Dig Deeper: Hardiness Zones, Heat Zones & Sunset Zones
Most packaged seeds and commercially grown plants are labeled with information that helps a novice and experienced gardeners better gauge whether the plant will survive and thrive in the spot it’s planted. Plant size, sunlight needs and watering instructions are usually accompanied by a growing zone recommendation of some sort. The most common source for identifying appropriate plant growing zone is the USDA Hardiness Zone.
The USDA Plant Hardiness Zone Map (PHZM) was first developed in 1960 to help gardeners determine which plants would and would not overwinter successfully in their geographic location. This was primarily done identifying the average low temperatures across the country and creating different zones based on those temperatures, with each zone representing a 10F degree range. In 1990, the map was revised to add a greater level of detail, and half zones (a/b) were added to the map to show growing zones just 5F degrees apart.
With the 2012 revision, (GIS) Geographic Information System data was used for the first time, allowing more precise measurements and mapping that better represented microclimates within larger zones, such as metropolitan heat islands or shaded valleys. This revision also doubled the data range, using a 30 year average of low temperatures from 1976-2005.
However, gardening isn’t just about extreme cold and low temperatures. A heat-injured plant may live on in a stunted form for several years before dying, so ensuring that plants can withstand a garden’s highest summer temperatures is also important when planning plantings. This was the thinking behind the Heat Zone map created by the American Horticultural Society, which also uses data about summer high temperatures. When you see a plant or seed tag with two zone ranges listed, the first range indicates the appropriate USDA Hardiness Zone range, and the second is the AHS Heat Zone range.
In some areas of the country, especially the west coast, you may see a “Sunset Zone” listed on the plant tag. This is a map developed by Sunset magazine that pulls together even more information into a map that helps gardeners determine not just where a plant will survive, but where it will thrive. According to the magazine’s website, their map takes into account the length of the growing season, timing and amount of rainfall, winter low temperatures, summer high temperatures, wind, and humidity. They measure not just these weather factors, but geographic ones, as well – elevation, proximity to the ocean, and topography, to help identify and label microclimates within larger zones.
Of course, each individual garden is its own microclimate, and no one zone definition can account for every possible variable or our ever-changing weather patterns. So don’t forget to check your most precise data source - your “Neighbor Zone.” Nearby gardening neighbors and neighborhood garden center are valuable sources of information about what will overwinter and thrive in your area!