This is usually the month for putting our trees into winter storage. You can wait until we get some consistently cold weather down into the low 30s. Protective cold frames are ideal for bonsai, especially on the north side of a building, where the winter sun doesn’t heat them up. Window wells may work also. Unheated garages or bulkheads will suffice for deciduous trees, but evergreens need some daylight, as they never go fully dormant (proximity to a window in an unheated structure will work). You can even bury your trees in the ground, pots and all, up to their first branches – good quality pots with well-drained bonsai soil seldom break from freezing. What’s important is not to expose your trees to biting winds which can suck the moisture out of the soil, or subject them to excessive freeze/thaw cycles. Temperatures of 30-40 degrees are ideal.
Keep in mind:
Check on moisture! Do not allow your trees to go dry. Frozen soil will not accept watering, but you can use ice cubes or heaped snow to melt in when temps permit.
Clean up your trees before putting them away, to counter insects and fungus.
Sunny greenhouses or cold frames can allow extreme temperature swings.
Rodents get hungry, and will gnaw the bark off bonsai, sometimes girdling the trunks and killing trees.
Some members here on Cape have been successful just leaving their trees in their regular places (if wind is not an issue).
Tropicals should spend the winter in an appropriate well-lighted area. Sun rooms are ideal if the temperature can be lowered into the 60s. Grow lights can also help. Provide foliage misting and pebble-filled humidity tray to promote a moist micro-climate.
Discontinue fertilizing for dormant trees. Evergreens can benefit with occasional light applications, and tropicals will need continued fertilizing, preferably with organics..
Apply Lime Sulfur sprays to keep down fungus and some diseases. Be sure to check label warnings, as they cannot be used on all species.