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In Japanese knotweed, axils of the upper leaves produce slender, fuzzy spikes 2-4" long, covered with white or greenish flowers; more than one spike can arise from an axil. The flowers arise in groups of 3-6 from papery sheaths on the spikes. Individual flowers are sessile, about 8 mm long, with a funnel-shaped calyx, five petal-like sepals with wings on their outer faces, no petals, 6-8 stamens, and a tripartite stigma on top of a bullet-shaped green ovary. The fruit is a single shiny, black, three-angled seed with three broad, whitish, wings. Japanese knotweed is a highly aggressive invasive, luckily uncommon in Jackson Park. I know of three places in Jackson Park where it grows (it is a perennial); the Park District has eradicated it in one of these locations.

Reynoutria

Reynoutria japonica

Japanese knotweed [Blooms: Aug-Sep]