Separate air hardiness from root hardiness
Cold tolerance is not one number. A tree may withstand low air temperatures when dormant but suffer root injury in an exposed container. Wind, moisture, freeze-thaw cycles, pot material and acclimation all affect risk.
Temperate trees need a winter cycle
Many temperate bonsai require dormancy. Protect them from damaging extremes while keeping them cool; a heated indoor room can interrupt the cycle. Check moisture during dry winter spells even when growth has stopped.
Tropical trees need bright warmth
Frost-sensitive species need protection above their safe minimum, with sufficient light and airflow. Avoid heater vents, cold windows and sudden moves. “Indoor” does not mean low light or stable conditions automatically.
Build a local record
Track minimum temperatures, where each tree was placed and how it responded. Consult species-specific sources and local growers. The right protection for one region or cultivar may be wrong elsewhere.
This page is reconstructed from historical BlogBonsai material. Horticultural details should be interpreted for the species, season, climate and condition of the individual tree.