Winter Care by Marty Weiser
General Information
Trees in pots are not as hardy as trees in the ground since they don't have as large a reservoir todraw from. This applies to all extremes - cold, heat, drought, and flood. Dehydration is often as big a problem as freezing to bonsai. Most temperate plants (maples, pines, junipers) require a period of cold weather for optimal health. Generally this ranges from 500 to 1500 hours below 35°F (2°C).
The move to dormancy in the fall is triggered by a combination of less daylight and lower temperature. Warmer temperatures and more daylight trigger the spring wakeup. Some bonsai growers advocate high phosphorous (P)fertilizer and low or no nitrogen(N) in late summer and fall as a method of preparing the plant for winter. Others disagree and cite horticultural studies showing no benefit of high phosphorous fertilizers except for blooming plants. I use lower nitrogen, higher phosphorous fertilizer (10-50-10 or similar) in the early fall and switch back to a more balanced (20-20-20) in the late fall so the roots can take up nitrogen to be ready for spring growth.
Soft, late season growth (promoted by a combination of high nitrogen fertilizer and pinching) is much more prone to winter die back. Therefore, don't pinch back in late summer/early fall; wait until top growth has stopped before cutting back.
Spokane and Coeur d'Alene are in USDA zone 5 (-10° to -20°F average annual low). Sunset Magazine lists them as sunset zone 2 (-2° to -34°F recorded) which is a more accurate indicator of how cold it can get.
All trees are different so the information below is a guideline. Trees that have been stressed( repotted, strongly pruned, heavily wired) need additional protection.
Tropicals and Subtropicals
Tropicals don't like temperatures below 50°F (10°C) and will probably die if exposed to any frost. This includes most ficus, fukien tea, buttonwood, etc. Subtropicals will not tolerate hard freezes, but may tolerate light frost. These include bougainvillea, serissa, etc.
Bring these into a very well lit, reasonably warm area with some airflow, but minimal drafts. Subtropicals normally do better if the nighttime temperatures drop below 60°F (15°C).
South facing windows or greenhouses are best but they can be grown under artificial light. Remember that your eyes are much more adaptable than a plant; what seems bright indoors is often rather dim compared to outside.
Lack of humidity is a major problem inside the house. Trays of water under the tree (water not touching the pot) add a little humidity, but humidifiers in an enclosed space are far better if you can keep the air circulating to avoid fungus.
