keith_nuttle wrote:

We have a rubber lined pond.

Are there any reason that flowers escaping their pots should not be
allowed to grow on the rocks and gravel in the pond?

There's no reason for the general health of the pond or plants, but it will
make it difficult to divide lilies in future - if the ponds getting either
too overgrown, or you just want to give a tuber to a friend.
--
derek