You may have come across the terms "hardiness zone" and "USDA zone" or a specific number like "Zone 8" when reading about individual plants. Many gardening resources, including some gardening facts articles on the Plant to Plate site, include this information.
What is a USDA hardiness zone? Does it always matter? Read on to find out.
Jump to: What it is | Why it matters | Where to find it
What is a Hardiness Zone?
USDA hardiness zones classify different areas of the United States by their average annual lowest temperature. In other words, they go by the coldest temperature that an area gets out of the whole year, on average.
Europe has a similar system known as European Hardiness Zones.
USDA hardiness zones are sometimes just called zones, such as Zone 5 or Zone 8, in gardening discussions and sometimes even on seed packets. Although it sounds like a normal short form, this abbreviation disregards heat zones, a lesser-known but equally important climate classification by the USDA. Heat zones, along with other regional climate factors, are why, for example, a gardener in Zone 9 in Oregon will not be able to grow all of the same plants as a gardener in Zone 9 in South Carolina.
Learn more about heat zones in this article.
When hardiness zones are referenced on seed packets or in other gardening resources, it can give you a general idea of the plant's minimum cold tolerance. Keep reading for more information about this.
Why It Matters (And Why It Doesn't)
Hardiness zones can be a factor in determining which plants will grow in your area.
As mentioned above, do NOT select plants based on hardiness zone alone. Know your heat zone, average rainfall, soil type and fertility, and any other local environmental factors that affect your climate and microclimate.
However, a hardiness zone can be a great starting point for garden planning. For example, some of our gardening facts articles contain statements like this one from Growing Tulsi:
"Tulsi is a perennial herb, meaning that it lives for longer than a couple years, when grown in Hardiness Zones 10-11 or when brought inside for the winter. It can live for 8-10 years in optimal conditions. In cooler regions, tulsi is grown as an annual, meaning that it starts and ends its life in the same year."
Likewise, our Growing Tomatoes article says this:
"Tomatoes are perennial, meaning that they live for longer than a couple years, in warm climates that do not have temperatures below 35°F or when brought inside for the winter. In colder regions, tomatoes are grown outdoors as annuals, meaning that they start and end their life cycle in the same year."
Does this mean that tulsi and tomatoes will never make it through a winter in Zone 9, where temperatures have an average lowest winter temperature of 30ºF?
Not necessarily.
Are they more likely to make it in South Carolina Zone 9 than Oregon Zone 9?
Probably, due to a longer summer with hotter temperatures and more direct sunlight as well as more evenly distributed annual rainfall. The Pacific Northwest tends to have 95% of its rainfall in the late fall, winter, and early spring months, followed by a months-long summer drought. The rainy months, especially combined with the constant winter cloud cover, are not friendly toward plants like tulsi and tomatoes, both of which prefer warm, sunny weather and an even, consistent amount of water. These are just a few examples of regional climate considerations.
Does this mean that you should try overwintering tomatoes outdoors in Hardiness Zone 5?
Probably not, unless you have some kind of heated beds. The fact that they are usually perennials in Hardiness Zones 10-11 means that Zone 9 and maybe Zone 8 might be able to get away with it, especially if expecting a mild winter, but tomatoes are unlikely to tolerate the winter temperatures of somewhere 5 zones lower. Hardiness Zone 5 average lowest temperatures are around -15ºF, rather than tomatoes' average minimum tolerance of 35°F, a 40-degree difference.
So zones give you a general starting point of what may or may not grow in your area. But they should not be the beginning and end of your garden-planning process.
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Where to Find Information on Hardiness Zones
We do not include a hardiness zone section as a regular feature of our gardening facts articles. If it is mentioned, it is usually in the Lifespan section, which discusses whether the plant is an annual, biennial, or perennial, as in the examples above. Some plants that would be perennials in warmer regions can still be grown as annuals in colder climates or if brought inside a home or heated greenhouse for the winter. This gives you an idea of the best winter care for the plant in your area.
We do not include a regular hardiness zones section for the reasons stated in the previous section. Basically, hardiness zones are relied upon far too often, and there are so many more factors to be considered when determining if a plant will grow in your area. As discussed above, a cold-sensitive plant may be more likely to survive winter in a place with long, hot summers and short winters than in one with short summers and long, dark winters, even if the 2 places share the same hardiness zone. Likewise, it may survive better in a place that has 1 cold week out of the whole year that gives it a low hardiness zone, while the rest of the year is warmer than the zone alone would indicate, than it would in a place with winter temperatures that routinely hover around or just above the lowest-temperature point for weeks or months.
If you would like to know your zone to use it as a starting point, find it in the official locator here: https://planthardiness.ars.usda.gov/
Read More
Check out these other supplementary articles:
- What is a Heat 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?