Poa annua is a seasonal plant usually first occurring in October or November and persisting into the spring. It tends to mature somewhat more slowly than other non-native grasses but more quickly, and at smaller sizes, then Polypogon.
In its earliest stages Poa annua develops long narrow ascending blades that are usually somewhat curled. In sandy soil these sprouts usually occur far enough below the surface that blades from the same sprout appear like separate plants. At this stage the basic morphology is similar to E. paupera. Note in the photo below right that the older leaves have begun to take on both a curved and curled aspect.
Small sprouts bear a striking resemblance to E. paupera, including narrow curled leaves, and a tendency for early leaves to die back, leaving a sheath from which the plant continues to grow. At this early stage Poa annua leaves are already softer and more flexible, with a tendency towards S-curved blades, compared to the single curvature and stiffer blades of E. paupera.
At later stages Poa annua takes on a dense clumping aspect and tends to be dark green. E. paupera, by contrast, takes on a linear radiating aspect and tends to be yellowish green.
Poa annua is also very similar to Polypogon interruptus. Polypogon typically has more slender blades and matures much more slowly. In the photo below left, 3 Poa are joined by a single Polypogon (rightmost plant). In the photo below right Poa is on the left. Note that all of the Poa are producing seed, while the Polypogon are not. Poa is fairly common on the trail from the main house to the beach and in Road to Runway RA, where Polypogon is absent, so this is a good place to get a feel for Poa.