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tarweed, any of several related resinous herbs (chiefly species of Hemizonia and Madia) of the family Asteraceae (aster family), having strongly scented and sticky herbage. Most North American species are found in fields and on dry hillsides of the Southwest and the Pacific region. They bear daisylike heads of yellow or cream-colored flowers. The heads of the common tarweed (M. elegans, also called common madia) are marked with an inner red ring and, like those of other Madia species, open in the evening and close before noon. Several species of this genus, notably M. sativa of Chile, are cultivated as oilseeds. Similar related Western plants are the rosinweeds (Calycadenia) and the gumweeds, or sticky-heads (Grindelia). Several gumweed species have become established in the East, where they are sometimes called tarweeds. The dried herbage of some gumweeds, containing resinous substances and essential oils, has been used in domestic remedies for treating burns and ivy poisoning. Tarweeds are classified in the division Magnoliophyta, class Magnoliopsida, order Asterales, family Asteraceae.
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