Regarding cooling the roots of N. rajah, or other highland Nepenthes, I have never taken steps
to do this, nor do I believe it to be necessary. I have plants that are many years old, very
large, and very healthy. The only cooling which their roots receive occurs when their entire
environment undergoes nocturnal cooling. Therefore, I don't recommend the application of
ice or unusually cold water to the plants or to their compost. Such extremes are needless,
and may, in fact, yield deleterious results!
Please keep in mind that D. californica, of the Sarraceniacea, does require that its roots
be kept cool, however....