Sweet-grasses (Glyceria spp., Poaceae) tend to get under-recorded because they are tricky to separate in the field. They grow in damp grassy places by ponds, ditches, streams, swamps, canals and river-banks. The trick is to break off the top of a flowering stem and take it home. 
It is all just a question of careful measurement (to the nearest 0.1mm), and it helps to have good magnification and a bright light source. It is good to have fresh anthers, but this is not essential.
Measure the length of a central lemma: 5.5 mm or more is Glyceria fluitans (the commonest species, with anthers > 1.4mm).
For spp. with smaller lemmas (3.5-5mm) you need to look closely at the apex of the lemma: distinctly 3-toothed is G. declinata (left) and untoothed is G. notata (right). In G. declinata, the lemma is exceeded by 2 sharply pointed teeth at the apex of the palea (above, left image)
This is a good genus in which to look out for hybrids. Signs include small, shrivelled, empty anthers (early in the season) and inflorescences that don't break up at maturity (later in the season). The huge water-side plant with tiny spikelets (<12mm) is Glyceria maxima.
