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Consider Micro-climates When Using a Climate Zone Map

By: Sandra Nelson

woman with computer outdoorsWe are having our typical February break from winter. The sun is shining and it’s even warm enough to trade the heavy winter coat for a light jacket. No matter what chores are looming inside, the pull is to be outdoors and in the garden. Obviously it isn’t time to plant (here in Missouri we are almost guaranteed another burst of winter soon), but it is a great time to plan. 

When we first moved into this house eight years ago, the backyard was a sad sight. There was one gnarled, old redbud tree and a few patches of sorry-looking grass struggling to survive. dead grassThere were rocks  – so many rocks. That first summer I spent hours in the blazing summer sun moving them out of what was going to be my future gardens. By late July I was ready to plant my first bed. It would be situated directly outside my office so I would be inspired by nature. I visualized my own tiny Garden of Eden, bursting with beautiful blooms and buzzing with dozens of pollinators. After all, I had accomplished just that at our last house, so why wouldn’t this be the same? Well, to put it bluntly, my new garden was, and still is, a miserable mess. I wish now that I had taken pictures of the bed; they would have been great shots of what not to do when planting a new bed.

 seed packet    Looking back, I realize that not only did I ignore a few basic design principles, I   skipped an essential element of the design process. I didn't take the time to truly   evaluate the site and select plants appropriate to the climate conditions. I   made a   beginner’s mistake of choosing simply by their USDA Hardiness Zone number. If the   tag read zone 6, then I assumed it would thrive in my yard. 

  For those who are unfamiliar with planting zones, a little background may help. In the mid 1950s, scientists at the Harvard University Arboretum began producing hand-drawn maps outlining the country’s isotherms, or regions with fairly constant winter time temperatures.

early climateThey contended that areas with the same temperature patterns would all support the same plants.  Because the maps they produced used inconsistent data sources, their regions, or zones, varied widely, so the public had trouble figuring out which zones they actually lived in. To make matters worse, other organizations, like the Arbor Day Foundation, also began producing their own climate zone maps which showed different zones. 

arbor day maps

In 1960, to help eliminate public confusion, the USDA produced its own Plant Hardiness Map based on data from a select and consistent group of weather stations across the country.

climate zones 1960The original map had 10 zones and each zone covered a 10-degree stretch of average wintertime lows. The zones were then subdivided into 5 degree units and labeled “a” and “b”. Since its inception in 1960, the map has been updated 4 times and an 11th zone has been added.

climate map 2023The newest USDA Plant Hardiness Map was introduced on November 15th, 2023. It is based on a 30-year wintertime “normal” period and includes data from 13,412 weather stations across the United States and Puerto Rico. To further improve the map’s accuracy, additional conditions that affect temperature were considered and interpolated into the data. The newest map is overall about 2.5 degrees F warmer than the previous 2012 version. About half of the 22 sub-zones shifted to a warmer zone.

Most experts in the field of horticulture agree that relying only on a planting zone map to determine “best” options, is dangerous. Every garden is unique, so to be successful, analyzing its particular microclimate is essential. A microclimate is defined as “the climate of a small area that differs from that of the general surrounding region.” Microclimates can cover large areas  – think of a city encircled by countryside – or spaces as small as a corner of your backyard.

city/yardThe effects of microclimates can significantly impact plants, sometimes causing them to flourish and sometimes to flounder.  When analyzing your area’s microclimates, some factors to consider are:

  • Topography  

mountain valley

Elevation changes temperature. During frigid night times, low areas can become significantly colder than surrounding patches as heavier cold air drops into even a pocket of low land. 


  • Nearby Water

city lake

Since bodies of water heat or cool faster than land, they tend to moderate temperatures. It’s cooler near the water during hot spells and a bit warmer during the winter.


  • Heat Islands

hot asphalt

Impermeable building materials absorb and radiate the sun’s heat throughout the day and the night. Even your house radiates nighttime heat.


  • Direction and Slope of the Land


The direction and slope of the bed faces determines how much sun it will receive. East facing gardens will get morning sun and will have somewhat cooler afternoons. West facing will receive hotter, afternoon sun.South facing slopes are the warmest and tend to be the driest.


  • Access to Shade

shaded bench

Trees, bushes and other structures can block sunlight from warming the surface of the soil, causing the bed to have cooler temperatures than the surrounding area. 


  • Protection from Wind

tree in wind

Persistent hot, dry winds not only raise surrounding air temperatures, they can also rob soil of moisture, robbing plants of necessary water. On the other hand, continual cool winds reduce temperatures. 


After watching my office bed suffer for seven years, it’s time to put it out of its misery and begin again. This time though, I will spend much more time thinking through the climate details that matter before I begin creating my own little slice of paradise.

 flower bed