That link in Julles' post contains a lot of good information. I also like the pictures showing pots that are more wide than they are deep. Much more in line with what waterlilies (and lotus for that matter,) really should be grown in.
I'm finally working on getting a similar guide together to post on my website. Some additional information I'll be including is as follows:
How do you know you really should re-pot waterlilies? If you use fertilizer tabs and it takes a hammer to get them down into the soil, that's a pretty good sign. If the rhizome of a hardy waterlily has grown out past the rim of the pot and now sticks out a foot into the water, that's another one that really needs attention.
There are three things to consider here. Do you want to reduce the size of the plant to fit the current size pot? Would you rather re-plant the waterlily in a larger pot to allow the plant to grow larger? And one thing not yet mentioned, do you want to end up with additional plants from this re-potting?
Most, if not all waterlilies, will perform much better when grown in larger pots than the ubiquitous one gallon "squat pot" that is probably the most common pot in use. I've always thought that the 2 1/2 gallon pot, shaped like the one pictured in the above link, should really be the "standard size" in place of the one gallon pot. Most waterlilies that are started off in that size pot should be able to grow for two seasons before needing to be re-potted. But, for many smaller ponds, that would be cutting back on the number of plants you would be able to grow in the pond.
As for what soil to plant waterlilies in, I stick with what one of Perry Slocum's grandsons told me. "If it will grow grass, it will grow waterlilies." That is true even of the native Florida sandy soil that I have a yard full of. You can even use pure builder's sand if you have to, thanks to the rather incredible fertilizers available today. You can experiment with all the various soil mixtures and additives you like, but when it comes down to it, whatever you have on hand will do just fine.
The same holds true for most of the "top dressings" you hear about. Some simple, such as a layer of pure, white sand, some more complicated involving multiple layers of different materials. After a couple of months in the pond, you won't see much of a difference in the looks of the planting. None of them really "keep nutrients" from leaching into the water, about the only real purpose of them is to please the eye, and as I said, that seldom last very long. A top dressing can help keeping fish from stirring up the soil, but the small rocks often used can easily be scattered about the pond by the fish and complicate adding fertilizers. If you need to keep fish from rooting around, consider using larger, flat rocks that you can pick up to put the tabs into the soil, then replace, at least that way you aren't pushing down a layer of gravel into the pot with every tab, giving the fish a place to dig.
Many times, I've found tropical waterlilies in need of re-potting that had more than one crown, or growth point where the pads seem to radiate from. When re-potting, these two (or more,) crowns can be pulled apart and give you two plants where you had only one. Each newly potted plant will be more productive when separated. (By "more productive," I mean they will put up more and generally larger blooms.) And anytime you re-pot a tropical waterlily, look through the soil in the old pot, and in the roots at the base of the plant for small corms, that can also be planted to grow new waterlilies. If you don't want those for growing new plants, you might have some friends who would want them. You can also use them to trade with others for new varieties for your pond.
That tuber that got cut away from the hardy waterlily can also be the source for several new plants if you want them. Sometimes I simply keep them in a bucket of water and wait for new plants to show up. Most of the time, the tuber will have some lumps or "eyes" on them, kind of like you'll occasionally see on a potato. Once the eye has produced a couple of small pads and some roots, they can be removed and planted.
Any plant that is "root-bound," needs to be re-potted. You will notice that both pad and bloom production has slowed down, and the new pads and blooms will be smaller. Will likely see roots coming up out of the soil, and into the water column. You will have a hard time pushing a finger into the pot to insert a fertilizer tab. This plant can be re-potted into a larger pot any time of the year, with little, if any, "re-potting shock." Just keep the root mass mostly intact, freeing up the roots a bit on bottom of the mass, and a little on the sides as well.
If it is to go back into the same pot, or one the same size, you will have to reduce the root mass. You should also remove a similar amount of pads, as well. Not much different from re-potting terrestrial plants. This waterlily will go through a period of re-potting shock, but growth should start again in about two weeks or so.
Just thought I throw in some more thoughts on the subject.
Mike S.
Spring Hill, FL
