Pigweed (Red Form)
Amaranth family (Amaranthaceae)
Description: This plant is a summer annual about 2-4' tall. It is unbranched, or branches occasionally. The central stem is greenish tan to red, round in circumference, ribbed, and usually hairy; sometimes the lower portion of this stem is hairless. The alternate leaves are up to 4½" long and 2¾" across (excluding the petioles), becoming slightly smaller as they ascend the central stem. They are ovate orelliptic-ovate, smooth or slightly undulate along the margins, and pubescent or hairless. The lower side of each leaf has elevated pinnate veins. The uppermost leaves are smaller, lanceolate, and pubescent. This form of Slender Pigweed has predominately red foliage, although the leaves may be green with blotches of red. The petioles are quite long, causing the leaves to droop downward somewhat; they are often pubescent like the stems.
The upper stem terminates in an elongated panicle of spikes with small reddish flowers. This panicle is up to 1½' long on large plants. From the axils of the upper leaves, there also develops panicles of spikes or simple spikes of flowers that are much shorter than the terminal inflorescence. The individual spikes have a bristly appearance because of the linear bracts surrounding the flowers. Slender Pigweed is usually monoecious with staminate and pistillate flowers on the same plant. Each type of flower has 5 pink sepals and no petals. The sepals are about 2 mm. in length and oblong with short pointed tips. Each staminate flower (male) has 5 stamens, while each pistillate flower (female) has an ovary with 3 styles. Each type of flower is surrounded by several red bracts that are about 3-4 mm. long and linear with long pointed tips. The blooming period occurs during late summer to early fall and lasts about 1-2 months. The flowers are wind-pollinated. Each flower is replaced by a seed that is contained in a bladder-like membrane (utricle). This utricle becomes brown and splits apart to release the seed. The small seeds are flattened, dark brown or black, circular, and shiny. They are produced in great abundance on a robust plant. The root system consists of a taproot that is short and stout; it is often tinted red. This plant spreads by reseeding itself.
Cultivation: This plant typically occurs in full sun, mesic to dry conditions, and a soil containing loam, clay-loam, or gravel. The size of individual plants is highly variable, depending on soil fertility and moisture amounts.
Range & Habitat: The adventive Slender Pigweed is a common plant that has been observed in most counties of Illinois (see Distribution Map). The red form of this species is less common than the green form (the Distribution Map combines observations for both forms). Slender Pigweed is originally from South America. Habitats include weedy meadows, cropland, fallow fields, farm lots, vegetable gardens, gravelly areas along railroads, and waste areas. Highly disturbed areas are preferred; the red form of Slender Amaranth often occurs along railroads.
Faunal Associations: Insects rarely visit the wind-pollinated flowers. The caterpillars of the skipper Pholisora catullus (Common Sootywing) feed on the foliage, as do the caterpillars of several moths, including Holomelina aurantiaca (Orange Holomelina), Hymenia perspectalis (Spotted Beet Webworm Moth), Spilosoma congrua (Agreeable Tiger Moth), and Spoladea recurvalis (Hawaiian Beet Webworm Moth). Sometimes Disonycha spp. (Flea Beetles) chew little holes in the leaves. The seeds of pigweeds are very popular with granivorus birds as a source of food during the fall and winter (see Bird Table). Pigs and cattle eat pigweeds readily, although the foliage can cause bloating and other symptoms of nitrate poisoning if an excessive amount of the foliage is eatened. Deer and rabbits eat pigweeds to a limited extent.
Photographic Location: A weedy meadow at Judge Webber Park in Urbana, Illinois.
Comments: The upper photograph displays the red form of Slender Pigweed. The 'redness' of individual plants can be quite variable – this is particularly true of the leaves, which can vary from green with dark purple blotches to to brilliant red. The flowering panicles and spikes are most likely to assume a bright red color. Another species that is often more or less red, Amaranthus cruentus (Red Amaranth), is occasionally cultivated as a garden plant. This species has been known to escape from cultivation, although this is uncommon. Some authorities regard Red Amaranth as a cultivated form Slender Amaranth, or Amaranthus hybridus cruentus. However, Mohlenbrock (2001) considers Red Amaranth a separate species that has bracts about 2-3 mm. in length and staminate flowers with only 3 stamens, rather than the usual 5. Another similar species, Amaranthus retroflexus (Rough Pigweed), is somewhat stouter and has longer bracts (about 4-6 mm.) than Slender Amaranth. To my knowledge, Rough Pigweed doesn't assume a red form.