Reed sweet-grass (Glyceria maxima) is a tall, waterside grass found by freshwater lakes and ponds, slow moving rivers and in ditches. It is quite common and until you know it even exists it would be easy to assume this is the common reed, Phragmites. Once you know then the differences are quite obvious.
Firstly, reed sweet-grass does not grow as tall as the common reed and it does not form such large, dense patches. The flower heads are much greener than common reed and actually are formed quite differently if you compare the two. The flowers are in full flow in July and August. The leaves are narrower and there is a brown mark on the stem by the leaf junction.
I can find no explanation as to why it is called sweet-grass but it is obvious where the reed bit comes from although it is a grass and not a reed. It is an attractive plant when in full flower and it can be an important source of food and shelter for many creatures. Sadly, in the wrong place it can be labelled as an obnoxious weed and attempts to control are deemed necessary.