Amaranthus watsonii Standl.

Plants pubescent and glandular-pubescent, especially on bracts. Stems ascending to erect, usually much-branched, 0.1-1 m; branches usually ascending. Leaves: petiole shorter than or equaling blade; blade ovate, obovate to elliptic, or oblong-elliptic, 1-8 × 0.5-4 cm, base broadly cuneate to nearly rounded, margins entire, plane or slightly undulate, apex obtuse to emarginate, usually with terminal mucro. Inflorescences mostly terminal, erect spikes to panicles, usually thick and uninterrupted, with few axillary clusters in basal part of plant. Bracts: of pistillate flowers with long-excurrent midrib, 3-4 mm, longer than tepals, apex long-acuminate or mucronulate; of staminate flowers 2.5-4 mm, usually equaling outer tepals, apex long-acuminate or mucronulate. Pistillate flowers: tepals with dark midribs not excurrent, spatulate to fan-shaped, 1.7-2.2 mm, apex obtuse, with terminal mucro; style branches spreading; stigmas 2(-3). Staminate flowers: tepals 5, equal or subequal, 1.5-2(-3) mm, apex acute or almost obtuse; inner tepal apex acuminate or mucronulate; stamens 3-5. Utricles light brown to brown, obovoid to subglobose, 1.5-2 mm, shorter than tepals, walls thin, smooth or indistinctly rugose. Seeds dark reddish brown to nearly black, (0.8-)1-1.2 mm diam., shiny.

Ariz., Calif.; Mexico (Baja California, Baja California Sur, Sonora); Europe (reported as rare, non-naturalized casual alien).

Flowering late winter-spring, summer-fall.

Coastal dunes, beaches, sandy inland areas, weakly saline flats; 0-100m.

Amaranthus watsonii is a species of amaranth known by the common name Watson's amaranth. It is native to the southwestern United States and northern Mexico, where it grows in sandy places such as deserts and beaches, and disturbed areas. It is also known as a rare introduced species in parts of Europe. This is an erect annual herb producing a glandular hairy stem to a maximum height of about a meter. The leaves are generally oval-shaped and up to 8 centimeters long, with a petiole of up to 9 centimeters. The species is dioecious, with male and female individuals producing different types of flowers. The inflorescence is a long spike cluster of flowers interspersed with spiny green glandular bracts. The fruit is a smooth capsule about 2 millimeters long that snaps in half to reveal a small shiny reddish black seed.

Amaranthus watsonii là loài thực vật có hoa thuộc họ Dền. Loài này được Standl. mô tả khoa học đầu tiên năm 1914.