Understanding and using zone maps are essential to ensure plants survive temperature changes. Of course, winter and summer are the seasons you must be most concerned with since temperatures are at high and low extremes in most regions. So, it’s worth reviewing the zone maps below to understand what you should look for during these seasons.
Winter/Cold Temperatures
There are several zone maps you can reference to understand the average annual extreme minimum temperatures in different regions. The full USDA zone map is most commonly used. But, other USDA maps are available, providing more detailed information and a micro-view of the different zones and regions. But, for simplicity, we are using the full USDA zone map to illustrate.
Let’s say you live in Indiana. It has two temperature zones. The upper part of the state experiences temperatures between -10 and -20 degrees Fahrenheit. The lower part of the state fluctuates between 0 and -10 degrees Fahrenheit. This means plants in the upper region can survive temperatures as low as -20 degrees Fahrenheit. While plants in the lower part of the state will survive temperatures down to -10 degrees Fahrenheit. So, plants classified as hardy in these zones should live through the cold winter temperatures.
Summer/Hot Temperatures
But what about warmer temperatures? How do you know if your plants can survive the extreme summer heat? Even though it’s not commonly used, the American Horticultural Society publishes a heat zone map. This map measures the average days per year temperatures rise above 86 degrees Fahrenheit.
Continuing with the state of Indiana example, it is split into three heat zones. The map illustrates that the upper part of the state has less than 30 to 45 days of temperatures above 86 degrees Fahrenheit. Although, there are a few pockets where temperatures are different. In these pockets, temperatures above 86 degrees Fahrenheit are between 45 and 60 days in some areas and 14 and 30 days in others.
Then moving down to the middle of the state, it shows there are about 45 to 60 days of average temperatures above 86 degrees Fahrenheit. While the lower part of the state shows 60 to 90 days. So, in contrast to the cold hardiness zone map, the heat zone map helps determine how many and whether plants can survive hot summer days above 86 degrees Fahrenheit.
But, despite the usefulness of the zone heat map, most plants are not classified with a heat zone rating. So it may be challenging to understand whether plants will perform well in a particular zone. Therefore, consulting garden centers is recommended to learn about plants’ heat tolerance. Another suggestion is to look up plants online before buying to ensure they can tolerate the summer conditions. But don’t give up on reading plant tags because the good news is more plant companies are starting to recognize the zone heat map. With that, some have even begun to include information on the plant tag. So also look for this information when buying.
Plant Grow Enjoy ™