The most aggressive of my water lilies is Augustus McCray, which has particularly fat rhizomes, grows quickly, and gets very large. Last spring I placed a large (20 gallon) fabric pot in my pond with a good-sized specimen in the middle and it produced lots of crowns, spreading to 10' across the water's surface. I let it overwinter in the pond, and this year when I saw it coming up from areas four feet apart, I knew I had problem.
It seems the rhizome branches had split the fabric in multiple places around the sides and rapidly shot out from the center. I went around the sides removing all of the thick rhizomes that were outside the original perimeter of the fabric pot. When I was done, it looked like this:
Here's a couple of the removed rhizomes:
I've heard many times that the best time to transplant is in the fall, so they get a chance to produce roots. I always end up dividing and transplanting my lilies in the spring, though, when I do my initial cleaning. This year I'm late, after the worst winter on record here, but the water is still pretty cool.
I've also been told to put lilies in poor nutrient clay material, in pots with no holes (so the roots can't escape), and to fertilize regularly. My happiest plants seem to be those in containers where I put a bit of organic material in the bottom, and then place silty material on top (our standard top soil here is silt/sand). This also seems to work best when there's some holes in the bottom, but it can be a source of excess nutrients in the pond. In the last couple of years I've experimented with fabric pots (such as root pouches), and the water lilies seem to love those. This latest evidence makes me wonder about their longevity, though. I've got a similarly sized pot on the other side of this pond with a younger division in it, so I'm going to try to keep its growth in check and see how the pot does.
In the meantime, lots of divisions to pot up. My main water lily pond has mostly been taken over by 'Almost Black'. It is a solid mat of fibrous roots from one end to the other, though, so hard to tell what all is in there. I'm aggressively tearing and dividing, though, in hopes of providing more room for everything I leave, and maybe discovering some suppressed rhizomes of 'Arc en Ciel' or some of the others that used to be in there. Guess I'll be hauling water lilies down to the spring plant sale in a couple weeks, but unfortunately won't be able to guarantee the identity of those.
For reference:
Hardy Water Lily (Nymphaea 'Augustus McCray')
Dwarf Hardy Water Lily (Nymphaea 'Almost Black')
