Like many other northern water garden enthusiasts we are faced with the problem of wintering over tropical waterlilies in an era of rapidly escalating energy costs. Although we have been successful in wintering over storage tubers of night bloomers, attempts to winter over day bloomer tubers has been marginal at best and stored tubers don't give us beautiful flowers on cold winter days.

At the outset a brief description of our wintering facilities might be in order. We have a 28' (8.5m) by approximately 8' (2.4m) greenhouse, with a southern exposure, attached to a sun room. The south face of the greenhouse is covered with twin wall polycarbonate, at a 60 degree angle, and there is a single wall between the greenhouse and sun room.

Inside the greenhouse are 100 gallon (379 liter) tubs on the outside wall to the left and 40 gallon (151 liter) tubs on the right. The shelf above the 40 gallon (151 liter) tubs holds up to 18 clear 4 gallon (15 liter) tubs. The greenhouse floor is 2' (.6m) lower than the sun room so that the shelf and tubs thereon don't block sunlight from the sun room tubs.

This view of the sun room shows 100 gallon (379 liter) and 150 gallon (568 liter) tubs on the left and the corner of a 7' x 8' (2.1m x 2.4m) pool to the right. There is a 3' x 12' (1m x 3.7m) skylight above the pool to supplement the sunlight filtered through two layers of polycarbonate paneling, each of which has a less than 80% light transmission rate.  

In the greenhouse we have seven unheated 100 gallon (379 liter) tubs plus five 40 gallon (151 liter)and one 25 gallon (95 liter) tub and up to 18 clear 4 gallon (15 liter) tubs on heat mats controlled by a very accurate thermostat set to come on when air temperature drops below 72° F (22 C).

On a sunny winter's day the temperature in the greenhouse can exceed 100° F (38 C) but on cold nights three 1500 watt heaters have struggled to keep the temperature above 65° F (18 C). Running all three cost us a fortune and the thermostats are so inaccurate that they often ran when the temperature in the greenhouse was 90° F (32 C) or more.

The sun room is warmed by a propane heater with supplemental heat provided via a fan that pushes hot air from a room heated by a wood stove on cold nights, but it is often a struggle to keep room temperature above our target minimum of 70° F (21 C).

We also have a dozen 40 gallon (151 liter) tubs in the basement that are heated with 250 or 300 watt aquarium heaters that ran almost constantly last winter because air temperature was only about 60° F (16 C).

We found 7.5 watt Hydor heaters for about US$10.00 each. Given that they can be used under gravel in an aquarium it seemed logical that they could be used buried in pots containing tropical waterlilies so we bought a couple to test. Not only did they raise the temperature of the dirt in the pots by up to 10° F (12 C) but they didn't get too hot to touch.

A little later we found a 7.5 watt "Junior" aquarium heater in the pet department at a Wal-Mart and bought a couple to test. They also warm the soil temperature in our 5 quart (5 liter) pots by up to 10° F (12 C) and don't get too hot to touch. Neither type of heater has a thermostat and thus run all the time unless power is cut. It should be noted that we have a generator that kicks on automatically, which is a must given an increasing number of power outages in our area.  

Given hope, based on brief testing of the two kinds of heaters in pots without plants, we have purchased half a dozen more Hydor heaters, cleaned several WalMarts out of Junior Heaters and are proceeding with a winter-long, fairly large scale test to see if the miniature heaters will allow us to winter over our beloved tropical waterlilies without declaring bankruptcy.

We have three areas that require supplemental heat in order to winter over tropical waterlilies inside. Area #1 is the attached greenhouse. Our dozen plus varieties of viviparous tropical waterlilies have wintered well in the 100 gallon tubs in the greenhouse, without heat mats or aquarium heaters, but with room temperature held above 65° F (18 C) by up to three 1500 watt heaters last winter. This area will not be directly included in the mini-heater test but we have put the "shrink film", sold for use on windows and patio doors, on the outer greenhouse panels. It is totally clear, easily applied with pressure sensitive tape and available in indoor and outdoor versions. Our hope is that the film will eliminate the need for one of the electric heaters that make our electric meter spin like a top. Although the non-viviparous day bloomers in tubs on the heat mats in the greenhouse will not be included in the test they will serve as a sort of control. If they are doing well, while their counterparts in the sun room and basement with mini-heaters in their pots are not; it will indicate that the mini-heaters are not keeping the lilies warm enough.

