USDA Hardiness Zone: How to use the information effectively

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Plants are often described as being “hardy” or “tender” based on their ability to survive in different hardiness zones. Hardy plants can withstand the colder temperatures of a given zone, while tender plants will only survive in the warmer zones.

When choosing plants for your garden, it is important to consider the hardiness zone of the area in which you live. This will help you select plants that are likely to thrive in your particular climate.

Hardiness zones are geographical areas that are defined by a range of climatic conditions, such as average annual minimum temperature, that are suitable for the growth of specific plants. The concept of hardiness zones was developed by the United States Department of Agriculture (USDA) in 1960, and has since been adopted by many countries worldwide. Hardiness zones are represented by a numbered scale, with Zone 1 being the coldest and Zone 11 the warmest.

Hardiness zones are typically represented on maps as bands of color, with each zone corresponding to a specific range of minimum temperatures. The zones are not static, but can shift over time as climate conditions change. For example, the USDA hardiness zone map of the United States has shifted northward by an average of almost one zone over the past 30 years.

Source: (1)

Hardiness zones are a useful tool for gardeners but are not perfect. They do not take into account factors such as rainfall, snowfall, wind, or elevation, which can all affect a plant’s ability to survive in a particular location. Additionally, microclimates can exist within hardiness zones, meaning that one location might be suitable for growing a certain plant, while another location just a few miles away might not be.

Despite their imperfections, hardiness zones are still a helpful guide for gardeners in determining which plants are likely to thrive in their area.

If you understand the hardiness zones in your area, you can pick plants that are most likely to survive the local winters. Annuals do not require zones because they will only survive one season, usually during the summer. Check the USDA zones before you plant perennials, trees, and shrubs. Western Americans are most affected by the USDA zones’ limitations.

Sunset climate zones may be useful if you live in this area. To determine which plants grow best where, this system considers more than just minimum temperatures. Besides summer temperatures, wind, humidity, and rainfall, they also consider the length of the growing season. In your own garden, you may have important microclimates that affect how plants grow, even if your zoning system is not perfect. You should always check your garden’s USDA or Sunset zones as a guide to maximize your chances of success.

1

Hardiness Zone 1

Hardiness Zone 1

Hardiness zone 1 is the coldest zone in the United States. It is located in northern Alaska and parts of Canada. The average minimum temperature in this zone is -60 degrees Fahrenheit. Only the hardiest plants can survive in this climate.

Temperature in Zone 1

  • 1a (-60 to -55 °F/-51.1 to -48.3 °C)
  • 1b (-55 to -50 °F/-48.3 to -45.6 °C)

What plants to grow in Zone 1?

Taking care of plants in Zone 1 can be challenging. It is rare for plants to survive in the tundra. It makes sense that native plants are excellent choices since they have already adapted to the area. In addition to annuals, winter isn’t a problem for them. Planting and caring properly will enable you to grow some non-native perennials in Zone 1.

When to Plant: Mid-June

2

Hardiness Zone 2

Hardiness Zone 2 is a zone being climatically suited to the growing of plants that are able to withstand minimum average temperatures of -50 to -40 degrees Fahrenheit. This zone is found in the northernmost parts of the United States and Canada, including Alaska. Plants that are hardy to Zone 2 include many trees, shrubs, and herbaceous perennials.

  • 2a (-50 to -45 °F/-45.6 to -42.8 °C)
  • 2b (-45 to -40 °F/-42.8 to -40 °C)

What plants to grow in Zone 2?

Zone 2 plants must withstand extreme cold and high levels of drought. The best plants for the environment are those that are native. There are some perennial plants that are suitable for this zone, but annuals are best suited for this zone.

When to Plant: Mid-June

3

Hardiness Zone 3

Hardiness Zone 3

Hardiness zone 3 is a zone where the average minimum temperature is -40 to -35 degrees Fahrenheit. This zone is found in the northernmost parts of the United States, as well as in parts of Canada. Plants that are hardy to zone 3 can withstand the coldest temperatures that occur in this zone.

Temperature in Zone 3

  • 3a (-40 to -35 °F/-40 to -37.2 °C)
  • 3b (-35 to -30 °F/-37.2 to -34.4 °C)

What plants to grow in Zone 3?

In Zone 3, plants must adapt to low temperatures due to the cold minimum average temperatures. Growing native plants across zones is possible as long as the growing conditions are similar, regardless of altitude. Vegetable and flowering annuals have a short growing season, but you can extend it by starting plants indoors or buying them from a greenhouse.

When to Plant: Mid-May

4

Hardiness Zone 4

Hardiness Zone 4 is a great place to garden. The climate is perfect for a wide variety of plants, and the soil is rich and fertile. There are many beautiful gardens in Zone 4, and it is a great place to live. The minimum temperatures in these climates range between -30 and -20 degrees Fahrenheit. Planting vegetables and flowers in this zone is easier than in colder zones, but the short growing season impacts both.

Temperature in Zone 4

  • 4a (-30 to -25 °F/-34.4 to -31.7 °C)
  • 4b (-25 to -20 °F/-31.7 to -28.9 °C)

What plants to grow in Zone 4?

Zone 4 has a cold climate suitable for growing plants hardy enough to withstand temperatures well below zero. Getting a head start on plants is possible by starting them indoors or purchasing them from a greenhouse to combat the shortened growing season. Some plants can survive cold winters by adding mulch and taking preventative measures.

When to Plant: Mid-May

5

Hardiness Zone 5

Hardiness zone 5 is a zone where the minimum average temperature is -20 to -15 degrees Fahrenheit. This zone is found in the northern and central United States, as well as in parts of Canada. Many plants and trees are able to grow in this zone, as long as they are able to withstand the cold temperatures.

