It sounds like you planted a rather large, strong growing variety.

Odorata hybrids can scoot around at the rate of six to fifteen foot a year, those can be a lark to trot after to keep up with.

Assuming its a natural pond with mud, gravel, sand, clay, bottom, the tether roots of the lily are likely to be four to six foot long and lash the rhisome to the bottom rather well...

To thin them out, a sharp knife (and something to sharpen it well) might be useful. Cut a line parallel to the rhisome downwards and inwards a few inches from the rhisome

When the tether roots are severed you will find the rhisomes wiggle out rather easy

Bad luck, if you planted those bare root on a liner pond...

While large fast growing varieties initially create an impression of 'growing great' and rather cheap, the long term liability is something that needs checking.

Alas a lot of unsuitable plants come on to the market rather cheap, with inadequate or misleading cultivation information

Regards, andy
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