This is from a post I left here recently. You may find some value in it:

There is wide genetically controlled variation in low temperature limits as they relate to growth in woody plants. Somewhere within the range of 32 - 40* actual soil temperatures is where you will find the start/stop growth temperature limits for a very high % of temperate trees. Actual growth is primarily influenced by temperature at temperatures below 60*, but once temperatures rise above that, the water/air ratio is the largest influence on root growth until trees approach the temperatures that limit growth in the upper part of the range.

I think you'll find the area you're describing to be much warmer than the range you describe, at least for a very large part of the winter - it almost has to be. You can over-winter evergreens, which really never truly go dormant, in total darkness with no noticeable ill effects, as long as the temperature stays under approx 40* F. I over-winter more than 100 evergreens in my garage, moving them back outdoors as soon as I'm pretty sure actual root temps will not fall below the 22-25* range.

If you have other questions I might have missed re. cold-hardiness or the physiology of dormancy, I'll try to answer.

Al

Al, thanks for your response.

The area I described really does stay quite cold. When the outside temp is over 27F/-3C the outside door is left open and the area is at the same temp as outside. When it is colder out, which is much of the winter here (near Toronto Ontario), both outer and inner doors are closed, and the temp in the plant storage area between them is intermediate between the outside and the adjacent part of the basement, which is also unheated and hovers around 45F/7C.

Do you think that what you say about overwintering evergreens (I assume you mean coniferous) in total darkness at temps under 40F/5C would also apply to, say, heucheras?

Irene

The reason I feel the temps are much warmer than you think, is because unheated basements are rarely cooler than 50* F because of the moderating effects of geothermal heat AND the heat gained from the first floor of the structure. Even if the basement is unheated & there was no heat to consider from the first floor, only geothermal heat, 60" soil temperatures in zone five are rarely below 40*, and are much warmer at 72" depths. This means the basement is radiating heat at around 45-50* from the entire floor in the dead of winter. I'm not trying to be argumentative, I just want to be sure you're working with reliable info. ;o)

Do you think that what you say about overwintering evergreens (I assume you mean coniferous) in total darkness at temps under 40F/5C would also apply to, say, heucheras?

Yes, I do. I overwinter lots of evergreen plant material in a garage that for all intents you may as well call dark. I overwinter troughs full of alpines, shallow hypertufa containers with sedum & house leeks (sempervivum), some hardy cacti, ..... you get the idea. As long as the temperature is low enough to keep the plant quiescent, no light is required and any light available is of little value.

What IS an issue is when the soil warms sufficiently to stimulate the plants to growth. THEN, light becomes an issue. Warm soil with no light means the plant will grow, but will be using stored energy, weakening the plant by producing foliage that is unlikely to survive exposure to the elements. In woody plants, it also produces etiolated growth with internodes long in contrast to normal growth, that unless removed will spoil the looks of the plant. Obviously, producing growth that needs to be removed before it has matured to the point of providing an energy return is an unnecessary misuse of the plant's energy.

Al

Al, thanks for the info and for the heads-up about the temperatures in my plant storage area. Obviously I should monitor the temperature closely, especially when both doors are closed for long periods. The area is roofed with a concrete slab (it is outside the walls of the house), but there is earth backfilled behind the concrete block side walls, so there is probably some heat radiation such as you describe from those walls and the floor. If necessary, I can crack the outside door if the temp goes too high.

Distilled, I think the key for you will be :If you see it's growing, it's not cold enough. Wishing you good luck. ;o)

Al