Slender Pigweed (Green Form)
Amaranth family (Amaranthaceae)
Description: This adventive plant is a summer annual about 2-6' tall. It is unbranched, or branches occasionally. The central stem is light green or tan-green, round in circumference, ribbed, and usually hairy; sometimes the lower portion of this stem is hairless. The alternate leaves are up to 5½" long and 3" across (excluding the petioles), becoming slightly smaller as they ascend the central stem. They are ovate or elliptic-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 green foliage, although their may be red tints along the margins of the leaves and elsewhere. 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 green flowers. This panicle is up to 2' long on large plants. From the axils of the upper leaves, there 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 cream 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 green 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 tan 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 conditions, and a loamy soil with high nitrogen content. However, it is adaptable and can be found in gravelly soil, clay-loam, and moist to dry areas. The size of individual plants is highly variable, depending on soil fertility and moisture amounts. The seeds can remain viable in the soil for eat least 30 years.
Range & Habitat: Slender Pigweed is a common plant that has been observed in most counties of Illinois (see Distribution Map). The green form of this species is more common than the red form (the Distribution Map combines observations for both forms). It 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 and degraded habitats are preferred.
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: This is the common green form of Slender Pigweed. Contrary to the scientific name, this species is not a hybrid, although it can hybridize with Amaranthus retroflexus (Rough Pigweed). These two species of Pigweed are both from South America and they are fairly difficult to tell apart. In this regard, a 10x hand lens can be helpful. Generally, the flowering spikes of Slender Amaranth are more green and slender than those of Rough Pigweed; the flowering spikes of the latter are whitish green and rather stout. The floral bracts of Slender Pigweed are about 3-4 mm., while the floral bracts of Rough Pigweed are about 4-6 mm. long. The sepals of Slender Pigweed usually have pointed tips, while the sepals of Rough Pigweed have tips that are more flattened. Rough Pigweed is generally more short and stout in its growth habit, and it is usually no more than 3' tall. Another common name for Amaranthus hybridus is Green Pigweed, but the foliage of this species is sometimes red.