I live in a zone where many plants I want have to be grown in containers and protected in the winter. I've read that to maintain dormancy you need to keep the plants below 40. I do not have access to 40. What I do have is 50 in the heated garage, where it is pretty dark. Or 50 in the sunroom, where it is not so dark and can get slightly warmer on sunny days. I've tried containers in both places. My results don't seem to match what I've read.
For example, I overwintered a japanese maple and a gingko bonsai in the 50 degree garage. They were left outside until fully dormant, then placed in a dark area of the garage, watered as needed, and have remained dormant all winter. They are just now showing signs of coming out of dormancy. I will move them to the sunroom when they emerge enough to need more light, and move them outside when temperatures stabilize.
I also overwintered Japanese maples in the colder, unheated garage. These pots freeze solid during the winter, and remain fully dormant until true spring.
I overwinter a cryptomeria japonica (conifer) in the sunroom. I've done this for at least 5 years now with no apparent ill effects. It is brought in before it freezes outside. It goes dormant (or quiescent, anyway) in the sunroom until early February, then resumes growth. I also do this with Rosemary and Corokia cotoneaster. Perhaps it would be better to let these three plants experience below freezing temperature before I bring them in, but that is not what I've done.
So from my experience, it appears that the standard below 40 degree rule is somewhat questionable. If the plants are fully dormant before placing at 50 degrees in the semi-dark garage, they remain dormant at least until near the end of winter. If the plants are not dormant when placed in the 50 degree minimum sunroom, they at least stop growing for a period of months through the depth of winter, and, at least in the case of the cryptomeria and rosemary (where I have done this for a significant number of years), they appear to suffer no ill effects.
I welcome your thoughts on this topic!
Wondering about dormancy
This is a reply I offered to a similar question about chill requirements/dormancy issues back in '05. If it seems a little off topic, it's because I simply copy/pasted it in its entirety. You may find something interesting in it:
Simplified, but: As nights lengthen, the primary trigger for onset of dormancy, photosynthesis decreases & the production of abscisic acid (ABA)increases. The reduction in auxin flow across the abscission zone of leaf petioles becomes inadequate to suppress the formation of of an abscission layer; it forms and leaves fall. High ABA levels, triggered by shorter photo-period and accompanied by dropping temperatures cause solutes (sugars, sugar alcohols, proteins, etc.) to concentrate in cell protoplasm. This is nature's anti-freeze and allows cells (in some plants) exposed to often very low temperatures to exist without freezing. Bound water (water contained within cell walls) is also drawn out of cells & into the inter-cellular spaces in living tissue. This, in effect, increases the concentration of solutes in remaining bound water & further enhances cellular cold-hardiness.
The major changes in cold-hardiness are in above-ground tissues. Roots also harden, especially the more lignified perennial roots, though to a lesser degree than above ground tissues.
So, "what's happening" is the plant responds to environmental stimuli and, as a defense mechanism, goes dormant. A varying length of time at (the generally agreed upon) temperatures between freezing and 45* F. is required to negate the effects of the physiological changes caused by the environmental triggers. Though I have never read a text that said definitively what occurs to release the trees from dormancy, many suggest that the high ABA levels are reduced to normal levels during the dormant period, which would then allow the plant to return to normal growth once cultural conditions are favorable.
After sufficient chilling occurs, the plant is technically released from dormancy, though in most cases it remains in a quiescent state until cultural conditions are favorable for growth (primarily temperature). Once the chilling requirement is met, soil temperatures need only rise above about 45* F, for as short a time as 3-4 days to initiate growth.
As you read the following, please consider that I am also including containerized plants:
Some temperate plants, if deprived of a dormant period, will still break bud in spring, usually very late. Signs of insufficient chilling are the late bud-break described, along with weak growth, primarily concentrated at/near the top of the plant and/or at branch tips. Weak, or no fruiting should be expected. Other temperate plants will simply not wake up in spring, or will continue to grow poorly through what would normally be their dormant period, only to go through leaf abscission and dormancy at a completely unexpected time - say in July. This is usually fatal to the plant, but in some cases can be reversed by providing a sufficient chilling period.
I've read extensively to try to discover the exact physiology involved in the actual release from dormancy at the molecular level & would be most interested if any can provide the info & cite a source.
Al