The following article contains information on weed biology and how to best manage large and smooth crabgrass infestations in field corn, soybeans, and alfalfa. Smooth - Digitaria ischaemum. Large crabgrass has a membranous ligule that is truncate and has a jagged appearance. The inflorescent consists of a panicle of branches with spikelets arranged in two rows of a flattened rachis. Large crabgrass may grow up to 3 feet in height.
Smooth crabgrass has a membranous ligule that is truncate and often has a smooth appearance.
The sheath has no hairs. The inflorescent consists of a panicle of digitate branches with spikelets arranged in two rows of a flattened rachis. Smooth crabgrass may grow up to 2 feet in height. Overall, smooth crabgrass is similar to large crabgrass with the exception that smooth crabgrass is not as coarse or tall, has no hairs around the sheath or on the leaves, and is more reddish or purple. Both germinate at higher soil temperatures, usually later in the season, when the residual activity of some soil-applied herbicides may have dissipated to the point that the grasses may escape.