Amaranthus species are annual (or occasionally perennial) herbaceous plants, native to the Americas, that can grow up to 2 m in height. Here, we discuss A. viridis (green amaranth) and A. retroflexus (redroot amaranth).
There are approximately 60 Amaranthus species worldwide, of which 20 are found in Australia. Specific identification is best achieved based on differences in leaves, branches and flowers.
Green amaranth leaves are alternate, between 3-9 cm in length and 2-6 cm in width, oval to triangular in shape, smooth, and darker green on the top of the leaf. Flowers are brownish-yellow, about 1 mm in length and found at the end of plant stems. Seed is discshaped, shiny brown or black, and approximately 1 mm in length. Plants grow up to 2 m in height.
Redroot amaranth can also grow up to 2 m in height, and has a red-pink tap root. Leaves are alternate, oval to egg shaped and up to 10 cm in length, with more prominent veins than green amaranth. Flowers are numerous, greenish, and seed is oval to egg shaped, somewhat flattened, shiny black or dark brown, and approximately 1 mm in length.
Young plants can bear some resemblance to blackberry nightshade (Solanum nigrum) and fat hen (Chenopodium album), but with experience can be identified by their cotyledons and first true leaves.