rye grass, short-lived perennial, leafy, tufted plant belonging to the family Gramineae (grass family). Two species are grown in the United States—Italian rye grass ( Lolium multiflorum ), the leading hay grass of Europe, and English, or perennial, rye grass ( L. perenne ). In parts of the United States where winters are mild, both are sowed, often mixed with other grains, for pasturage. Italian rye grass is much used for lawns in warmer regions of the United States. Perennial rye grass was probably the first of all perennial grasses to be cultivated pure for forage. Poison rye grass, or darnel ( L. temulentum ), reputed to be poisonous, grows in grain fields and waste places; it is thought to be the tare of the Bible. Rye grass is classified in the division Magnoliophyta, class Liliatae, order Cyperales, family Gramineae.