Among the hardy waterlilies 'species' lilies such as Odorata will seed, the hybrids (cross bred) mostly have very low fertility, you might see one or none fertile seed pod among a thousand crowns.
There are five or six 'rhisome' growing types among the hardies, Odorata hybrids amble across a pond quite fast, perhaps up to six foot a year, while Marliac type rhisomes are mainly slow growers, it might take a Froebeli ten years to spread one foot in distance, though, you will find a lot of crowns that flower in that small space are combined with a very high flower to leaf ratio
Whoa, Bill has an Atropurpurea, gosh! that is a bit special... Where grazing seems to put too much pressure on waterlilies, a fence of chicken mesh encircling the positions should buy time for the crowns to strengthen up. With a good volume of 'bio mass' outnumbering the fish, grazing levels can strike a happy medium
I don't know how grass carp compare with koi carp, habit wise, however with low fish density to high plant density, waterlilies cope quite well with koi, where there is one adult koi per 4,000 gallons on a heavily planted pond
Regards, andy
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