The same plants are often listed in different reference materials with wide ranges in hardiness zones. After asking around in the trade, I’ve found that most hardiness levels seem to be assigned essentially on hearsay, with little scientific evaluation. Many plants are listed as Zone 5 in our catalogue because we are able to grow them here, but a good number of them are probably much hardier than listed.
Many factors influence plant hardiness.
One of the most important hardiness factors is snow cover. Because snow (especially dry snow) traps so much air, it is an excellent insulator. A friend who gardens considerably north of Acton grows plants with which I’ve had difficulty. He has deep, reliable snow cover, while the winter snow here is anything but reliable. Quite often it is the recurrent early spring thaws that leave a plant exposed and weakened. Once established, plants seem to be hardier.
Mulching around plants will help protect the roots from the rollercoaster ride of spring temperature fluctuations. Fall mulching can help catch snow, thus providing even more insulation.
Mulching also has year-round benefits: water retention, weed control, and protection from the baking sun.
Another factor in plant survival is soil moisture. Some plants, especially those from intercontinental areas, survive much better in well-drained soils in which excess moisture can drain away. Similarly, we have found many bog plants will grow in very moist to wet soils but won’t survive being frozen to -20C. The same plants often survive just fine in a drier winter soil.
Our advice about hardiness
So don’t be put off by the zone listings in this catalogue; they are only very general guidelines. If you grow any of these plants in lower hardiness zones, we would be happy to hear from you so that we may provide more accurate information. Thanks.