the carrot family which tend to have flat-topped umbellets, adjacent or overlapping.