Peter's right we had problems with blanket weed at out old house & the use of natural water plus plenty of plants is the best way to get rid of it. We succeeded after a couple of years; I think we used a chemical as well though I'm not a fan of putting chemicals into ponds. I found duck weed far more of a problem, I got a scoop for taking chips from a chip fryer that had been thrown out by the local chippie the best thing for skimming the water, the trouble was you could be clear for a year if you worked hard then suddenly it would be back again.
I must bear all this in mind for our pond, it was going to be a slow process digging out 25` x 15` x 4`- 0 for a bog area when most of the area is full of rubble and the ground rises up by around 18" over the proposed site, the highest area is thankfully where I want the bog area to be or there would have been far more digging to do. I love the hard work but struggle with the time, however next door has a mini digger in tomorrow clearing the site where his hay barn burnt down, so I may see if he'll give me a couple of hours to dig it out.
I've Water Avens, Water Cinquefoil, Bog Bean, Water Mint, Flag Iris, Purple Loosestrife, Marsh Marigolds & Marsh Valerian all waiting to go in. I'd also like some Common Reed which is beautiful with its purple flowering heads, Branched Burr Reed & Flowering Rush plus plenty of Sedges..