Bonsai are real trees. Trees have developed over millions of years to become what we see today, and are adapted to their particular conditions which, if one wishes success in bonsai, one must understand and work with, not against. No amount of complaining of limited space, or wishing one lived in cooler or warmer climes, will affect a tree's needs for survival and vigour.

     Trees adapted to colder, winter-freezing climates actually need that cold temperature season. Winter isn't merely something they can survive. Well, perhaps they do merely survive winter, but it is accomplished by responding to it and to the subsequent changes to the change to spring. The problem may be the absence of the changes in seasons.     

     I'll use the terms "hardy" (for winter-needing plants) and "tender" (for tropical or semi-tropical plants that may actually need to be protected from freezing temps); The term "hardy" is actually a bit more complex, as a plant will be hardy for a certain climate's extremes, and not for another. For this discussion, it means that it needs winter dormancy.

     It seems that all trees and plants undergo dormancy, or a rest period, if you will. The difference between tender and hardy trees may be the triggering of the response to come out of dormancy. Hardy trees are obviously urged into dormancy by fall and winter; they emerge from dormancy due to either the increasing temperatures or shortening nights of the coming spring. Tropical-zone trees enter and emerge dormacy due to factors with which I am unfamiliar.

     However, a hardy tree will not just ignore dormancy and thrive in warm environments, as indoors; it will, eventually, enter dormancy. The problem lies in the fact that the genetic triggers for emerging from dormancy will be absent.

     A frequent problem with bonsai that I've heard is that a tree thrived for perhaps several years, and then just quit growing and died. The answer to that problem maybe that it was a hardy tree being grown indoors. They will eventually enter dormancy, and will have no trigger to tell them to start growing again. (I've read that freezing temps break down a chemical inhibitor to growth, thus enabling spring growth when spring returns.)

     So, get to the point --- how to winter bonsai?

     Tender trees must be brought indoors (usually before freezing, perhaps before cool temps for best growth), with adequate light, humidity, and air movement for their health and maintenance. They may not actually thrive in these conditions, but they can be sustained until spring returns outside. (The problem is more difficult for the bonsai enthusiast raising trees entirely indoors, but it can be done.)

     Hardy trees must be exposed to the winter conditions for at least some minimum that they require. I'm ignorant on various species' minimum requirements --- persons more widely experienced than myself have given data in the newsgroup rec.arts.bonsai on winter temp and interval requirements, on which I am not qualified to comment. But in general, the following advice will serve.

     One must determine how hardy a tree is for one's particular climate. If it grow in the ground in an area with winters more severe than you, it will probably survived potted life outside in your area; if it survives winter in areas slightly warmer than yours, it probably needs winter temps.

     The primary problem in this situation is not merely temperature extremes, but rather the stress on the upper part of the tree due to the small root system. (There may be some limit to the low temperature extremes that the root system can survive, but again I plead ignorance.) Because the smaller, above ground root system will be more hampered by freezing than a spread-out, below ground root system, it cannot supply moisture to the branches and twigs as readily; twigs, branches, and even entire potted trees can be "freeze-dried" by the dry, winter winds.

     Protection from these winds is in order, as is as much warmth for the rootball as is possible.

     Bonsai should be place on the ground during winter, for thermal contact as well as lowering their profile to the wind; they should be placed in a position with protection against wind, as well. A wall, house, sheet of plastic, tarp, fence, or bales of straw can protect them from the north and west (in the northern hemisphere) winter winds. This is critical to the survival of bonsai outdoors.

     For bonsai that are perhaps not as hardy for your area as you wish, but still need winter, one can follow the above advice up to the point at which temps plummet and winter becomes severe even for your area for a few days; one can then bring the bonsai indoors (preferably to a cool area, like a garage or basement) until the worst extremes of winter pass. Return them to their outdoor sheltered spots as soon as temps return to reasonable. This achieves a happy medium of dormancy with protection from the extremes of winter. This is the "half-way" treatment that some of my bonsai get. I have several bonsai that remain outside until the temp drops below what I consider the average for our area, get moved to the garage for up to a week, then go back outside.

     My winter experiences? The following plants have withstood temps down to zero degrees F in a location sheltered from the winds: pines, junipers, acer palmatum (japanese maple), premna japonica (japanese musk maple), lagerstronium indica (crepe myrtle), juniperus virginiana (eastern red cedar), miniature roses, azalea species, wisteria... your mileage may vary. Bonsai that I give the half-way treatment: anything in smallish pots or small on-rock plantings, favorites (don't EVER say a bonsai is your favorite where it can hear you), gardenias, etc.