People have created weeds by carrying plants in boxes or as seeds in the soles of their shoes as they traveled. Thousands of types of green herbaceous plants with long slender leaves carpet lawns, feed grazing animals in pastures and provide ornamental accents in landscapes. Weed grasses do exactly the same thing except that, for one reason or another, people have identified them as invaders.
Begin identification by first eliminating sedges, because grass herbicides will not work on them. Sedges have triangular stems and long, narrow blades that grow in three rows up the stem, while grasses have round or flat stems.
Find summer annual weeds when they germinate in very early spring; they will grow all summer and die with the first hard frost. Crabgrass, foxtail, barnyard grass and goose grass are coarse summer annual grasses. Although most of the summer annual grasses grow throughout the U.S., some sub-groups have limited range. Large crabgrass, for example, does not grow well in the Deep South, crowfoot grass grows in the South and along the East Coast and foxtails favor fertile soils like lawns. Most annual grasses are difficult to control after germination; pre-emergent herbicides like MSMA (monosodium methyl arsonate) prevent seed germination.
Identify winter annual or biennial grasses as they begin growth in the fall and die the following fall. Italian ryegrass, often called annual ryegrass, crowds out perennial grasses. Jointed goat grass has a tall, jointed stem and poses problems in wheat fields because of its resemblance to the grain. Cheat grass is an invasive weed in the Western U.S. Urban dwellers may notice winter annual grasses along roadways, which also require pre-emergent control.
Identify perennial grasses by the fact that they regrow each year, developing larger crowns and deeper roots each season. Quackgrass is often mistaken for crabgrass; it is found across the U.S. except for the Deep South. Knotgrass grows only in the South and along the West Coast. Nimblewill resembles a fat Bermuda grass and forms dense mats in lawns. Tall fescue, creeping bentgrass and Zoysiagrass, all lawn grasses in the right place, become weeds in Kentucky bluegrass turf. Pre-emergents have limited effectiveness on these grasses.
Classify the grass using its growth habit, height, number of nodes, construction of collar and ligule and height and shape of seed head. Some grasses have folded leaves and others are curled leaves. Some, like tall fescue, grow in clumps; others, like Johnson grass, develop rhizomes from a crown. Crabgrass spreads above the ground, putting down roots from stolons and branch nodes.