Characteristics of Crab Grass

Identifying Crab Grass

• Two species of crab grass are found in North America. Smooth crab grass (Digitaria ischaemum) occurs primarily in the Northern regions, while large crab grass (D. sanguinalis) grows throughout North America. Both weeds produce thick, tough stems that grow parallel to the ground from the center of the plant. Leaf blades emerge from these stems, and the stems may become maroon as they age. Distinguishing between the two types of crab grass isn't essential before implementing control measures.

Life Cycle

• Crab grass is an annual grass, meaning that it grows anew from seed each spring. Crab grass germination begins when the soil reaches and maintains temperatures between 55 and 60 degrees Fahrenheit for a week to 10 days. Germination continues until the soil reaches a temperature of 95 F, meaning that new crab grass plants can continue emerging through the summer and into the fall. Crab grass goes to seed in July, producing between 150,000 and 180,000 seeds per plant before dying with autumn's first frost. In addition to reproducing vigorously by seed, crab grass also spreads vegetatively. Whenever one of its stems touches the ground, it produces a new set of roots capable of sustaining a new plant, allowing it to take over a space even before the seeds germinate in the spring.

Preferred Environment

• Despite its reputation as one of the homeowner's most despised weeds, crab grass does not compete well with lawn grasses. Instead, it tends to germinate in bare spots where it doesn't have to compete with neighboring plants. It can also utilize light applications of water better than turf grasses, which allows it to thrive under rain and irrigation conditions too light to promote the growth of most turf grasses.

Crab Grass Control

• These characteristics suggest control measures effective in limiting crab grass in your lawn and garden. Because crab grass competes poorly with other grasses, a dense, healthy lawn represents the best crab grass prevention. Lawns mowed taller than 2 inches are better able to resist crab grass. If you irrigate your lawn, use infrequent, deep soakings over frequent light sprinklings that water the crab grass but not the turf. If you've had crab grass problems in the past, the millions of seeds those plants released are waiting in the soil to colonize bare and thin spots where they can take hold. A pre-emergence herbicide applied before the soil warms in the spring will stop those seeds from germinating.