Posted: 12/03/2014 at 21:40
I bought my barley straw from a QD store for very little. It depends on the size of you pond how much to put in...Monty Don did give the ratio last year but I've forgotten. I stuff an old leg of a pair of tights or an onion bag full of the straw, tie off the end and then tuck it away under overhanging foliage at the side of my pond. You can weight it down so it is just below the water line. As it rots down it releases harmless chemicals in the water that deters the blanket weed. It takes several weeks before it really starts to work (rotting down) but continues to work for months on end after that.
As has been said, take out as much of the blanket weed as you can by hand in order to help in its reduction.