Crab grass grows along with ordinary grass, and early in the growing season crab grass may not even be noticeable. In late summer, though, when many lawns dry out and become brown, patches of crab grass remain green, creating unhealthy patches. Why does crab grass do so well when ordinary grass struggles in late summer? Is this a phenomenon ? Or is this based on a scientific principleso, what is it that causes crab grass to thrive in relation to the different type of plants ?