How to Overwinter Tropical Water Lilies
Tropical water lilies (Nymphaea spp.) truly are tropical or nearly so, only thriving outdoors permanently in hardiness zones 9 to 11. In colder zones, they are often treated as annuals, but since they are very attractive and also very expensive, you might want to try and keep them from from one year to the next. Conserving them is however a bit difficult. Here are two methods you can try.
In this method, take advantage of the plant’s natural tendency to go dormant when temperatures drop or when there is a drought. Bring the pot indoors after the first frost and remove the foliage. Place the pot in darkness, keeping the soil moist but not wet. It is important to maintain a minimum temperature of 60˚F (15˚C). In April, place the pot in an aquarium or tub, making sure the top is covered with 4 to 6 inches (10-15 cm) of water. Use an aquarium heater to keep the water at about 68˚F (20˚C). This will stimulate the plant to start a new growth cycle. When outdoor water temperatures also reach 68˚F (20˚C), move it back into the summer water garden.
Keep It Growing
The other method is to keep the plant growing indoors during the winter. Unless you have a lot of space, this method will be most practical with miniature tropical water lilies.
You’ll need a warm, brightly-lit location (a greenhouse or intense artificial lighting would be quite appropriate). Bring the pot indoors and cut off the larger leaves. Place the pot in a tub or basin and keep the crown covered with 4 to 6 inches (10-15 cm) of water. You’ll probably need an aquarium heater, as you have to keep the water temperature above 68˚C (20˚C). With this method, you may even get a few blooms indoors in the fall, but the plant will usually stop blooming and go through a short dormant period around Christmas, then start a new growth cycle a few weeks later. Again, you can move the plant back outdoors when the water temperature in your pond is above 68˚C (20˚C).
Best of luck!