USDA Cold Hardiness Zones or AHS Heat Zones for Gardening?

Backyard Spruce


Are you a gardening enthusiast in the New England region? Do you struggle to select the right plants for your garden with the unpredictable climate and varying temperatures? Well, worry no more! In this article, we will be discussing two popular systems used by gardeners to determine plant suitability – USDA Plant Hardiness Zones and AHS Heat Zones. These planting zones are crucial for selecting which crops to plant during spring, summer, and fall in your vegetable garden.

Both of these systems provide valuable information about temperature ranges that different plants can tolerate. However, there are some key differences between the two that may affect your decision as a gardener. So, let’s dive into the details and find out which system is best suited for your gardening needs in New England!

What is the Difference Between USDA Hardiness Zones and AHS Heat Zones?

USDA Plant Hardiness Zones help measure the “cold tolerance” of the temperatures that plants in the area can be exposed to — that is if the plant will survive lower temperatures during early spring and late fall.

AHS Heat Zones are on the opposite spectrum of USDA Hardiness Zones. It helps to measure the “heat tolerance” of our plants so that gardeners can know if their plants will survive the heat of summer.

So to answer your question simply, AHS Zones track heat patterns while USDA Zones track cold hardiness.

How are USDA Hardiness Zones Broken Up?

USDA Hardiness Zones are divided into subsections based on the average annual minimum temperature, ranging from Zone 1a (-60°F to -50°F) to Zone 13b (65°F to 70°F). Each zone is divided into 10°F increments that are then split into two subsections of 5°F labeled as subzones a and b.

For example, Zone 2a has an annual minimum temperature is -50°F to -45°F, while Zone 2b has an annual minimum is -45°F to -40°F.

A plant rated as hardy in Zones 4 through 9, for instance, can thrive despite the winter season it will be subjected to.

How Are AHS Heat Zones Broken Up?

American Horticulture Society (AHS) Heat Zones have the same concept as the USDA Zones with a range from Zone 1 to Zone 12. Where Zone 1 areas have fewer than one “heat day” per year and Zone 12 that have more than 210 heat days each year. A heat day is a day with temps rising above 86°F.

A plant rated at AHS Heat Zones 6–9, for example, can take the heat of summer in those zones but would suffer in summers that are colder.

Which Zone System Should You Use for Gardening?

The simple answer is – both! You’ll want to select a crop that can tolerate both the winter cold temperatures as well as the summer high temperatures. This is particularly useful when planning your garden for spring, summer, and fall plantings.

Many nurseries will actually sell plants with information providing both cold hardiness zones and heat zones data so you can ensure you are buying something that can survive both extremes of high and low temperatures.

When reviewing the information of having both zone systems listed, the first range of numbers usually indicated the cold hardiness zones, while the second range of numbers will be the hot heat zones. You’ll need to reference both the USDA hardiness zone maps and the AHS heat zone maps to make sure that your location falls within the specified zones.