Why “unheated” does not mean stable
The historical article says garage and outdoor temperatures will not differ much and implies that storage nearly guarantees safe roots. Neither assumption is reliable. An attached or detached garage can swing with sun, insulation, door openings and local weather. A container exposes roots to colder, faster-changing conditions than the ground, while a warm dark room can interrupt dormancy.
Check species and dormancy first
Only consider this approach for an outdoor temperate species whose winter needs you understand. Tropical and subtropical bonsai do not belong in cold storage. Confirm that the tree has entered dormancy before moving it; the right cues and cold tolerance vary by species and region. If the tree is evergreen, its light needs also depend on temperature: darkness can be damaging if the storage space is too warm for full dormancy.
Measure before relying on the garage
Place a min/max thermometer near the pots and record readings for several days during the coldest and sunniest conditions. A single room reading is insufficient. Compare the observed range with trusted guidance for the species and your climate. If you cannot keep the conditions within a suitable range, use a more controllable protected outdoor setup or seek local expert advice.
Harvard’s Arnold Arboretum provides a useful example of an institutional system, not a recipe to copy blindly: its collection uses a concrete-block storage structure held at 33–36°F, with plants fully dormant and moisture checked. That tightly managed temperature, building and collection care differ from an ordinary garage.
Keep roots safe without leaving them wet
- Insulate pots or group them so root balls are less exposed to rapid temperature changes.
- Check moisture regularly; dormant trees still need water if the root ball dries.
- Avoid standing water and persistently saturated substrate, which increase fungal risk.
- Ventilate and protect from rodents, exhaust, de-icing salts and accidental impacts.
- Do not use monthly snow addition as a substitute for checking the actual root zone.
Bring the tree back gradually
When conditions permit, move the tree back outdoors before growth starts, following local seasonal guidance. Avoid a sudden transition from dark storage to intense sun or wind. Continue monitoring moisture and temperature; a garage can change quickly during a spring warm spell.
Know the species, confirm dormancy, log actual temperature extremes, protect roots and inspect moisture. If any of these are unknown, choose a more predictable wintering method.
Sources and further reading
- Bonsai & Penjing Collection — Arnold Arboretum of Harvard UniversityInstitutional overview of winter care and controlled cold storage.
- Bonsai & Penjing Collection Care — Arnold ArboretumDetailed description of dormancy, cold-storage temperature, moisture monitoring and limits of dark storage.
- Bonsai: Growing Miniature Trees — RHSGeneral seasonal and species-dependent bonsai care.