A beginner’s guide to plant hardiness

First, a simple definition: hardiness in plants refers to the ability of a tree, shrub or perennial to survive winter in a particular geographic area.  Hardiness does NOT refer to any other quality of plants like health, durability, etc. 

When you buy a plant at a nursery, its tag will usually designate hardiness in a zone rating; the most widely used system of zones among gardeners and nursery personnel and that which is used on all plant tags is the USDA system.  This system is also used in most gardening books, although the very popular Sunset Western Garden Book uses a more complicated system with entirely different zone designations.

Our region (west Puget Sound) is variously rated as zone 7 or 8 (4 or 5 in Western Garden!).  As a general rule, plants labeled zone 7 are “safe”.  Zone 8 plants are iffy, and only people with very favorable local microclimates (e.g. right on the water in a protected cove) can assume they live in zone 8.  However, sometimes a plant’s potential vulnerability can be offset by siting and extra protection: plant it close to a building or a brick or stone wall or walkway, or surrounded by trees; add extra mulch in winter; wrap in burlap or reemay when temperatures are expected to dip severely.

To make things just a little more complicated-  wholesale nurseries (which provide most of the tags), gardening experts and writers—are by no means unanimous in their assessments of the hardiness of plants, or in the zone ratings of some geographic areas. 
More difficult still: nurseries in our region widely sell plants known to area horticulturists to be somewhat tender here. These include: Hebe (many species and varieties, with varying degrees of tenderness); Escallonia, some species and varieties of Ceanothus (Vandenburg, Dark Star).

Some winters can be hard on even the tried-and-true, hardiest of plants.  Last fall, we had a long, warm spell in November, followed by an abrupt hard freeze, in which nighttime lows plunged something like 20 degrees in a matter of days.  When that happens, plants that have been “fooled” into continued active growth may suffer considerable winter damage, even when the rest of the winter proves fairly mild.  Human error can also be a factor in winter damage. Fertilizing trees, shrubs or perennials, or pruning trees or shrubs late in the season can stimulate new, tender growth that may then be damaged with the onset of cold winter temperatures.