Which aquatic plant has flattened, winged, red-brown brittle stems that grow up to 6 feet long?

The aquatic plant characterized by flattened, winged, red-brown brittle stems that can grow up to 6 feet long is indeed curly-leaf pondweed. This plant is notable for its distinctive stem structure and its ability to thrive in various aquatic environments, often forming dense mats that can impact local ecosystems.

Curly-leaf pondweed is particularly recognized for its unique morphology, which includes the broad, wing-like extensions along the stems, contributing to its overall flattened appearance. The reddish-brown color can further help in its identification. This species is also known for its rapid growth in the spring, which can outcompete native vegetation and disrupt local habitats.

In contrast, the other options, such as clasping-leaf pondweed, brittle naiad, and southern naiad, do not possess the same stem characteristics as curly-leaf pondweed. Clasping-leaf pondweed has different leaf shapes and arrangements. Brittle naiad and southern naiad have distinct stem and leaf features that set them apart from curly-leaf pondweed. Understanding these physical traits is crucial for accurate identification in aquatic pest management.