I believe there was somebody who had a problem with the variegated Phramgites just recently. I will try to find the thread. Others will give you good suggestions for plants soon.

Lisa

Here is the link.

Here is a link that might be useful: Invasive bog plant

Pondlifetony,
If my math is correct the liner 30 mils EDPM. You don't state what the weight of the underlayment is or how thick the clay and type of clay is in the pond.
Mike

Hi
Apparently you intend to grow the plants directly on the bottom? IME almost all water plants are VERY invasive . Growing in containers goes a long way toward avoidiing a lot of problems.
makes arranging and fertilizing MUCh easier also and a backsaver when a cleanout is needed. Only plant I've had make it through the liner is "Strangler fig" but then I've seen them penetrate driveways lol. gary