Where do I begin?
For several years, I avoided growing water lily, because I was afraid of winter care, potting, and dividing. But the past couple years I took the dive and found that it was not as intimidating as I first thought. Hardy water lilies need little winter care and are a good lily to grow and learn about lily growing. Tropicals need a little more attention, but they are well worth it! All lilies go dormant in wintertime, but first you must have the right size lily pot. Below are different methods that work out well...
To maximize size and bloom, tropical water lilies should be planted into large pots at least 12” in diameter or larger.
Do not fertilize!
Water lilies should not be fertilized at the end of the growing season. August 1st for cold regions and September 1st for mild to warm regions is a good target date to end fertilizing. This will promote dormancy and form a hard over-wintering tuber. A good rule of thumb is to restrict fertilizing two months before frosts begin. This will allow the lily to use up available fertilizer and prepare for dormancy. After the plant has died back and gone dormant, it will produce a smaller, hard tuber (or rhizome) different and separate from its normal root structure. This tuber is hard and dense enough that it is not easily crushed between thumb and forefinger, almost like a nut.
Note: Begin heavy fertilizing again after warm weather arrives and the plant is growing vigorously. Keep in mind that tropical water lilies need 2 to 4 times more fertilizer than hardy lilies.
Store after first 2 frosts:
It is good for the lilies to go through 1 or 2 frosts. The cool weather helps force dormancy. After the first or second frost remove the plants from the pond. Put the pots in a cool but protected place such as the garage and let the pots dry out somewhat until the soil is barely moist. Wrap each pot in a garbage bag and close the top of the bag so that they will not dry out much more. Do not close the top tightly; this way the plant will be able to breathe just a little. Sealing up the bag tight will also promote mold and mildew which will damage or even kill your lily plant.
Place the lilies in the house, garage, or basement with a consistent cool temperature of 55-60 degrees. For vivacious tropical water lilies that produce new plants from their leaves (such as Charles Thomas), the room temperature should be 60-65 degrees. When spring comes, take the plants out of storage. Place them back in the pond when water temperatures reach 65 degrees or more. They may also be forced in heated water in direct sunlight or with plant lights; low wattage submersible aquarium heaters work great for this in an aquarium or whiskey barrel size liner keep the heater off the plastic!).
After first 2 frosts:
Take each tuber out of the pot and gently clean it off (spraying off with normal water pressure, but do not scrub). Trim off mature roots and leaves. Tiny budding leaves can remain if present. Place each tuber in a sealed sandwich bag or glass jar (with or without a little fungicide) and cover it with water.
Place in the house, garage or basement with a consistent cool temperature of 55-60 degrees for regular tropical lilies and 60-65 degrees for vivacious tropical lilies. Tubers may also be placed in a glass jar without a lid and placed on a cool (but not cold) windowsill. In the spring, place each tuber into a 3"-4” starter pot with soil and ½ -1 tab of fertilizer; use 65+ degree water in direct sunlight or with plant lights. After the water lily fills out the starter pot, plant in a 1 gallon lily pot.
After first 2 frosts:
Take each tuber out of the pot and gently clean it off (spraying off with normal water pressure, but do not scrub). Trim off any roots and leaves. Gently towel dry and pack each tuber in damp, almost dry sand (with or without a fungicide powder). The sand should be dried out to the moisture level of pipe tobacco. Store in a sandwich bag or glass jar.
Place in the house, garage or basement with a consistent cool temperature of 55-60 degrees for regular tropical lilies and 60-65 degrees for vivacious tropical lilies. Plants should be left in darkness for a few months to keep them dormant. In the spring, follow the above directions.
Reference: Oregon Aquatics http://oregonaquatics.com/Overwintering%20tropical%20lilies.pdf