What Are Growing Zones, and What Do They Mean for Your Garden?

When determining which perennial flowers or fruit trees you can grow at home, the growing zone is one of the most important pieces of information. Look at a seed packet or read through one of our plant guides, and you’ll see the growing zone listed. And while you may know that growing zones relate to whether or not you can grow that gorgeous flowering perennial, do you know what they really mean?

I’ll explain how scientists determine growing zones and describe how you can best use your growing zone to guide your gardening practices. Then, I’ll explain other factors to consider when determining which plants you can successfully grow in your area.

What Are Growing Zones?

Growing zones, also known as hardiness zones, are established and published by the United States Department of Agriculture (USDA).  While many people think hardiness zones relate to the number of frost-free days, they actually explain a location’s average low temperature. Scientists look at the average lowest winter temperature across 30 years and then use this data to form the map. 

The map contains 13 different zones, each containing a range of 10°F (~5°C). Along with these numerical zones, each zone includes two half zones indicated by “a” and “b.” Zone 1a experiences average annual low temperatures between -60°F to -55°F (-51°C to -48°C) while zone 13b only drops down to 65°F to 70°F (18-21°C). The rest of the growing zones fall somewhere in between these two.

Since growing zones relate to a location’s temperature, the map changes over time. In fact, experts just updated the map in 2023. Many locations bumped up into a higher growing zone, but some locations remained in the same zone. Not only did the zones change, but they also became more accurate. When experts created the 2023 map, they utilized data from 7,983 weather stations; this number increased to 13,412 stations in 2023.

USDA Plant Hardiness Zones

As I mentioned above, the USDA breaks the United States into 13 different plant hardiness zones. Since these zones are separated by average low temperature, factors including longitude, elevation, and proximity to the ocean impact which growing zone a specific location falls into.

Here is a brief introduction to the different growing zones you can find in the US.

Zone 1

• Average low winter temperature: -60°F to -50°F (-51°C to -46°C)

Zone 1 is reserved for the coldest areas of Alaska. The plants that grow in zone 1 are well adapted to survive its harsh conditions.

Zone 2

• Average low winter temperature: -50°F to -40°F (-46°C to -40°C)

Zone 2 also occurs in Alaska. While it can support more plants than zone 1, many plants struggle to grow in this location without winter protection.

Zone 3

• Average low winter temperature: -40°F to -30°F (-40°C to -34°C)

Parts of Alaska and sections of northern Montana, North Dakota, Minnesota, and Maine fall in zone 3. Although this zone is undoubtedly cold, you can grow perennials well-adapted to long and cold winters. Some popular flowering plants that thrive in zone 3 include yarrow,  columbine, coreopsis, and delphinium.

You can also grow many cold-hardy vegetables in zone 3, especially with the help of row covers and high tunnels. Starting seeds indoors in the spring is another way to make the most of this zone’s short growing season.

Zone 4

• Average low winter temperature: -30°F to -20°F (-34°C to -29°C)

Many of the northern parts of the Midwest, Central Plains, and New England belong to zone 4. Although these areas experience very cold winter temperatures, their growing seasons are long enough to support a wide variety of crops. 

Roots, greens, and perennial flowers will produce without protection, and summer crops like tomatoes and peppers are possible with the help of row covers and high tunnels.  You can also grow cold-hardy apples, grapes, peaches, and pears as long as you select an appropriate variety.

Zone 5

• Average low winter temperature: -20°F to -10°F (-29°C to -23°C)

Zone 5 consists of much of the Midwest and Central Plains as well as portions of New England and the Rocky Mountains. Many perennials can survive in zone 5, including a wide variety of flowering plants and fruit trees.

Since the frost-free growing season in zone 5 is around 150 days long, you can also successfully grow long-season crops like tomatoes and eggplant. But since the first frost arrives in early fall, plant fall crops in midsummer so they have time to mature.

Zone 6

• Average low winter temperature: -10°F to 0°F (-23°C to -18°C)

This zone expands across the country from Coastal Maine all the way to Eastern Washington. It also includes much of the Central Plains, Mid-Atlantic, and high elevations in the Southwest. You can get away with growing lots of different vegetables, flowers, and trees in zone 6, but you should cover or bring inside cold-sensitive plants.

Zone 7

• Average low winter temperature: 0°F to 10°F (-18°C to -12°C)

If you live in zone 7, you can grow a wide variety of plants, including those that require winter chilling hours, as well as those that need a long summer growing season. So go ahead and experiment with crops like figs and okra, as well as cold-tolerant perennials.

Zone 8

• Average low winter temperature: 10°F to 20°F (-12°C to -7°C)

Zone 8 includes much of the South, Southeast, and Pacific Northwest. These areas experience regular winter freezes, but they also benefit from long growing seasons.

While it’s easy to grow many common fruits, vegetables, and flowers outside in zone eight, you still have to be aware of killing frosts. Along with cold temperatures, pay attention to chilling hours and choose fruit tree varieties well suited to this warmer zone.

Zone 9

• Average low winter temperature: 20°F to 30°F (-7°C to -1°C)

Parts of Florida, Texas, and the South belong to zone 9. These areas typically only experience a handful of frosts each winter and remain warm for much of the year.

You can easily grow most vegetables, fruits, and flowers in zone 9, but you may need to plant cool-weather crops earlier in the spring to avoid hot temperatures that cause plant stress. 

Zone 10

• Average low winter temperature: 30°F to 40°F (-1°C to 4°C)

Zone 10 is reserved for the warmest areas of the continental United States, including California, Florida, and Texas, plus parts of Hawaii. Locations in zone 10 deal with few or no frosts each winter.

