After two years of trying to outwit my koi, I now know what *doesn't* work to stop them from digging up water lilies:
1) a 1" layer of pea gravel over the planting medium.
2) a 2" layer of river rocks over the planting medium.
3) filter pads, cut to fit over the surface of the container with room for the lily to grow, with the fabric secured by landscaping fabric pins. I will say that it took the koi longer to uproot the lilies planted this way, but in the end the (very expensive!) lilies were still floating free in the pond.
I never had this problem until I added koi to the pond, and even then it wasn't until the last couple of years. Now I have two that are 12" long and one that is about 8", and they are an unstoppable lily-uprooting force.
Any suggestions will be gratefully accepted!
Koi digging up water lilies
I would try a basket, wire mesh as pictured here, overturned on top of the pot. Cut just enough space in the top (bottom) for the foliage to be carefully fed through. It lets water circulate, but stops larger fish from actually digging. It's also easy to weigh down with heavy rocks set on top.
