In an effort to avoid bankruptcy heating waterlilies
over the winter, Charles Leach has designed -
An Experiment to
Save Energy with Mini-Heaters
by Charles Leach, Ohio USA - Zone 6
Click images to enlarge
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.
Greenhouse and sunroom
A newly potted 'Mrs. Martin
E. Randig' in one of the
unheated 100 gallon tubs
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
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).
Small tubs on the shelf above
the thermostat that controls
the heat mats
Given the growing cost of keeping our tropicals happy in the
winter we were seriously considering giving up when an article
on the Victoria-Adventure web site revealed that it might only
be necessary to keep the roots warm (see Thermoplanters).
Figuring that it wouldn't require much energy to keep the roots
of tropicals in our standard 5 quart (5 liter) pails sufficiently
above a room temperature of approximately 60 to 65° F (18
C) which we can maintain, we went searching for alternatives.
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.
"Junior" aquarium heater
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
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
Two newly potted N. coloratas
Day blooming tropicals being tested with mini-heaters in the
sun room tubs include: 'Mrs. Edwards Whitaker', N. colorata,
'Albert Greenberg', 'Pamela', 'Hickory Bend Ruth', and 'Star
of Zanzibar'. There will be two pots of each, one with and one
without a mini-heater for comparison. As N. colorata does
as well as our viviparous varieties in temperatures in the low
60's° F (16 C), testing it with a mini-heater is probably
useless but it had to winter over somewhere.
Night blooming tropicals being tested with mini-heaters in the
sun room tubs will include: 'Red Flare', 'Trudy Slocum', 'Frank
Trelease' and 'Charles Tricker'. There will be two pots each
of the first two, one with and one without mini-heaters for comparison,
but only one pot each with mini-heater of the last two which
are new to our collection.
Area # 3 is our basement in which we have a dozen 40 gallon (151
liter) tubs that were heated last year with 250 or 300 watt heaters.
The heaters ran almost constantly during extremely cold weather,
when the temperature sometimes hovered around 60° F (16 C).
Sets of two tubs are illuminated by 2' x 4' (.6m x 1.2m) three-bulb
T8 fluorescent lights hung approximately 12" (.3m) above
the water. In a further attempt to reduce energy cost, we are
going to try heating half of them with 7.5 watt mini-heaters
in the pots.
Four tubs will house two five quart pails each containing two
'Albert Greenberg' and one each of the following: 'Trudy Slocum',
'Red Flare', 'Texas Shell Pink', 'Midnight', 'Leopardess' and
'Hickory Bend Ruth'. The remaining two tubs will each contain
six smaller size pots with mini-heaters in the center surrounded
by three or four small lilies. We will see how well the small
heaters work in wintering over constricted lilies. No fertilizer
will be used in these smaller pots because we want to preserve
the spark of life over the winter rather than having the plants
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
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
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
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.