Heating beds can help you overwinter recently collected Yamadori to improve root growth and prevent freezing of the roots during the winter. They are also useful in boosting root growth after repotting a bonsai tree that required heavy root manipulation, or was done towards the end of the growing season.
For trees that require dormancy in the winter, using a heat bed is something you would want to do for one season with a tree, not on an ongoing basis year-after-year.
There is an excellent discussion and example of heat bed construction starting 36 minutes into this Winter Care video from Bonsai Mirai. If you are not a member of Bonsai Mirai, there is also a PDF offered by Rocky Mountain Bonsai Society titled How to construct a bonsai heating bed.
You'll want the following supplies:
- 2" thick foam board insulation (common brand name Foamular, in pink color)
- Plywood between foam board and propogation mat
- Propogation heat mat
- Digital thermostat
- Thick plastic sheeting
- Pumice
- How to construct a bonsai heating bed PDF by Rocky Mountain Bonsai Society
A nice guide that explains the construction of a heat bed. A note is included that these are intended for indoor use, but they have also been used outdoors in colder climates.
- Winter Care video by Ryan Neil (36 minutes into video)
This video breaks down the construction of a heat bed and includes a helpful Q&A about the technique.
- Heat Bed Construction video by Bonsai Mirai
This video shows construction of a large heat bed using heating wire, which did not stand up to the test of time. You may have more success with the propogation mat technique described above.