Whever I left my water hyacinth and water lettuce in the pond after we had the first frost it would rot and sink before I could pull it out. As soon as the frost hit it the leaves would just fall off.

I don't think anything will kill pennywort. This stuff took over my pond this summer. I had a little boat with two fake frogs fishing from it. The pennywort totally covered the boat. As a matter of fact I forgot about it. You know how it is: Out of sight, out of mind. When I recovered the boat in the overgrown pennywort I had to cut it out. It took me three days to cut though the pennywort and everything it grew through i.e. floating planters,totally choked out my water lilies, etc. We have had several killing frosts and 1/2 inch of snow and that stuff is still growing strong along my pond edges. It grew between the rocks under my pond liner. I have a bad feeling that I am going to be fighting this stuff for years. Someone posted on this forum that it's not invasive unless it gets to land, well it overgrew the edges of my pond and was rooted into the soil. So I guess in answer to your question both you and your wife are right.

Jenny