Lapland mountain sorrel

Rumex lapponicus is a herbaceous perennial plant with several or a single branched erect stem. Branching in the distal portion of the stem and inflorescence differentiates R. lapponicus from other species of Rumex, namely R. acetosa, of which R. lapponicus was initially mischaracterized as a subspecies. It has stems usually reaching 20-60 centimeters (8-24 inches) high, and broadly ovate, terminally sagittate leaves that are 3–10 cm long and 1–4 cm wide. It is dioecious.

Rumex lapponicus occurs between 0–2500 meters in elevation in montane and submontane meadow and outcrop environments throughout Canada (British Columbia, Alberta, Yukon, and the Northwest Territories), the United States (Montana, Wyoming, and Alaska), Greenland, and throughout North Eurasia. It is most successful in silty, fine soils along rivers and streams in montane and tundra zones.

The Norwegian name for Rumex lapponicus, setersyre, has led to some speculation that it was used on Mid-Norwegian summer farms as an acidifying agent. The United States Department of Agriculture has listed Rumex lapponicus as a noxious weed in the state of Arkansas, indicating that members of the genus Rumex "can directly or indirectly injure or cause damage to crops (including nursery stock or plant products), livestock, poultry or other interests of agriculture, irrigation, navigation, the natural resources of the United States, the public health, or the environment", in an official profile on the USDA website, and thus subject to interstate travel restrictions within the US.

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