What type of weed is characterized by having stems that are flattened and can be easily identified in late spring?

The correct response highlights a specific characteristic of large crabgrass that sets it apart from the other weed types listed. Large crabgrass, known scientifically as Digitaria sanguinalis, typically displays flattened stems, particularly as it grows and matures in late spring. This distinctive stem shape can help in identifying this weed, especially as it spreads across lawns and other turf areas.

In contrast, the other options present different types of weeds that do not prominently exhibit the flattened stem characteristic in the same manner. Henbit, for example, is a winter annual that features a more upright growth habit and distinct purple flowers, making it easily recognizable but not aligning with the flattened stem trait. Bittercress is another species often identified by its small white flowers and tends to present straight stems rather than flattened ones. Gripeweed, known for its narrow leaves and a different form, also does not share the flattened stem feature that defines large crabgrass. Understanding these defining traits is essential for proper identification and management of turf and ornamental weeds.