Flower:

Tiny flowers are tightly packed in small clusters (glomerules) in a spike-like arrangement at the tip of branching stems and arising from leaf axils, with a few to several flowers in a cluster. Flowers are either male (staminate) or female (pistillate), both on the same plant (monoecious) and usually mixed within the same glomerule.

Male flowers have 3 yellow stamens, female have a 3-parted style at the tip of an oval green ovary. Both lack petals and have 3 green sepals that are less than 2 mm long, slightly unequal in size and have a green midrib that extends to a pale spine-like tip. At the base of each flower is a bract that is similar to the sepals but about twice as long, or longer. The sepals, bracts and stalks are all smooth and hairless.

Leaves and stems:

Leaves are alternate, elliptic to spatula-shaped, mostly widest above the middle toothless, hairless, wedge-shaped at the base tapering to a stalk about half as long as the blade. Edges are commonly wavy, weakly to strongly so. The tip is mostly rounded and usually has a small spine-like extension at the apex. Early lower leaves may be up to 3 inches long and 1 inch wide, but most are less than half that size, an inch or less long.

Stems are whitish to straw-colored to light green, usually hairless, sometimes glandular-hairy, erect (rarely prostrate), much branched, the branches ascending to spreading. A plant can become quite bushy and large plants may break off near the base, forming tumbleweeds to spread their seed.

Fruit:

Fruit is a dry seed enclosed in the persistent ovary shell (pericarp) that has a wrinkly texture and a seam around the middle, the top coming off like a cap to expose the seed. Mature fruit is about as long as or longer than the sepals

Seeds are lens-shaped, .6 to 1 mm long, smooth, shiny dark reddish-brown to black. 

Notes:

Tumbleweed Amaranth is found in gravelly or sandy disturbed soils such as roadsides, railroads, cultivated fields, gravel pits, construction sites and vacant lots. While considered native to the central US and Canada, it has spread world-wide and has naturalized on all continents except Antarctica. It is distinguished by the erect bushy growth, small often crinkly leaves with a small spine at the tip, separate male and female flowers on the same plant, flowers with 3 sepals shorter than the floral bracts, and shiny lens-shaped seeds about 1 mm in diameter. A similar species is Prostrate Pigweed (Amaranthus blitoides), with has a prostrate growth, leaves that are usually flat not with crinkled edges, 4 or 5 sepals, bracts and sepals all similar in size, and dull black seeds 1.3 to 1.6 mm diameter.