Cold protection of tropical container plants

Q: I am long time customer of yours, I live in San Diego California and while the summer and Fall temperatures are warm to mild, the winter temperatures dip to a point where some tropicals die off. We are experimenting with different variations of way to heat and insulate the pots we have the tropicals planted in as a way to keep them alive during the colder winter months. I was wondering if you knew of the ideal soil temperature for these tropical in order to look their best year round. All of the heaters we have installed have thermostats and temperature adjustments so we can now keep the soil anywhere between a range of between 65-75 degrees. Any advise you can provide would be greatly appreciated.

A: This is a very interesting concept you are working with. Indeed, keeping pots/roots warm, may help a lot! We've been experimenting with cold protection for a long time - for the above-ground plant parts. For sure keeping roots protected (even with a thick layer of mulch) will benefit tropical plants during winter. In case with container plants, this may help dramatically.
The guideline is, tropical plants slow down or stop their metabolism at 65F. As long as you can keep soil above that temperature - this should work great. Of course, the higher the better.
Optimum temperature for growing tropical plants in general - 70-85F. Above 90F, metabolism stops too, unless it's a heat tolerant, desert plant.

More information on winter cold protection of tropical plants and zone pushing:

Greenhouse in Virginia
Plumeria cold protection
Ghost Cold Protection
Seven rules of cold protection for tropicals
About Cold Protection
Cold protection - winter action for your plant collection
Tropical Treasures articles on zone pushing.