Common Cord-grass (Spartina anglica)
In Poole Harbour where the water is more subdued and less wave action than on exposed sea coast, silt from the rivers various that flow in to the harbour settle and creates mud. This mud is covered by salt water for around 12 hours a and uncovered the rest of the time Anything growing here has to be able to withstand these changing conditions and be resistant to salt water.
As with Marram on sand so Cord-grass can cope with unstable mud. It is a very successful plant of the shore line mud flats and is actually planted to achieve stability in these conditions.
Not an attractive plant but an essential part of the process where vegetation recolonises lost ground.