That means you can successfully grow frost-sensitive plants like lemons, oranges, and lantana year-round. However, the lack of frost means plants like coneflowers, apples, and hellebores won’t thrive here.

Zone 11

• Average low winter temperature: 40°F to 50°F (4°C to 10°C)

Zone 11 occurs only in Hawaii and Puerto Rico. Since these areas receive no frosts, they’re well suited for growing tropical plants as well as annual vegetables.

Zone 12

• Average low winter temperature: 50°F to 60°F (10°C to 16°C)

Zone 12 occurs in much of Puerto Rico and parts of Hawaii. Its continuous warmth means it’s a great environment to grow tropical crops and common annual plants.

Zone 13

• Average low winter temperature: 60°F to 70°F (16°C to 21°C)

The coastal edges of Puerto Rico belong to zone 13. The year-round warm temperatures mean this climate can support a variety of tropical plants.

Why Are Growing Zones Important?

These zones are an important factor when deciding which plants to add to your garden. Since these zones relate to the average low temperature, they are a great indicator of whether or not a perennial plant will survive the winter outdoors in your area.

For example, let’s say you live in St. Louis, Missouri, and want to grow a fig tree outdoors. Your first step is to determine your growing zone by looking at the USDA plant hardiness zone map or searching online. You’ll see that St. Louis belongs in zone 7a since the average winter low ranges between 0°F and 5°F (-18°C and -12°C).

Next, check the hardiness zone of fig trees. Like with most fruit trees, different fig varieties have varying abilities to survive cold temperatures. While most are well suited to survive winters in zone 7, a few can successfully overwinter in zone 6.

When you put the two aforementioned pieces of information together, you’ll learn that you can successfully grow figs in St. Louis! However, it’s important to remember that hardiness zones are based on the average winter low. If an unusual cold snap strikes, you may need to take extra steps to help plants survive the winter.

Annuals and Growing Zones

While growing zones let you know which plants will survive the winter in your area, they’re also useful for determining whether or not annuals will grow well in your area. Since annual plants complete their life cycle and die in one year, having them die during the winter isn’t a big deal. However, not all annuals are well-suited to all hardiness zones!

Hardiness zones technically only tell us about the average low temperature, but they also correlate with factors such as the length of the growing season. For example, areas in zone 9, like Seattle and San Antonio, have a growing season that’s 230-260 days long. When you move into zone 6 locations like Cincinnati and Denver, you’ll find that the growing season is only 150-180 days.

As you might expect, long-season annuals have a harder time growing in lower hardiness zones. While it’s not impossible to grow crops like tomatoes, winter squash, and sweet peppers if you live in a lower zone, you should take steps to increase the likelihood of a successful harvest. For example, starting seeds indoors before the last spring frost and covering young plants with row cover can help you make the most of your shorter growing season.

Growing Zones and Tender Perennials

Many plants we consider annuals are actually tender perennials. Although these plants often die after a single year of growth in cold environments, their death is caused by cold rather than the plant’s natural lifecycle. If you were to plant a tender perennial in, say, zone 11 instead of zone 6, it would live for multiple years. These growing zones are just an example—the exact location where a perennial can overwinter varies between individual plants.

Some examples of tender perennials include impatiens, begonias, and fuchsias. Since these plants die back with frost, many people purchase and plant them each spring.

Factors to Consider Beyond Your Growing Zone

Although the growing zone plays a big role in whether or not a plant will thrive in your location, it’s not the only factor to consider when looking for new plants! Just because a plant is suited for your hardiness zone, it isn’t necessarily suited for your precise location. Check the following factors before adding a new plant to your home.

Frost Free Days

When it comes to growing vegetables, annual fruits, and cut flowers, the number of frost-free days plays a big role in whether or not you’ll obtain a good harvest. You can calculate your frost-free days by counting the amount of time that occurs between your average last spring frost date and average first fall frost date.

Since plants like zinnias and sweet peppers can take up to three months to reach maturity, ensure they have enough time to produce their sought-after flowers and/or fruits. In areas with short growing seasons, this means planting transplants outdoors as soon as possible after the last spring frost.

Although growing zones are associated with the number of frost-free days, it’s not an exact correlation. The best way to determine whether or not you can grow a specific long-season crop is to look up the length of your growing season rather than your hardiness zone.

First Spring and Last Fall Frost Dates

As mentioned above, the time between your first spring and late fall frost date determines the length of your growing season. Knowing the bookends of your gardening season is just as important as the length of the season.

Let’s say you’re wondering when to transplant tomatoes outside in the spring or when to start vegetable seeds indoors. Since both of these activities are dependent on temperature, you want to look at your area’s average last frost date. It’s true that lower-growing zones have later spring frost dates, but the exact date can vary by a few weeks within a single growing zone.

For example, both Philadelphia and Boise are located in zone 7. However, their average last spring frost dates vary by almost a month—Philadelphia’s occurs on April 4, and Boise’s occurs on April 30. That means you can set tomatoes outdoors three weeks earlier in Philadelphia than you can in Boise, even though they’re in the same growing zone.

The last fall frost date helps you determine when to plant fall crops so they’re ready before the bitter cold arrives. As with the last spring frost, this date relates to the growing zone but varies between different locations within a single zone.

Sun Exposure, Rainfall, and Soil Type

Even if a crop is well-suited to your growing zone, it doesn’t mean it will thrive in your garden. Remember to look at important environmental conditions, including light, soil drainage, humidity, and soil moisture. You should also consider the exact location you’re planting since items like brick walls and low-lying pockets can create microclimates that differ from the great environment.