For landscape designers whose gardening education (either formal or informal) began with the Sunset Western Garden Book, the concept of climate zones is second nature. But for plenty of our clients, it’s about as foreign as, well, a foreign land. What is a climate zone, anyhow, and why should you care?

Horticulturally speaking, a climate zone is simply an area where the growing conditions are consistently unique from any other area. Back in 1960, the U.S. Department of Agriculture defined 10 “plant hardiness zones” throughout the country, based mainly on winter low temperatures, to help growers understand where crops would, and would not, survive the winter. Over time those definitions have been refined and the zones subdivided, and today there are 26 USDA zones. You can see a map online here.

Here in the West, Sunset Magazine took the idea a step further, defining “Sunset climate zones” that take into account not only winter low temperatures but also summer highs, wind, rainfall, humidity, elevation, latitude and ocean influence. The result is a finely detailed picture of where plants can truly thrive. From its original 13 zones in the western states, Sunset now has identified more than 50 unique climate zones throughout the U.S. and Canada. These climate maps have become indispensable tools for garden designers, landscape architects, builders, and of course homeowners. Plant growers, who want gardeners to have success with their products, routinely label their plants with the USDA and/or Sunset zones those plants are best suited to.

But why do zones matter, anyway? Because—whether for a budding home gardener or a licensed landscape architect—the first rule of planting design is “right plant, right place.” So if you're designing a garden in San Francisco, you can know that there in Sunset Zone 17, even though it’s mostly frostless, summers also might not get warm enough for some citrus trees to bear fruit. On the other hand, if you're designing for Palo Alto, you can assume that the Agave attenuata that thrives up in the mild coastal air will turn to mush during our freezing Zone 15 winters.

Sure, plenty of people are in "zonal denial": gardening on the edge of reality, insistently planting frost-tender plants in cold-winter areas (and high-water turf grass in what’s really a desert). They may skirt outright failure enough to be emboldened in their efforts, but their gardens aren't really thriving. And plants draped with flannel sheets and holiday lights probably aren't the curb appeal most people want.

So don't live in that state called denial. Know your zone and plant accordingly. Your garden (and your neighbors) will thank you.