Area #2 in our winter facilities is our sun room that is located between the greenhouse and the house itself. A combination of a propane heater and heat from a wood stove blown in on very cold nights keeps the temperature above 65° F (18 C). That, combined with filtering out of sunlight by multiple layers of polycarbonate panels, has made the environment unacceptable for some of our more tender tropical lilies. Nymphaea 'Midnight', 'Pamela', and 'Green Smoke' had to be moved from sun room tubs to tubs on heat mats last year to keep them happy. 'Red Flare', 'Trudy Slocum', 'Emily Grant Hutchings' and N. rubra barely survived. 'Albert Greenberg', which is supposed to be somewhat shade tolerant, had to be moved from the sun room pool under the skylight to a tub on the heat mat as well, which makes me believe that the problem was more too low a temperature than too little sunlight.

All the plants moved to the tubs on the heat mats in the greenhouse perked up except 'Midnight', which went dormant and was presumed lost. Even putting it in a well heated and lit aquarium failed to revive it so the lights and heat were finally turned off. When I eventually got around to cleaning the aquarium in late spring, I noticed a couple of tiny sprouts in the pots just before dumping the dirt out. These were repotted and placed in one of the tubs on the heat mat. Once outside, in above ground patio tubs for the summer, these two 'Midnights' became star attractions and were in constant bloom until mid-October.

A pleasant surprise was 'Mrs. Edwards Whitaker' that remained happy and bloomed all winter in both the pool and a tub in the sun room, which leads me to believe that it should be put on lists for both shade- and cool-temperature-tolerant tropicals. Two night bloomers, 'Mary Francis' and 'Texas Shell Pink' were also happy in the sun room pool last winter. 'Texas Shell Pink' didn't bloom frequently but 'Mary Francis' did. This year they will be joined in the pool by 'Wood's White Knight' and what we are calling "Hickory Bend Pink Knight", both new additions to our collection. Each will have a mini-heater in its pot.  

Notes on the mini-heaters  

Although a little more expensive, costing about US$10.00, the Hydor 7.5 watt heaters have been trouble free so far. The flat miniature heating pad shape lends itself well to placement on the bottom of pots. They appear to be practically indestructible and the fairly light cords are easy to conceal.

Several of the "Junior" aquarium heaters, available from WalMart for under US$8.00, have been defective and, rather than finding this out after they have been placed in pots, I have started to test each one prior to use. I simply place them in water, plug them in and wait a few minutes. If upon grasping them firmly after the wait they feel warm, I use them, and, if they don't feel warm, I return them. It may be my imagination but some feel hotter than others, which could be a problem. But in no case has one been too hot to grasp comfortably so I anticipate no burning of plant roots. Although it may border on nit-picking, the cords on these heaters are far heavier than they need to be for a 7.5 watt heater which makes them far harder to conceal or manipulate. Given their shape and apparently more concentrated heat emission they do not lend themselves as well to horizontal use at the bottom of pots. Attempting to use them in that manner might increase the odds of failure. The good news is that, given their shape and rigidity, it is very easy to push then into the soil next to plant roots and to adjust the distance from them if required.

To minimize heat loss via conduction, I use insulation at the bottom of the pots. I do this by cutting appropriately sized circles out of the same (approximately ¼" [.6 cm] thick) insulation we used under the greenhouse tubs. The image at the left shows a circle cut from this material being placed in the bottom of one of the 7 cup (1.7 liter) size pots. This material is commonly used under house siding and is readily available from any home improvement center.

To keep track of soil temperature we will use a digital thermometer with a probe that can be stuck into the dirt. I could get by with sticking an old-fashioned aquarium thermometer into the dirt but that wouldn't be convenient when making multiple readings. Even lower tech monitoring could be done by sticking a finger in the dirt. As long as the dirt around the crown feels warm things are OK, but adjustments can be made if it feels too hot or cold.

Because we hope to keep the lilies in the 5 quart (5 liter) pails growing all winter, a heaping tablespoon full of 12-12-12 fertilizer was put at the bottom of each container, on top in the insulation and under the dirt. Experience indicates that roots of some varieties potted in this manner seem to remain shallow thus avoiding the fertilizer, while others seem to seek it out. There is little difference in leaf or flower development. Some tropical varieties such as N. colorata and hardy varieties such as 'Vesuve' seem to require the addition of fertilizer in order to bloom. Others seem to bloom almost as well without it. We don't use fertilizer when we want to just keep lilies alive in a confined space over winter. This seems to aid in tuber development.

In the next issue of WGI Online we will give a mid-winter update on the project. As we are flying blind I have no idea what will happen. I hope to be able to report more than that the mini-heaters didn't work. If things go well we'll to have a final report and conclusions in the spring issue.