No, hardiness zones summer heat is not what the USDA system measures at all. USDA zones only track winter low temps and ignore how hot your summers get. A plant that survives your coldest night might still die from heat stress in July or August.
I learned this lesson when I planted some hostas that were rated hardy to my zone 6. They made it through winter fine but turned crispy brown during a hot summer with weeks above 90 degrees. The zone rating told me nothing about heat tolerance and I lost several plants that year.
The AHS heat zone map fills this gap by tracking days above 86 degrees Fahrenheit. This temp marks when plants start to feel heat stress from the weather. The map splits the country into 12 heat zones based on how many hot days each area sees per year on average.
Recent research in iScience from 2024 found that about 50% of tree species face more stress from heat than cold. This means half the trees you might plant care more about your hot summers than your cold winters. Yet most gardeners only check cold hardiness before they buy.
Heat tolerance plants often come with two zone ratings if the seller does their homework. You might see "USDA 4-8, AHS 8-1" on a tag which tells you both the cold and heat limits. The AHS numbers count down from 12 to 1 as you go from hottest to coolest areas.
Plants from cool climates like northern Europe struggle in hot humid summers even if they handle your winters well. Delphiniums, lupines, and many primroses fall into this group. They may survive winter in zone 6 but suffer when summer temps stay above 85 degrees for weeks.
Check both ratings when you shop for perennials if you live somewhere with hot summers. Southern gardeners need to watch heat zones since many popular plants come from cooler regions. A plant rated for USDA zone 8 might still fail due to intense summer heat in your yard.
Look for plants bred for your full climate range and not just your winter lows. Native plants from your region handle both local cold and heat since they evolved there naturally. Ask local nurseries what grows well through summer in your specific area. This advice can save you from repeat losses to heat stress each year in your garden beds.
Read the full article: Hardiness Zone Map: Find Your Growing Zone
