Amaranth family (Amaranthaceae)
Description: This adventive plant 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.
Cultivation: This plant is typically found in sunny areas where the soil is moist or mesic, loamy, and fertile. It is quite weedy. The seeds can remain viable in the ground for at least 10 years.
Range & Habitat: Prostrate Pigweed is quite common in most areas of Illinois, except the NW and SE sections of the state, where it is apparently less common or absent. Prostrate Pigweed is native to the western United States. Habitats include fields, cropland, gardens, vacant lots, construction sites, landfills, areas along railroads and roads, and waste areas. This plant prefers highly disturbed habitats with bare open ground; it is not invasive of natural habitats in Illinois.
Faunal Associations: Flea Beetles (Disonycha spp.), the caterpillars of the skipper Pholisora catullus (Common Sootywing), and the caterpillars of several moths feed on the foliage of Amaranthus spp. (Pigweeds, Amaranths). Among the moths, this includes Spilosoma congrua (Agreeable Tiger Moth) and Holomelina aurantiaca (Orange Holomelina). The seeds are an important food source to many granivorous songbirds and some upland gamebirds (see Bird Table for a listing of these species). The foliage of Prostrate Pigweed is eaten by rabbits, pigs, and poultry.
Photographic Location: A construction site near the Champaign County Courthouse in Urbana, Illinois. The plant in the photograph isn't blooming.
Comments: This is a typical field weed. Prostrate Pigweed is easy to identify because of its prostrate habit and spoon-shaped leaves. Other Amaranth spp. (Pigweeds, Amaranths) in Illinois are more erect and their leaves are more lanceolate in shape. An exception is Amaranthus tuberculatus var. prostratus (Prostrate Water Hemp). This rare plant has a prostrate habit, but its leaves are lanceolate, rather than spoon-shaped, and its female flowers have fewer sepals. Prostrate Pigweed superficially resembles Portulaca oleracea (Common Purslane). However, this latter species produces small flowers with yellow petals and its leaves lack the conspicuous lateral veins that can be found on the leaves of Prostrate Pigweed. Some authorities use the scientific synonym Amaranthus graecizans for Prostrate Pigweed.