The following is taken from the North American Heather Society's bulletin Heather News #58, Spring issue '92.  The author is Barrie Porteous of Muskoka which is 100 miles north of Ontario, Canada.  He obviously has extensive experience with extreme weather.  His heather garden contains over 600 heathers. 

1.  Winter.
a. Summer and Fall.  Some heathers can tolerate higher summer temperatures than others.  Moisture is obviously a key factor, and the soil must never be allowed to dry out.  Since it is probable that the ability of a plant to winter over successfully is related to the degree of ripening of the wood, which takes place in late summer and fall. Late fertilization or a cool, wet fall will most likely result in the plants being unprepared for a hard winter.
b.  Winter.  Clearly the lower the temperature, the greater the potential for plant damage. However, it's not that simple.  An extreme low temperature in January is not nearly as likely to cause damage as one in November when the ripening of the wood has not reached its maximum. Drops in temperature of a short duration are not as likely to cause problems as those which last an extended period  Perhaps the most critical factor is the earliness and depth of the snow cover.  Even if the air temperature is -25 degrees F it will be a balmy +25 degrees F under two feet of snow.  By applying pine needles or evergreen boughs it is often possible to raise the snow level and help bring plants through difficult winters.  However, there is another side to that coin, which is covered later.
c.  Rain. Heathers must have an adequate supply of moisture at all times of the year.  While a mulch will help keep the soil damp, it may be necessary to water even during the winter months if the water used is alkaline, care must be taken to keep the bed acidic.
d.  Sun. In March and early April, it is critical that the plants be shaded from the sun. The ground will likely be frozen and the plants therefore cannot draw up moisture.

2. pH and Soils.
Calluna heather like to grow in soils which are moist, peaty, and well aerated, with pH values in the range of 5 to 6. They do not like wet or boggy conditions. The most important factor is the generous application of peat moss which contributes acidity, moisture retention and aeration.  Plants grown in the wrong soil will invariably go into winter in a stressed condition and will likely be lost.  For the same reason, it is vitally important that potted plants have their roots teased out, so they will grow into the new soil and not back into the root ball.  Many plants are lost not because they succumbed to winter conditions, but because they were never property planted in the first place.

3. Siting.
Heathers, though always in danger of having their leaves dry out, thrive in open, sunny, moist, warm sites.  Larger plants can be used as windbreaks, or evergreen boughs can be applied in early winter after the ground has frozen. Beds located in a slope will be able to shed cold air and the plants will better survive the winter and escape the worst of spring frosts.  Wind and lack of soil moisture are the real enemies, and steps must be taken to insure that the beds are sheltered and kept moist. Never place the bed near maple, torch, beech, cedar, spruce, hemlock or any other trees that have vigorous surface tools which can suck the moisture and nutrients out of the

soil.