Welcome to an introduction to the United States Department of Agriculture Plant Hardiness Zones. This is a multi-part series that will walk you through the different plant hardiness zones throughout the United States. This intro post will explain what the USDA Plant Hardiness Zones are, how many are designated, discuss terminology, as well as a variety of other information.
Table of Contents
Making Planting Easier
To understand what plants grow in your climate, you must first know what planting zone you live in. I touched upon this subject in my post “How to Determine Your Area’s Frost Dates’. When you buy plants at the store, they sell plants that are compatible with your area. There is one caveat to that though. Many times they are selling plants way too early for them to be able to survive weather conditions. For example, this past year (2022), some local stores I had gone to were already selling tomato plants in April. For my area, April is way too early to plant tomatoes, unless you can keep them in a greenhouse. In the beginning of May last year, we had flurries. The year prior we had freezing temps around that time of year. Tomato plants don’t really do well below 50 degrees Fahrenheit.
It’s good practice to check the outdoor temperatures at least 2 weeks out before planting to make sure it’s safe. Our neighbor decided to plant around Mother’s Day since he was anxious to get his plants in the ground. That happened to not be a good idea and lost all of his plants to cold temperatures. Knowing your climate and understanding when to plant each plant will save you a lot of heartache and double work.
What are the USDA Plant Hardiness Zones?
The United States Department of Agriculture, or USDA, has a guide for plant hardiness zones. It is only a guide and not an exact science. There are other elements, such as microclimates, that you’ll encounter. (We’ll review microclimates later in this post).
The USDA defines the the plant hardiness zones (or gardening zone) as,
“the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.”USDA Plant Hardiness Zone Map. usda, 2022. https://planthardiness.ars.usda.gov/
These zones are determined based on the following:
“Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future.”USDA Plant Hardiness Zone Map. usda, 2022. https://planthardiness.ars.usda.gov/
There are a total of 13 zones in the US. Zone 1 has the coldest winters, while zone 13 is a low or subtropical desert. These zones are divided further into designated ‘a’ and ‘b’. For example, in zone 7 there is 7a and 7b. Within each of these zones there are microclimates.
The Importance of Knowing Plant Hardiness Zones
There are a lot of reasons why you need to know your plant hardiness zone, even if it’s a guide. You need to know what plants can survive in your climate and which ones won’t survive. Some weather in your area may be either too hot or too cold for plants.
For example, in my area, tropical plants like dragon fruit trees won’t be able to survive our winter. They prefer temperatures between 60 to 80 degrees Fahrenheit, but can tolerate down to 40 degrees Fahrenheit. Our winters would be too cold. To produce fruit it needs either full sun or at least 6 hours of sunlight. If I planned to have a dragon fruit tree, I’d have to be prepared to move it inside and located in a warm spot.
1. Knowing How Long Your Growing Season Is
To understand the length of the growing season for your area, you’ll need to know your first and last frost dates. Identifying these two dates helps to gauge how many frost free days there are. To extend the growing season, some may opt to either grow in a greenhouse, indoors under grow lights, or row covers that are rated to protect against frost.
In my area, the last threat of frost this year was towards the end of May and frost hit our area in October. We have roughly 122 days of growing season.
To extend the season and get a head start, I grow my plants from seeds indoors and put them under grow lights. When it warms up, I’ll later put them in the greenhouse.
Putting plants outside when it is too cold for them can either cause damage or even kill them entirely.
First and last frost dates
In my post, How to Determine Your Area’s Frost Dates, I Let’s take a look at the steps to determine your plant hardiness zone. It’s super simple!
- Start out by checking the USDA Plant Hardiness Zone interactive map.
- In the upper left hand corner of the map, there’s a search bar. Enter in either your zip code or address.
- Click on the magnifying glass to conduct the search.
- The map will then identify zoom in on the area you typed in. Use the legend on the right hand side of the map to pinpoint your hardiness zone.
- If the legend does not appear, click on the 3 lined toggle.
Jot down your plant hardiness zone that you just identified. You’ll need this information if you’re buying plants or seeds.
Now that you have your plant hardiness zone, we can now zoom in on your area.
Farmer’s Almanac is another helpful tool as it gives you an estimated, or average, first and last frost date based on your zip code. Again this is only an estimation To use their frost date calculator, navigate to the link provided above.
The calculator is easy to use – type in your zip code and click ‘search’.
Write down the two dates that Farmer’s Almanac provides. To plan your garden, you’ll need those dates too.
2. What Else to Know: Heat Zone and Sunset Climate Zone
Heat Zone maps are a relatively new concept. Each area has its own plant hardiness zone designated, but there are now heat zones too. The map indicates how many days of the year each area has temperatures above 86 degrees Fahrenheit. Temperatures above 86 degrees Fahrenheit start to stress plants out. When you’re buying plants, seed packets, etc. look out for the newly added heat zone indications on the packaging or tag.
A Sunset Climate Zone, derived by Sunset magazine, takes into account not only temperatures but also latitude; elevation; ocean influence; continental air influence; mountains, hills, and valleys, and microclimates. All of these conditions have an effect on the overall health and productivity of plants.
3. Get to Know Your Garden’s Sun Exposure
Map out the sun exposure on your property. Which areas get sun and which areas get shade? How much sun and shade is important to know if you want your plants to grow and produce fruit. There are different types of exposure so I’ve bulleted out each one with a brief definition.
4. Terminology to Know
There are varying degrees of both shade and sun. Shade for instance can be categorized as full, dense, light, bright, and partial. Sunlight can be divided into two – partial or full sun. A key factor to keep in mind is that there is a difference between morning and afternoon sun. Morning sun is less intense and cooler, while afternoon sun can be very hot and intense.
