Growing in and near wet places, Muhlenbergia racimosa has upright, puffy seedheads and extra wide leaves. Look for the little half-inch bundles of "seeds" (spikelets). The lowest bundles are often separated from the others. As in all Muhlys, each spikelet contains only one floret, containing the single seed (this is the give-away). Marsh Muhly is perennial and grows up to three feet high.
Orchardgrass or Dactylis glomerata
is similar. Take a close look at the fluffy seed bundles (the puffy branches) to see the obvious differences in detail
(several florets per spikelet).
Polypogon looks similar but is smaller, shorter,
and the awns are longer.
If in doubt, check the Puffs page.