Water Hemp
Amaranthus rudis
Pigweed family (Amaranthaceae)

Description: This native annual plant is highly variable in size, ranging from 2-8' tall. Larger plants branch frequently and are broader at the base than at the top. The hairless stems are either round or ridged, and green or pinkish red. The hairless alternate leaves are up to 6" long and 1" across. They are lanceolate, ovate-lanceolate, or oblong, with smooth margins and a somewhat shiny upper surface. The petioles are long and narrow, sometimes as long as the leaves, but usually shorter. The flowers are arranged in spikes, or narrow panicles of spikes, up to 1' long. These spikes may develop directly from the upper branches, or they may appear from the axils of the upper leaves. 

Water Hemp is dioecious, with individual plants producing either staminate flowers (male) or pistillate flowers (female). Both types of flower are less than 1/8" long. Each staminate flower consists of 5 sepals, 5 stamens, and no petals; it is surrounded by 1-3 narrow bracts with pointed tips. Each pistillate flower consists of 0-2 sepals (usually 1), an ovary with 3 feathery styles, and no petals; it is surrounded by 1-3 narrow bracts with pointed tips. The bracts of both staminate and pistillate flowers vary in color from green to reddish pink. The flowers and bracts are arranged along the spikes more or less densely, sometimes occurring along the spikes in loose bunches. The blooming period usually occurs from late summer to early fall, and lasts about 1-2 months. There is no floral scent. The pistillate flowers are replaced by ovoid capsules that split cleanly in the middle to release the seeds. Each capsule contains a single small seed. The seeds are flat, round, shiny, and dark brown to black. The root system consists of a taproot. This plant spreads by reseeding itself.

Cultivation: The preference is full or partial sun, wet to moist conditions, and fertile soil consisting of loam or clay loam. This plant will grow in drier sites with poor soil, but it will be smaller in size. This plant is rather weedy and can aggressively reseed itself in disturbed areas.

Range & Habitat: Water Hemp is a common plant that occurs in most areas of Illinois (see Distribution Map); it is somewhat less common in southern Illinois. Probably it is more widespread than the Distribution Map indicates, even though this map is derived from a recent source (from Mohlenbrock, 2001). Natural habitats include swampy areas, mudflats, and borders of small lakes, ponds, or rivers. It also occurs in cultivated fields and waste areas. This plant definitely prefers disturbed areas, and is probably spreading throughout the state.

Faunal Associations: The flowers are wind-pollinated and don't attract many insects. The caterpillars of the skipper Pholisora catullus (Common Sootywing) feed on the foliage of Amaranthus spp., as do the caterpillars of several moth species, including Holomelina aurantiaca (Orange Holomelina), Spilosoma congrua (Agreeable Tiger Moth), Hymenia perspectalis (Spotted Beet Webworm Moth), and Spoladea recurvalis (Hawaiian Beet Webworm Moth). Another insect that feeds on Amaranthus spp. is Piesma cinerea (Ash Gray Leaf Bug). The seeds are an attractive source of food to the Mourning Dove, ducks, sparrows, and other songbirds (see Bird Table). This is partly because the little seeds are produced in great abundance and can persist through the winter. Because the foliage can accumulate high levels of nitrates, it can be toxic to livestock when eaten in abundance.

Photographic Location: A mound of soil that was recently deposited by earth-moving equipment in Champaign, Illinois.

Comments: This despised plant is beneficial to wildlife, especially birds. This is just one more example why weedy plants should not be destroyed on sight for trivial reasons. However, it can invade cultivated fields and cause allergic reactions in humans from the wind-borne pollen. It is very similar in appearance to Amaranthus tuberculatus (Western Hemp), and both species share the same range in Illinois and have similar habitat preferences. However, the seed-containing capsules of Western Hemp split open irregularly with jagged edges, while the capsules of Water Hemp split cleanly and evenly in the middle. This is easy to observe by rubbing the flowering spikes with one's hand, as many of the capsules (along with the bracts and released seeds) will fall readily into one's palm. These two species are sometimes assigned to the Acnida genus because the pistillate flowers usually have a single sepal, whereas the pistillate flowers of other Amaranthus spp. usually have 5 sepals, like the staminate flowers.

Return