Crab grass and Johnsongrass are invasive weeds that can be classified as grasses. They differ in many ways, including size and the seasons in which they grow. Crab grass is a name that indicates one of two types: smooth or hairy. Hairy crab grass is sometimes called large crab grass. Johnsongrass is one type of grass, a taller, larger weed grass that generally invades crop fields.
Vastly different in size and appearance, crab grass and Johnsongrass can easily be differentiated by growth height. According to the University of California Integrated Pest Management, hairy crab grass grows up to 2 feet tall if it is not mowed. Unmowed smooth crab grass grows to about 6 inches in height. According to the University of Missouri Extension, Johnsongrass height ranges from 6 to 8 feet tall.
Crab grass propagates by seed, while Johnsongrass mainly propagates through rhizomes. Propagation is a plant's means of reproduction. Rhizomes are underground stems that extend from the original plant and root into a new plant.
Crab grass is an annual plant, while Johnsongrass is a perennial. An annual is a plant that germinates, grows, blooms and goes to seed in one year and begins the cycle again the following germinating season. A perennial plant can survive several growing seasons in a row. According to the Texas A & M Agricultural Department, a perennial plant might not bloom until its second year of life. Because their growth and reproductive patterns are divergent, meaning their reproductive patterns do not align, perennial and annual weeds require differently timed weed control techniques to succeed in eradicating the invasion.
According to the University of Missouri Extension, infestations of Johnsongrass can damage crops in fields of grain plants like those that grow corn and wheat, as well as many other crops worldwide. In contrast, crab grasses are landscaping and gardening pests and can invade lawn areas in expanding patches, but do not generally affect crops.
Crab grass can be found almost anywhere, as long as it receives some rain in the summer. In the United States, crab grasses are found in every state in the country except Alaska, according to the United States Department of Agriculture. The presence of crab grass spreads north and reaches mid-northern Canada before it tapers off.
Johnsongrass is typically found in the southern parts of the United States. Thought to be from Asia or Africa, Johnsongrass is a multinational invasive crop weed with reported sightings in more than 50 countries.