Zonal Information

Heat, Cold, and Frost information

🔨 How‑To Garden Guide | by Guy Saldiveri | January 20, 2026

USDA Plant Hardiness Zone Map

https://planthardiness.ars.usda.gov/

The USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which perennial plants are most likely to thrive at a location. The map is based on the average annual extreme minimum winter temperatures. 

It does not, however, provide you with adequate information regarding regional growing conditions. While this is a good reference and starting point, it’s very important to utilize the local zonal information in order to be successful.

Plant Heat Zone Map

While the USDA Plant Hardiness Zone Map is a familiar and well-used tool, there is another lesser-known map that can be just as useful.

https://www.usbg.gov/blog/heat-zones-plant-health-and-ahs-heat-zone-map

This map, created in 1997 by the American Horticultural Society, provides similar zonal information, but instead of average cold, it shows the number of heat days in each area. 

Heat days are defined as days when temperatures reached or exceeded 86°F. 

Daily high temperatures recorded by the National Weather Service between 1974 and 1995, were used to create the map. 

— Specific information for my state —

Northern Mississippi - Zone 8 - 90 to 120 heat days

Southern Mississippi - Zone 9 - 120 to 150 heat days

That's a lot of heat!

The most recent update to the USDA zone map occurred in 2023. At that time, the map was changed to indicate a warming trend over most of the zones. Because the most recent data for this map dates back to 1995 it's likely that updated data would show more heat days across many areas.

So, what's the importance of 86 degrees?

Plants, like any other living thing, have strong reactions to temperature. Whether it's cold or hot, plants react and attempt to protect themselves. Most folks are aware that freezing temperatures can damage plants fairly quickly. Heat damage is more variable and often more subtle. One of the first signs of heat stress in plants is wilting.

As temperatures increase, the plant loses more water through the process of transpiration. Transpiration is the process where plants release water vapor into the atmosphere, primarily through pores called stomata on their leaves. If the plant's water uptake does not keep pace, it becomes dehydrated and wilts.

Plants may continue to survive in this dehydrated state, but their growth rate and 

overall health decline. 

Curled leaves are the plant's way of hiding from the sun, but that also slows down photosynthesis—limiting the sugars and nutrients the plant needs to stay healthy.

If prolonged, heat stress can cause irreversible damage to the plant.

Most effects of rising heat can be mitigated until temperatures reach about 86°F. At this point, stress becomes more severe.  

Many vegetables, such as tomatoes, peppers, squash, and eggplants shut down around this stage: flowers stop forming, buds drop, and fruit production halts.

While you cannot control ambient air temperature, you can help plants cope by ensuring they are adequately watered, moving them into a more shaded area, or providing a protective cover, such as shade cloth.

First and Last Frost Dates

Experts advise planting within a specific time frame of either your first or last frost dates. You can reference this chart to see when those times occur in your specific area.

Image courtesy of FastGrowingTrees.com

https://www.fast-growing-trees.com/blogs/seasonal-guides/first-and-last-frost-dates-by-zone

 Using all three maps gives you a valuable head start. Combined, they will not only help you understand and resolve issues, but aid in selecting plants that will thrive in your local environment.

Happy Gardening 🌱