What It Is and How It Differs From Hardiness Zone

Heat tolerance in plants is commonly classified by the American Horticultural Society’s (AHS) Heat Zone Map. While the USDA Hardiness Zones focus on the cold extremes that plants can tolerate, the AHS Heat Zone Map provides useful information about the other end of the temperature spectrum — the heat.

Hot Soil Can Impact the Entire Plant Ecosystem

Soil temperature plays a significant role in determining soil health and the survival of organisms living within it. When it gets too hot, both can be adversely affected.

The AHS Heat Zone Map, similar in concept to the USDA’s Hardiness Zones, breaks down North America into (now) 13 zones based on the average number of days each year that a region experiences temperatures above 86 degrees Fahrenheit (30 degrees Celsius).

This temperature threshold is crucial as it is the point at which many plants begin to suffer from heat damage and stress.

Just as the Hardiness Zones help gardeners understand which plants can survive their region’s winter, the AHS Heat Zones help them understand which plants can survive and thrive during their region’s summer.

How Temperature Affects Your Soil and Plant Ecosystem

1. Microorganisms: Most soil bacteria and fungi thrive at moderate temperatures, typically between 68-95°F (20-35°C). Their growth can slow as temperatures approach the limits of this range, and many can be killed or go dormant at sustained temperatures above 104°F (40°C).
2. Macroorganisms: Earthworms and many soil-dwelling insects can experience stress above 77°F (25°C) and may die if exposed to temperatures above 86°F (30°C) for extended periods.
3. Soil Organic Matter: The rate of organic matter decomposition typically increases with temperature. While this can vary depending on several factors, decomposition rates often start to increase at sustained temperatures above 77°F (25°C).
4. Plant Health: Most plants prefer soil temperatures between 65-75°F (18-24°C) for optimal root growth. When soil temperatures get too high, generally over 90°F (32°C), plants can experience stress and reduced growth.

These are naturally broad generalizations, and the exact thresholds can vary depending on the specific organism or plant species, as well as other environmental conditions. But generally, keeping soil temperatures in the moderate range is beneficial for soil health and plant growth.

Tips for Keeping Plants and Soil Cooler

• Adding mulch to the soil surface to help cool it and conserve moisture
• Providing shade, such shade covers.

See also, our article on desert garden ideas.

By considering both types of zones (heat and cold), gardeners can better choose plants that are suited to expected climatic conditions they may face throughout the year.

See more on the ravaging effects of heat on soil in our article on soil temperature.

Never Heard of the Heat Zone Map?

If you’ve never heard of the Heat Zone Map, don’t worry, you’re not alone. Many gardeners haven’t as it’s used less frequently than the Hardiness Zones and is less well-known amongst casual gardeners.

But depending on your area and the plants you’re growing, or would like to grow, you may also benefit by checking the Heat Zone Map. This is naturally especially relevant for those in the hotter zones.

If you’re in a drought prone area, you may also enjoy our article on drought tolerant plants.

A Note About USDA Hardiness Zones

The opposite of the heat tolerance is cold tolerance, also known as cold-hardy. You can read all about that here in our article on hardiness zones.

The AHS Heat Zone Map divides the United States into 12 zones based on the average number of “heat days” each area experiences — days that reach temperatures above 86°F (30°C), the point at which many plants start to suffer from heat stress. The zones range from zone 1 (less than one heat day) to Zone 12 (more than 210 heat days).

Meanwhile, the USDA hardiness zone map divides the U.S. into 13 zones, with Zone 1 being the coldest (average coldest annual temperature below -50°F/-45°C) and Zone 13 the warmest (average hottest annual temperature above 60°F/15°C).

How to use Heat and Hardy Maps in Conjunction:

1. Identify your zones: First, find your location on both the AHS Heat Zone Map and the USDA Hardiness Zone Map to determine your heat zone and hardiness zone.
2. Select appropriate plants: When selecting plants, check their heat and cold hardiness information, usually available on the plant label, catalog description, or online. This information is often given as a range of zones where the plant is likely to thrive.
3. Cross-reference: Make sure the plant is suitable for both your heat zone and your hardiness zone. For instance, a plant that is rated for Heat Zone 8 and Hardiness Zone 5 would be a good fit for a garden that is also in Heat Zone 8 and Hardiness Zone 5.

While these maps are helpful, remember they are just guidelines.

Other factors, such as soil type, sunlight, wind exposure, humidity, pests, and how well the plant is watered and cared for, also greatly influence whether a plant thrives in your garden.

When it comes to heat measurements, it’s also advisable to measure your soil temps below the surface. You can read more on soil temperature here.

Local garden centers and extension services can also be a valuable resource for information tailored to your specific area, so don’t hesitate to call them. Most are delighted to speak to gardeners and are glad to help.