What Climate Change May Bring to Northern Arizona Gardens

By Laura Huenneke

Surveys repeatedly show a large majority (70 – 85 %) of Americans are very concerned about climate change impacts. As gardeners, we are more intimately connected and attuned to the environment than most. What can we and our gardens expect from the shifting climate here in northern Arizona?

Thinking of climate change only as “global warming” is an oversimplification. Averaged across the earth’s surface, the globe has indeed warmed in the last 50 years. In fact, according to the National Oceanic and Atmospheric Administration, the years 2016 to 2025 have been the 10 hottest years since 1850. But no place is actually “average,” and our gardens are affected more by extremes than by averages.

Average annual temperatures across the globe, 1880 to 2024. The zero point is average temperature for 1901 to 2000. Years before 1940, shown in blue on the left, were cooler than that average. Years after 1980, in red, have been increasingly hotter than that average. Data and image from an archived 2025 version of NOAA’s Climate.gov website.

Minimum winter temperatures define the plant hardiness zones mapped by the US Department of Agriculture and listed on nursery plant tags and seed packets.  The hardiness zone map published in 2012 averaged low winter extremes from 1976 to 2005; the recent map, published in 2025, used average winter lows from 1991 to 2020. Flagstaff’s growing zone has shifted from 5b, where average winter lows range between 10 and 15 degrees below zero, to 6a (average lows 5 to 10 below). Gardeners now have a wider selection of plants to try that might make it through a Flagstaff winter.

Of course, our altitude is still near 7000 feet, so we cannot count on escaping late spring freezes. Earlier, hotter springs combined with continued risk of late frost may scramble both our expectations of short growing seasons and our planned planting dates. I don’t want to waste weeks of potential growth by waiting too long to plant, but I need to continue to watch for late freezes.

Cold weather in winter is actually necessary for some plants. Fruit tree varieties differ considerably in their “chill hours requirements” – the minimum number of hours with temperatures between 32 and 45 degrees they must experience in a winter before they will flower. Some varieties flower after as few as 50 chill hours, while others require 1000 or more. Growers in mild winter areas choose varieties with low chill requirements, but in some recent years regions from the southeast to Phoenix to California have failed to meet even those requirements, and crop yields have fallen. Here in Flagstaff we choose varieties with high chill hour requirements, hoping our trees flower late enough to escape late spring freezes. However, our winters may continue to warm, putting that accumulation of many cool hours at risk. And horticulturalists elsewhere have observed that unseasonably warm temperatures (over 60 degrees F) in midwinter or early spring can even “erase” the impact of chilling hours already experienced. I fear that may have happened in my warm Doney Park garden this past winter, as my cherry tree failed to leaf out and flower normally.

Maximum temperatures, too, are expected to be higher. National Weather Service records for Flagstaff show no annual high temperature greater than 95 degrees F from 1899 through 1969, but four years with highs of 95 or greater in the past dozen. We’ll also probably experience more sustained record heat waves, like most of the western US this past March. Gardeners can prepare for higher extremes by choosing more heat-tolerant varieties or providing shade during the hottest days.

Basic physics dictates that during hotter summers the warmer air can hold more moisture or water vapor. That means greater evaporative demand and more rapid water loss from plant leaves and from soil. Drought stress is altering our forests as ponderosa and pinyon pines die out. In our gardens this drying means more watering to keep plants healthy, and perhaps choosing more drought tolerant species and varieties. It will become more important to use mulch or other moisture-conserving techniques.

Warmer winters will likely mean that many storm events that would previously have brought snow will instead bring rain, which may run off or evaporate quickly. We’ll find ourselves doing more winter watering and trying to protect plants and soil which might previously have been blanketed under long-lasting snow.

Many of us consider gardening to be part of our personal strategy for resilience – the ability to survive and even thrive through multiple disruptions. However, conventional wisdom about Flagstaff’s high-elevation climate – cool temperatures, short growing season, dependable snow cover – is being challenged by climate change. Whether planning for resilient landscapes or for reliable food production, considering our likely future climate is a wise part of preparation.

Laura Huenneke, coordinator for the weekly Gardening Etcetera column, has been gardening east of Flagstaff for more than 20 years. Gardening Etcetera is written for the community by certified Master Gardeners of the University of Arizona's Coconino County Cooperative Extension Master Gardener Program. To learn more visit https://extension.arizona.edu/programs/coconino-county-master-gardener.