Postrate Pigweed is a summer annual with branched stems up to 2' long; it is more or less prostrate. The rather succulent stems are rather terete, smooth, and glaucous; they vary in color from whitish green to pale red. The alternate leaves are up to 2" long and half as much across; they are dark green, glabrous, obovate (spoon-shaped), and smooth along the margins. Each leaf tapers gradually to a slender petiole. Short clusters of light green flowers develop from the axils of the leaves. Each plant is monoecious and produces separate male (staminate) and female flowers (pistillate). The male flowers have 4-5 sepals, 3 stamens, and no petals, while the female flowers have 4-5 sepals, an ovary with 3 styles, and no petals.

The sepals of both kinds of flowers are about 1/8" in length or a little longer and oblong-lanceolate in shape. Underneath the flowers, are several bracts that are the same length or a little larger than the sepals. Like the sepals, the bracts are light green and oblong-lanceolate; their tips are pointed, but not spiny. The blooming period occurs from early summer into the fall; individual plants can bloom for about 1-3 months. The flowers are wind-pollinated. Each flower is replaced by a bladder-like capsule (or utricle) containing a single seed; this capsule is globoid and smooth while young. Later, it splits open around the middle to release the seed. The seeds are 1.1-1.7 mm. across; they are dark brown or black, shiny, round in circumference, and somewhat flattened. The root system consists of a stout taproot. This plant spreads by reseeding itself.