by Ken Lain, the mountain gardener
Find your garden’s hardiness zone by Zip Code
Gardeners rely on USDA Hardiness Zone Maps to tell us which plants survive and thrive locally. The USDA released updated Plant Hardiness Zone Maps for the United States. Plant zones are based on the average annual minimum temperature over a 30-year period, not the lowest temperature ever in each zone.
Zone maps don’t dictate steadfast rules. Instead, they guide other garden factors like soil quality, sun exposure, moisture, and sudden or prolonged temperature swings. These combined variables for their respective zones indicate how well plants grow in each zone.
Variations within a hardiness zone are called microclimates. They can exist even within the radius of a single yard! Trust the familiarity of your property and your gardens to know where microclimates exist.
You can download the USDA Plant Hardiness Map Site here.
The 2012 map was the first designed for the internet. Zones can be zoomed in on for a closer look at specific zones. A more sophisticated algorithm was used to compile low-temperature values from actual weather reporting stations.
The zones are based on 1976 – 2005 weather data. This date range was chosen to provide year-to-year fluctuations and variations. A trial check on more recent data showed no measurable difference between the earlier and more recent zone evaluations.
Interactive Map – This link to the USDA interactive map can use a zip code to get a zone number, the average temperature, or temperature range. The longitude and latitude of a zone are also available. Playing with the map can be addictive!
In the mid-1900s, the U.S. Department of Agriculture (USDA) mapped out the entire United States, Mexico, and Canada by lowest annual minimum temperature groupings. Each zone represented a 10-degree F. difference. This was invaluable advice for the agriculture industry. Plants could be rated by hardiness zones, taking costly guesswork out of choosing the best plant varieties for different areas. Home gardeners now had a reliable guide, other than practical experience, for selecting plants.
1990 the zones were divided into ‘a’ the lower temperature end of the zone, and ‘b’ the higher. Unfortunately, plant breeders have yet to start using these distinctions on their plant labels, so they are most useful if a gardener wants to push the envelope a bit. A gardener in zone 6b will be tempted to dabble in 7a plants. Given the variability within any climate, it’s a hit and miss by microclimate, but I define it as gardening.
The American Horticulture Society (AHS) introduced a Plant Heat-Zone Map in 1997 to supplement the hardiness map. Heat-related problems are much harder to quantify. High summer temperatures only told half the story. Plants don’t usually react to a day or two of heat how they might respond to a frost. However, plants subjected to a two-week heatwave could very well not survive. Other variables that weren’t considered were humidity levels, nighttime temperatures, and rainfall.
The Southwest map is shown here. You won’t get as much detail from the regional maps. Still, you get a sense of microclimates and how zones are affected by their geography. Within miles of each other, higher altitudes in Arizona’s mountains and coastal areas can be 2-3 zones cooler than urban and desert areas.
The only real hardiness zone gardeners care about is their own. Each state has an individual map that shows the gradual changes in hardiness from one area to another. There’s an interactive map where you can get your zone, average temperature, the temperature range for your zone, and longitude and latitude of your area. It includes a Zip Code Lookup at the top of the page.
Here’s the link where you can download all the individual state maps. Central Yavapai County is a zone 7 to 8, with the ridgelineS dipping to zone 6. This means we need plants that can take winters below 10 degrees F. Chino Valley, Prescott Valley, and Prescott are all the same zone. As Watters’ shoppers read plant labels, hardiness zone numbers are useful guides to making good selections.
Variables occur within each garden. A north-facing garden will be far colder than east or south-facing gardens because of the sun’s focus during winter. Likewise, a garden shaded by trees or a beautiful two-story house will be far colder in winter than a garden that gets sun most days.
Big mistakes are made by gardeners from desert or coastal areas and not realizing how cold affects plants. Cactus brought up to the mountains from desert nurseries thrive in the summer heat but quickly turn to black mush as freezing weather hits them, never to live again.
The best advice for Arizona mountain gardeners is to verify your hardiness zone. As you shop for plants at the garden center, confirm the plants hardiness zone. Plants with a lower hardiness zone will grow in your gardens. Those with higher zone numbers should be treated as annuals; they generally will not survive winter conditions. Stick with plants that can grow in zones 4b to 8a, and they should tolerate our mountain winters. Those plants rated 8b and higher are likely to die from the winter cold.
Until next week, I’ll be helping gardeners plant the hardiest gardens here at Watters Garden Center.