- Full shade: receives less than 3 hours of shade. It can also be dappled throughout the day because of tree canopies.
- Dense shade: an area that has little to no shade. Dense shade can occur around buildings that were built closely together and under dense trees and bushes.
- Bright shade: also known as “partial filtered sun” or “light shade”, receives sun for half of the day (between 4 to 6 hours of direct sun). It can be a bright location with a high, diffuse canopy of trees.
- Partial shade: or partial sun, receives at least 2 hours of shade of direct sun per day or shaded for at least half of the day.
- Full Sun: receives 6 to 8 hours of direct sun per day.
5. Microclimates in Your Area and On Your Property
A microclimate is a set of local atmospheric conditions that differ from those in the surrounding area. For example a hill can cause different types of microclimates. On one side there is a sunny slope which can have higher temperatures, and on the other side it can be shady with lower temperatures.
Each property has its own microclimates that may differ from plant hardiness zones. Remember, plant hardiness zones are only a guide. So paying attention to the weather in your area Start making notes of how the sun touches (exposure) each part of your property. Make note of the following as well:
- What part of your property is the last to dry out after rain?
- What part of your property has snow on it the longest and takes the longest to melt?
- What part of your property has a lot of shade?
- Are there any areas of the property that have a lot of wind?
- Is there an area that is sheltered from wind?
- What direction does the wind come from?
- What is each area’s position in relation to the sun?
There are 4 different microclimates to be aware of on your property:
- Northern exposure: a Northern exposure usually has the least amount of sunlight and the slowest to warm up. It can vary from little or weak sunlight to no sunlight at all. Unless this area of your property gets 8 hours of direct sunlight, it will most likely not be suitable for planting most fruits and vegetables.
In our Northern facing garden area, we do not have any fruits or vegetables. Instead we have a variety of plants, like ferns that enjoy cool, many times wetter, conditions.
- Southern exposure: A southern exposure may be the most ideal for planting a vegetable garden, depending on your climate. This type of exposure gets the most amount of sunlight and heat. It may even get sun all day long. It’s the warmest and hottest part of your property.
For example, my garden faces south, plus it is cornered by our house (see below for reference). It is the hottest area of our property and gets the most amount of sun. However since it is right next to the side of our house, it is shielded by the late afternoon sun.
- Eastern exposure: Eastern exposure provides an early morning sun and shaded most of the time in the afternoon.
- Western exposure: A Western exposure is shaded during the morning and possibly early afternoon. It gets afternoon, late afternoon, and evening sun. These times of the day will be the hottest.
Other elements on your property
Additional elements to make note of that can affect microclimates on your property:
- Any type of sloping
Natural sloping has an effect on water and moisture retention. Depending on your climate, slopes can either be an advantage or a disadvantage. Practices like permaculture for instance takes sloping into account to be an advantage to the garden for irrigation. However, sloping and moisture retention can become a problem when the soil is waterlogged, leading to issues like rot.
Windbreaks can come in handy when you have plants that are sensitive to winds and may need a sheltered place to be planted. Make note of any trees that block wind, buildings, hills, bushes, etc.
- Rocks, cement, and walls
These different elements absorb heat, which can either help or be an issue for your plant. Heat loving plants will love being planted near heat absorbing materials that block cold temperatures. Plants that can only tolerate heat to a certain temperature may not be suitable planted next to those types of materials.
Fencing is both a barrier to wind, but can also reflect heat depending on the type of material. For example, vinyl fencing reflects heat. Keep this in mind as some plants may be heat sensitive. The amount of heat can either cause damage or kill your plants.
6. Tips on Planting the Correct Plants for Your Area
So you know whether you’re planting the correct plants for your area, always be aware of your plant hardiness zone and the appropriate time to plant. As I mentioned earlier, you may see stores selling plants very early in the season. Like in my example, tomato plants being sold in April or even early May is far too early to plant them outside. It can lead to a variety of health issues of the plant or even lead to its demise.
When buying plants or seed packets check the following:
- The tag on the plant –
When you’re buying a plant at the store, it more than likely has a tag attached to it. Read the planting instructions carefully.
- Sun exposure – how many hours of sun the plant needs per day
- The plant’s common name and scientific name
- How much water per week it needs
- When to expect fruit
- Hardiness zone
- Estimated height
- On a seed pack –
When buying seeds, there is similar information listed to what you’ll find on a plant tag.
- Common name
- Scientific name
- Number of days until seeds sprout (germination)
- Ideal temperatures for the plant to thrive
- How deep to plant the seeds
- Whether to start indoors or outdoors
- Plant spacing
- Whether the plant is frost hardy or not
- Number of days the plant bears fruit after transplant
Breakdown of Each of the Plant Hardiness Zones in the US (coming soon!)
The below posts are not yet available. Check back soon as you’ll be able to click on each of the zones to learn more about them.
- Quick Guide to Zone 1
- Zone 1a
- Zone 1b
- Zone 2
- Zone 2a
- Zone 2b
- Zone 3
- Zone 3a
- Zone 3b
- Zone 4
- Zone 4a
- Zone 4b
- Zone 5
- Zone 5a
- Zone 5b
- Zone 6
- Zone 6a
- Zone 6b
- Zone 7
- Zone 7a
- Zone 7b
- Zone 8
- Zone 8a
- Zone 8b
- Zone 9
- Zone 9a
- Zone 9b
- Zone 10
- Zone 10a
- Zone 10b
- Zone 11
- Zone 11a
- Zone 11b
- Zone 12
- Zone 12a
- Zone 12b
- Zone 13
- Zone 13a
- Zone 13b
Printable Workbook Coming Soon!
Not from the United States?
If you’re not from the United States, what do you use as a general guide for planting your garden? Share in the comments section!