Don’t look for your planting zone in the Sunset Western Garden Book

As winter approaches, thoughts turn to curling up beside the fire with a warming beverage and a good gardening book to plan next spring’s projects.

For many, that book is likely to be the Sunset Western Garden Book, the wonderful and widely popular gardening encyclopedia published by Sunset magazine.

However, for all of its many fine attributes, the Sunset Western Garden Book has a signature flaw: it’s hardiness zone maps. While geographically accurate, these maps use a numbered hardiness zone system of their own, completely at odds with the U.S. Department of Agriculture zone numbers that everyone else uses.

The Sunset Western Garden Book says Spokane is in their zone 2, while the USDA—and nearly everyone—calls this zone 6. It’s the same place, with the same annual minimum temperature of -10 Fahrenheit, but using two different scales for context.

The USDA system is a 12-point scale ranging from Zone 1 interior Alaska to Zone 12 tropical Hawaii, while the Western Garden Book uses a 28-point scale, most of which is dedicated to mapping tiny climate variations on the California Coast where Sunset magazine is headquartered. Which may not matter if you never chat with nursery people or other gardeners, and never read other gardening books or websites, and never order seeds online, but it can be downright confusing if you do.

Most of the world uses the USDA hardiness zone system. You’ll find it used in plant hardiness maps of Europe, Asia, Australia, South America and Africa. It is the global standard, like a gardener’s version of the metric system.

All you need to know is that Spokane is USDA zone 6b, meaning the warmer half of zone 6. Go north to Deer Park or west to Reardon and you’ll be in slightly chillier zone 6a, while a little further to Metaline Falls takes you to zone 5.

Yet the Western Garden Book is so popular that even Costco regularly sells it, so the confusion persists, especially with those new to garden literature.

So, when you meet a well-meaning soul who insists that this is zone 2, just smile, nod and move on. They’ll learn soon enough.