Bulrushes aren't poisonous, the roots are a bit too stringy this time of year, you can eat the shoots as well. There is a similarity with irises which at moment have bright yellow or blue flowers. The roots of iris are knobbly and stink of pig poo when cooked, they make a purple die.
The main danger from bulrushes is that they absorb pollution from water very easily. They are used as toxic sponges in the wirral to soak up dioxin and heavy metals. Personally I like to know what the industrial history of the area I pick them from and the natural history of the water course that feeds them.