For members with just a few trees, here is a container that you can regulate the potting soil's temperature during the winter months, to prevent winter leaf drop (WLD), and to promote year around tree growth.
This ingenious new heated planter is a plant-saver. It allows you to place tropical plants in cool rooms during the winter months without concern for root rot, winter leaf drop, or other problems associated with low root zone temperatures. The ThermoPlanter regulates soil temperature with its thermostatically-controlled 15-watt heater that switches on during cool days and nights and off during warm sunny days, creating an evenly warm soil temperature despite fluctuating air and room temperatures. The stable soil temperature promotes larger, healthier plants with increased flowers and fruit. Most tropical plants can thrive in lower air temperatures (not below freezing) as long as the roots are kept warm. - Millet (1,236-)
http://www.thermoplanter.com/thermoplanterspecs.html

For members growing in extra fast draining mediums such as CHC and bark, and want to maintain high root zone temperatures through out the winter months, in order to avoid winter leaf drop (WLD), and to insure good winter tree growth, but desiring to water, and or fertilize, their trees by submerging, I post a reply in answer to this question asked on the UBC forum . The answer is by Mr. Mark Prescott, the inventor of the ThermoPlanter. The UBC question was, paraphrased "Can I soak my tree when using a ThermoPlanter"? - Millet (1,229-)

Answer: --- "I am the person who developed the ThermoPlanter so I thought I would try and help you with your question. If you are needing to submerge the pot for watering you can do that with your ThermoPlanter. The heater was made for running fully submerged and has passed all the UL 499 tests for doing so as well as all the CE requirements in Canada. You can add a few holes in the bottom of the outer black pot with a small 1/4 inch drill to help the water get into the pot quicker. These holes will not effect how the pot operates, just stay clear of the electric cord when drilling. (unplug it before you attempt it) You will have to submerge it a little deeper than a regular pot because the inner pot is on short legs so you will have to put it under about 2 inches.

I am not sure if you are aware of exactly how the ThermoPlanter functions so let me explain it quick. You will find the drenching method will work quite well with the pot because of the way the heater is designed. The pot functions by creating an evaporative lift of moisture that will take that water and move it up the soil media. This happens because the heater gets quite hot, evaporating the water in an upward flow. the purpose of this is to give heat with out drying out the soil to quickly. As the water lifts up it pulls air in from the bottom of the pot allowing the soil to breath. The shape of the inner pot allows soil to heat evenly. That is the basic difference between the ThermoPlanter and heat tapes or mats which will dry a pot out quickly. Let me know if you have any more questions and good luck with your plant.
Mark Prescott

It's ashame I just started my makeshift root pruning experiment because I would really like to try one of these.