Flower:

Tiny flowers are tightly packed in small clusters (glomerules) in a spike-like arrangement at the tips of branching stems and arising from leaf axils, with a few to several flowers in a glomerule. The spikes are thick, dense, relatively short, erect to ascending but not stiff, and usually numerous. Flowers are either male or female, both on the same plant (monoecious) and usually mixed within the same glomerule.

Both male and female flowers have 5 tepals that are 2.5 to 3.5 mm (to 1/8 inch) long, thin, translucent whitish with a green midrib, more or less oblong and rounded at the tip, with a minute, sharp point at the apex (mucronate). Male flowers have 3 to 5 yellow stamens, female have a 3-parted style at the tip of an oval green ovary and are shorter than to about as long as the tepals. At the base of each flower is a bract that is longer than the tepals, green, tapering to a pointed tip, with a midrib that exends to a pale, spine-like tip.

Leaves and stems:

Leaves are alternate, 1 to 6 inches long, ½ to 3 inches wide, egg to diamond-shaped, widest at or below the middle, hairless to sparsely hairy, more densely hairy along the veins on the underside, bluntly pointed at the tip, wedge-shaped to somewhat rounded at the base, on a hairy stalk half to about as long as the blade. Edges are toothless and often somewhat wavy.

Stems are few-branched to many-branched, moderately to densely hairy especially in the upper plant, green to red to striped green/red, and have a pink to reddish taproot, which is the source of the common name. Plants are typically erect but when subject to mowing can be low and more sprawling, the branches spreading from the base and rising near the tips (decumbent).

Fruit:

Fruit is a dry seed enclosed in the persistent ovary shell (pericarp) that has a seam around the middle, the top coming off like a cap to expose the seed. Mature fruit (including the remains of the styles) is shorter than to about as long as the tepals. Seeds are lens-shaped, 1 to 1.3 mm long, smooth, shiny, black to dark reddish-brown.

Notes:

Redroot Pigweed is found in all manner of disturbed soils such as roadsides, railroads, cultivated fields, weedy shores, vacant lots, old homesteads and backyard gardens. It's origin is murky, though it may be native to eastern North American and/or Central America. In any case, it's not considered native to Minnesota, has spread globally, and is generally treated as a weed pretty much everywhere.

Redroot Pigweed is usually tall and erect, few-branched or many-branched; plants subject to mowing may be low and more spreading. It is distinguished by the hairy upper stem; sparsely hairy leaves that are egg to diamond-shaped, hairy leaf stalks that are half to as long as the blade; usually numerous thick, dense, relatively short, spike-like clusters that are erect to ascending but not particularly stiff; separate male and female flowers on the same plant (monoecious); 5 tepals 2.5 to 3.5 mm long, rounded to straight across at the tip with a minute point at the apex (mucronate), equal to or longer than the flower/capsule.

Two very similar Amaranthus species are Smooth Pigweed (A. hybridus), which is essentially hairless or only sparsely hairy in the upper plant, and Powell's Amaranth (A. powellii), which has stiff spikes, and both of which have tepals that taper to a pointed tip. The invasive Palmer's Amaranth (A. palmeri) is hairless or nearly so, has leaf stalks longer than the blade, flower spikes that are much much longer and more slender, and male and female flowers on separate plants (dioecious).