The following was written for you by Donald Mackay, editor of the North American Heather Society bulletin Heather News.
Although some "not-quite-so-hardy" heaths often appear in books about the hardy heaths, it is clear from a map of the natural distribution of European heaths that Calluna vulgaris--the Scotch Heather- is the most widespread and northerly of them all, that Erica tetralix (bog heather) and Erica cinerea (bell heather) are less able to move inland from the moderation influence of the Atlantic Ocean, and that Erica carnea occupies the alpine ground of Central Europe. The other not-quite-so-hardy ericas are found naturally in the warm southwest of England or west of Ireland or around the Mediterranean sea. As expected, Erica vagans, Erica ciliaris, Erica erigena, Erica mackaiana and the hybrids they form (E. x williamsii, x watsonii, x praegeri, x darleyensis) are generally less likely to survive a hard winter than the alpine Erica carnea or the northern plants, i.e., Scotch Heather, bog heather, bell heather trio.
In terms of simple survival ability, the not probably go to Erica carnea, but at a price. Practically speaking, the New England garden does best under extreme conditions with Calluna vulgaris, followed by Erica tetralix with Erica cinerea a poor third. Each of these species may be available in large numbers of cultivars. In such large numbers, a wide range of winter hardiness is to be found, and much overlapping, so that even among Calluna vulgaris, there are some cultivars whose southern origin can be a big impediment to Canadian, New England or Midwest gardening.
The biggest enemy for heather gardening is extreme cold (20 degrees of frost or more). The biggest friend is snow. The most important question for you is does the period of deepest snow you get in winter overlap the periods of deepest frosts?
In Vermont ski country, with a continuous snow cover all winter from two to four feet deep, Calluna vulgaris is perfectly safe. Erica carnea is also safe, but by thetime the snow cover has thawed, it may have finished its blooming unseen, often bleached in color, under the snow. Even in snow country one can get the January thaw, but this will affect Erica tetralix and Erica cinerea much more than Calluna vulgaris, usually by splitting their stems whch, while it does not kill the plant, does drastically cut back on the bloom in summer.
If you live in more civilized parts of New England like the Connecticut River Valley, the snow cover is less reliable, or persistent, and tend to disappear long before the cold night frosts allow the ground to warm up. Under these conditions, W. Dickey grows heathers successfully using a combination of sloping bank to spill the frost Reemay®, porous synthetic cloth, to cover the ericas, pine needles and branches over everything, and special attention paid to drainage of the more cold-susceptible heathers. He finds that digging lots of sand into the ground under the plant seems to work well. The pine needles and Reemay® (this can be called Harvest Guard and is widely available) come off gradually in spring so that the carnea blooms can be seen, while the callunas remain shielded from bright sun and cold dehydrating winds so long as their roots are imbedded in the frozen ground. It is also important to avoid alternate freezing and thawing in fall, and especially spring, which leads to frost heaved