Amaranthus tuberculatus -- Oregon

Amaranthus tuberculatus (waterhemp, tall waterhemp, common waterhemp, roughfruit waterhemp; Amaranthaceae) is a tall (to 2 m), taprooted, dioecious, annual forb.

The nomenclatural history of A. tuberculatus is somewhat complicated (see Costea et al. 2005, Canadian Journal of Plant Science 85: 507-522, for a fuller discussion). Two varieties are currently recognized, var. rudis (some older sources refer this taxon to the species level as Amaranthus rudis) and var. tuberculatus. Var. rudis was historically found west of the Mississippi River, from Nebraska to Texas, while var. tuberculatus was east of the Mississippi. Most sources refer to var. rudis, or do not distinguish between varieties.

The native range of A. tuberculatus is the midwestern and eastern United States. However, the USDA PLANTS Database also ascribes native status for A. tuberculatus in California, Washington, and Idaho (but absent/ no record in Oregon), as well as in Colorado, New Mexico, the southeastern U.S., and the Canadian provinces of Quebec, Ontario, and Manitoba. Other sources count A. tuberculatus as introduced west of a line of U.S. states comprised of Texas, Oklahoma, Kansas, Nebraska, and the Dakotas, in the southeastern U.S., and in Canada. A. tuberculatus is classified as a Class A noxious weed (eradication required) by the Washington State Noxious Weed Control Board, indicating that that body does not consider A. tuberculatus native to the state.

Outside of North America, A. tuberculatus has been introduced into eastern and central Canada, much of Europe, Israel, and Australia. Initial detections in the Eastern Hemisphere are frequently associated with ports or elsewhere where contaminated grain shipments are likely.

A. tuberculatus exhibits a high growth rate, produces a large number of viable and easily dispersed seeds and develops a persistent seed bank. These characteristics enable the species’ populations to expand rapidly after initial establishment. A. tuberculatus is a weed of concern in agricultural landscapes, even within its native range in the U.S., where it has been shown to dramatically reduce yields in corn, soybeans, and other crops, and has exhibited resistance to a number of herbicides. In wildlands, the species has been reported invading riparian areas in Portugal, Croatia, Italy, and Palestine, forming dense stands and displacing native plant species.

A. tuberculatus’ ability to establish and become a species of ecological concern in Oregon is dependent on the species ability to tolerate a Pacific Northwest climate. The species’ PRE score rises only to Moderate Potential Risk in part because its global distribution lies primarily outside this type of climate. Still, A. tuberculatus’ native range includes occurrences in areas in north Texas, the Oklahoma panhandle, southwestern Kansas, and eastern Colorado that ARE similar to Oregon’s climate, suggesting that this species deserves continued monitoring. Washington state’s rating of A. tuberculatus as a Class A Noxious Weed indicates that that neighboring state has determined that the species is a threat to its natural and agricultural ecosystems.