Tropical waterlilies are spectacular, but not easy to overwinter! Photo: http://www.aquascape.com
From the gardener’s point of view, there are two main categories of waterlilies (Nymphaea spp.): hardy waterlilies, which tolerate the cold and can therefore overwinter outdoors in a water garden in temperate climates (at least, as long as the water doesn’t freeze completely), and tropical waterlilies, which don’t tolerate frost nor even cold temperatures!
And tropical waterlilies are spectacular, blooming more abundantly and longer than the hardy waterlilies, with flowers often larger and more fragrant. It’s hard to resist their charms! But what can you do with such tender beauties in winter? Unless you live in a very mild—even tropical—climate (hardiness zones 9 to 12), their future doesn’t appear very rosy!
What Experts Say
Expert waterlily gardeners usually say the best thing to do with tropical waterlilies is to let them freeze, because keeping them alive over the winter is a hassle. Most aquatic plant catalogs agree: they usually recommend treating tropical waterlilies as annuals. Just toss them into the compost in the fall and, next summer, buy new ones, they recommend.
And that is great advice if your budget allows it, but they are expensive plants, costing much more than you’d normally think of paying for a simple annual. Are there no other options?
Fortunately, there are. There are even two options. You can either push tropical waterlilies into dormancy and keep them “dry” (relatively speaking) in a basement or keep them growing in a warm, well-lit room for the winter.
Keeping Tropical Waterlilies Dormant
Even in the wild, most tropical waterlily species have a dormant period during which they lose their foliage and flowers and retreat into their tuber for a few months. Sometimes this corresponds to a period of the year when temperatures are cooler or to a dry season when their pond dries out partly or completely. You can try to induce that kind of dormancy in your tropical waterlilies.
In this case, you can leave the pots outdoors until frost has killed back the foliage. At this point, the water will be cold and the plant will already be dormant.
Remove the pot from the pond and cut off both dead leaves and any that are struggling to grow. Bring the pot indoors and seal it in a clear plastic bag. Place it in a dark spot at a minimum temperature of 60 °F (15 °C), perhaps in a basement.
This is not the kind of dry dormancy you’d give to a gladiolus corm: you need to keep the soil moist at all times, so check every now and then to make sure it’s still humid.
In April, it’s time to wake the plant up. Place the pot in an aquarium or in a bucket or tub of water. It needs to be covered in water. Use an aquarium heater to keep the water at 70 °F (21 °C) or more. This will tell the plant that it’s time to wake up. Soon, new leaves will rise to the surface. If nothing happens, give it a shock treatment at 85 °F (30 °C) for a few weeks: that will wake it up!
At the beginning of the summer, when the water of your pond also reaches about 70 °F (21 °C), you can put the plant into its summer home.
Keeping Tropical Waterlilies Growing
The other method of keeping tropical waterlilies alive over the winter is to keep them growing.
In this case, bring the plant in early, while nights are still warm. Place the pot in the bottom of a tub or bucket and keep it filled with warm water. You’ll need to maintain strong lighting and summer temperatures all winter: essentially, you’ll want “greenhouse conditions”. Even in my greenhouse, I found I needed to add artificial light to supplement the sun’s rays, otherwise I lost them. You could also grow them under artificial light alone, for example, hanging LED plant lights just above the foliage.
You need to maintain a minimum water temperature of about 70 °F (21 °C) as well, and since water is generally cooler than the air above, “room temperature” water is likely to be too cold, so you’ll have to heat the water as well. For that purpose, an aquarium heater will be useful.
Under such conditions, the plant often continues to grow and even to bloom for a while, but often does start to decline in early winter (around Christmas) and may go into a short period of dormancy or near dormancy, keeping only a few leaves. If so, don’t be too concerned. It will likely start a new cycle of growth a few weeks later.
As with reawakened dormant tropical waterlilies, don’t even think of moving your plant outdoors to your pond for the summer until the water there has warmed up to about 70 °F (21 °C).
Tropical waterlilies: they’re so beautiful they may well be worth some special care during the winter months!