You may have heard hardiness zones mentioned in gardening discussions. But there is another type of growing zone to be aware of when planning your garden — your heat zone.
What is a heat zone? How much does it matter? Read on to find out.
Jump to: What it is | Why it matters | Where to find it
What is a Heat Zone?
Heat zones classify different areas of the United States by the number of days with temperatures of 86ºF or higher. In other words, they go by the total number of hot days that an area gets out of the whole year, on average.
Some areas, particularly in the eastern half of the country, have similar heat zones and hardiness zones. For example, Greenville, SC is in both hardiness zone 8 and heat zone 8.
However, other areas are quite different. Olympia, WA is also in hardiness zone 8 since winter low temperatures do not fall below 20ºF on average. But it is only in heat zone 2, similar to northern Maine and the Upper Peninsula of Michigan, due to its mild-summer climate with very few days above 86ºF.
Learn more about USDA hardiness zones in this article.
So why does a heat zone matter? Let's get right into it!
Why Heat Zone Matters
Heat zones can be an important factor in determining which plants will grow in your area.
This is especially true if you live in a low heat zone or a heat zone that greatly differs from your hardiness zone, as in the Olympia example above. This is true for much of the maritime Pacific Northwest, Alaska, and high-altitude regions, among other areas.
Some plants may not grow or may not produce fruit in lower heat zones. Extra measures to add light and heat, such as grow lights and heat mats, can combat these challenges in many cases.
For extra-heat-loving plants like okra, these measures still may not be enough. But other plants like tomatoes can be worked around by starting seeds inside ahead of the growing season and choosing the sunniest spot in your yard when transplanting. Another strategy is to choose smaller-fruit varieties like cherry and Roma. Smaller fruits require less plant energy — and therefore less sunlight — for production.
Similar measures can be taken for other nightshade family plants such as eggplants and peppers, both of which need even more heat than tomatoes do. For example, you may choose mini bell pepper varieties instead of the larger, supermarket-size varieties, or you might select always-smaller peppers like poblanos or jalapeños instead. Eggplants also have mini varieties, and Asian eggplant variety fruits tend to be narrower, requiring less energy output from the plant.
However, being in a higher heat zone can also affect plant growth and selection. Some plants may become heat-stressed or sun-scalded in hot areas. Of course, cool-season plants that bolt quickly in hotter temperatures will have a shorter seasons in higher heat zones. For example, our Growing Chard article makes this comment:
"Most chard varieties do not tolerate heat as well as they tolerate cold. They are quick to bolt in temperatures above 80°F, especially in a sudden temperature increase like a spring heatwave. Preventative measures like moving the plant to a cooler, shadier, or north-facing area; covering it; or encouraging taller plants to provide more shade can help it to survive summer in warmer areas. Some chard varieties such as perpetual spinach (a chard, despite the name) are specifically bred to be more heat-tolerant, so seeking these varieties is another option for warm to hot regions."
Strategies like these help a lot in hot-summer regions. You can extend the growing season of heat-sensitive plants by providing shade or planting them in a container that can be brought inside or moved to a cooler spot. If you live in a high enough hardiness zone, you may choose to plant them in the winter and skip them in the summer as well.
Some plants are sensitive to both too much heat and too little heat. Tomatoes are a good example of this. In areas with short and/or mild summers, tomatoes may struggle to produce large fruits or enough (or sometimes any) fruits before the end of the growing season. However, in temperatures of 85-90ºF and above, their growth becomes stunted, and they stop producing fruit. As noted in our Growing Tomatoes article, the best temperature range for tomato fruiting is 65-80ºF.
Heat zones are just 1 reason why selecting plants based on your hardiness zone alone is not the best garden-planning strategy. Cold hardiness is just 1 factor in determining plant selection. Learn more in our What is a USDA Hardiness Zone? article here.
However, heat zones should not be your only consideration either. They are simply 1 factor to consider when planning out your garden.
For example, in addition to trying the strategies mentioned above, you may also decide to try plants not recommended for your heat zone in case you end up having a hotter summer than usual.
Need to organize your garden-planning thoughts? Grab a FREE printable garden planner below! Enter your email address in the box, and we'll send it directly to your inbox:
Where to Find Information on Heat Zones
We do not include a heat zone section as a regular feature of our gardening facts articles. However, we may mention it if it is relevant to the plant being discussed. If so, it is usually in the Lifespan section, which discusses whether the plant is an annual, biennial, or perennial, or in the Ideal Temperature section, as in the chard example above.
You can also get an idea of whether a plant is right for your heat zone by looking at the Ideal Temperature section and comparing it with the days to maturity noted on your seed packet. For example, that section in the Growing Eggplants article here notes that eggplants grow best in temperatures of 70-95ºF. Your seed packet might note that your eggplant variety matures in 100-120 days. If you are in a low heat zone but routinely have high temperatures in the 70s, you might be able to grow this variety, as long as your warm season lasts at least 120 days. If not, this plant or variety may not be the best fit for your area, or you may need to take extra measures such as those noted in the previous section.
If you would like to know your heat zone, find it in the official American Horticultural Society heat zone map here: https://ahsgardening.org/about-us/news-press/cool_timeline/heat-zone-map-developed/
Read More
Check out these other supplementary articles:
- What is a USDA Hardiness Zone?
- How Often to Water Plants
- How Much Sun Does My Plant Need?
- Choosing the Right Planter for Your Container Garden
- Companion Planting Basics
- What is a Plant Family?