Crab grass goes by a variety of names such as Bermuda grass and Saint Augustine grass. Not an actual grass but, rather, a weed, crab grass is among the most common type of weed to invade a lawn along with dandelions. Crab grass has two forms: large crab grass and smooth crab grass. Both take hold during warm moist weather and will remain until killed by frost if allowed. Chemical treatments such as Roundup or Scotts can help eliminate crab grass if applied directly.
Look at the grass in question. If it does not look like the surrounding grass it could be a weed; if it is a weed it could be crab grass. Crab grass grows in wet lawns and in crowded lawns with random bald spots around the crowded areas.
Examine at the weed in question. Crab grass grows new growth in an upright formation that resembles a crab; older blades or stems fall to the ground, giving the weed a star formation on the ground. Crab grass has many branches with blades as opposed to regular grass which only forms one branch. Crab grass often has purple stems.
Check the size of the weed to determine what type of crab grass you are dealing with. Large crab grass can grow to 3 feet in height. Smooth crab grass will only reach up to 15 inches. Large crab grass will take on a matted appearance.
View the leaves. Crab grass leaves are stubby. The edges are rough and have sharp points. The leaves can have a fold down the middle and are ¼ inch wide. Identify the leaves of large crab grass by the pale green or bluish color. Smooth crab grass has a dark, dull green color.
Observe the total area of coverage. Crab grass can overrun a lawn quickly. Crab grass grows faster than regular grass because the energy used to grow are at the base of the crab grass, where regular grass has all the energy stored at the top which is cut off. If the lawn is mowed too low the crab grass will grow faster and cause shadows over the regular grass, which will stunt or kill the regular grass.
Look for bare spots around crowded areas as this is a sign of crab grass, exhausting the moisture from the surrounding area.