Temperature in Zone 5

  • 5a (-20 to -15 °F/-28.9 to -26.1 °C)
  • 5b (-15 to -10 °F/-26.1 to -23.3 °C)

What plants to grow in Zone 5?

Gardens in Zone 5 have a variety of environments. No matter where they grow, plants need to be not only cold-hardy, but also adapted to the growing environment they are in, whether they are in coastal waters or woodlands or the wide plains of the Midwest. A longer growing season results in a higher vegetable harvest and an extended bloom period in gardens.

When to Plant: Mid-April

6

Hardiness Zone 6

Hardiness zone 6 is a zone where the average minimum winter temperature is -10°F to -5°F or -20.6°C to -17.8°C. This zone is found in the northern and central parts of the United States, as well as in southern Canada. Plant hardiness zone 6 is characterized by cool, dry winters and hot, humid summers. Gardeners in hardiness zone 6 must be prepared for a wide range of conditions, from drought to deluge.

Temperature in Zone 6

  • 6a (-10 to -5 °F/-23.3 to -20.6 °C)
  • 6b (-5 to 0 °F/-20.6 to -17.8 °C)

What plants to grow in Zone 6?

There are many plants that are ideal for gardening and landscaping in Zone 6. The options available to Zone 6 gardeners are endless thanks to seed companies and nurseries as well as garden centers. Depending on the region, you may be able to find blooms and vegetables in spring, summer, and fall.

When to Plant: Mid-April

7

Hardiness Zone 7

Planting Zone 7 is an area that encompasses roughly 15 states in the United States. As you might expect from the name, this zone has a cool winter climate with temperatures ranging between 0 and 10 degrees Fahrenheit on average. There is a wide selection of plants available to gardeners in this zone, from seed catalogs, local home improvement stores, nurseries and greenhouses.

Temperature in Zone 7

  • 7a (0 to 5 °F/-17.8 to -15 °C)
  • 7b (5 to 10 °F/-15 to -12.2 °C)

What plants to grow in Zone 7?

There are many different types of growing climates within Zone 7, so it encompasses a wide range of landscapes, from the coastal region in the east through the prairies of Oklahoma to the arid regions of the southwest and up into the forests of Oregon and Washington. As a result of the region’s wide variation of climates, other considerations often need to be made in order to cater for drought tolerance, and soil conditions need to be appropriately adjusted to accommodate for drought tolerance. Zone 7 hardy plants are capable of being successfully grown in a number of different locations after some adjustments are made to suit their specific needs in each location.

When to Plant: Mid-April

8

Hardiness Zone 8

Hardiness Zone 8 is a zone that is located in the southern United States. This zone is characterized by a warm climate with an average annual temperature of between 10 to 20 degrees Fahrenheit. The warm climate of this zone makes it ideal for growing a variety of plants and flowers. In summers that are hot and winters that are mild, growers usually have a long planting season.

Temperature in Zone 8

  • 8a (10 to 15 °F/-12.2 to -9.4 °C)
  • 8b (15 to 20 °F/-9.4 to -6.7 °C)

What plants to grow in Zone 8?

This zone is ideal for growing plants that love the mild winters and the long, hot summers of Zone 8. Considering that many types of plants require different levels of moisture and sunlight, growers should narrow down their list of potential plants by zone, and then by climatic zone.

When to Plant: Mid-March

9

Hardiness Zone 9

Zone 9 is the warmest zone on the map, and includes parts of California, Arizona, and Florida. In this zone, average low temperatures range from 20 to 25 degrees Fahrenheit. In Zone 9, you can enjoy a long growing season and a wide variety of plants. Many flowers and vegetables will thrive in this climate, including roses, tomatoes, and squash. With a little care, you can grow just about anything in zone 9.

Temperature in Zone 9

  • 9a (20 to 25 °F/-6.7 to -3.9 °C)
  • 9b (25 to 30 °F/-3.9 to -1.1 °C)

What plants to grow in Zone 9?

In this zone, the summers are long and hot, while the winters are mild, which means that the heat is more of an issue than the cold. Zone 9 is an ideal place for tropical plants with low water requirements. There is a much longer growing season in the area because of the extreme heat, so spring gardening begins much earlier and fall gardening also produces much longer than in other areas.

When to Plant: Mid-February

10

Hardiness Zone 10

Hardiness Zone 10

Hardiness zone 10 is a zone that covers the hottest parts of the country. In general, plants that are hardy to zone 10 can handle temperatures up to 40 degrees Fahrenheit. This means that hardy plants in this zone can withstand heat and drought better than plants in other zones.

Temperature in Zone 10

  • 10a (30 to 35 °F/-1.1 to 1.7 °C)
  • 10b (35 to 40 °F/1.7 to 4.4 °C)

What plants to grow in Zone 10?

Zone 10 is best known for its tropical plants, which are some of the easiest plants to grow there. As there are few frosts in summer, growers have to contend with high temperatures and humidity due to the lack of frosts.

When to Plant: Mid-January

11

Hardiness Zone 11

Hardiness Zone 11 is the warmest of the hardiness zones. It extends along the coast of California and includes parts of Hawaii. This zone is known for its warm, sunny weather and its Mediterannean-like climate. The average minimum temperature in Zone 11 is 40 degrees Fahrenheit. This means that the coldest weather this zone experiences is still above freezing. This makes Zone 11 a great place to grow tropical plants and flowers.

Temperature in Zone 11

  • 11a (40 to 45 °F/4.4 to 7.2 °C)
  • 11b (45 to 50 °F/7.2 to 10 °C)

What plants to grow in Zone 11?

The highlight of Zone 11 is its tropical plants. Zone 11 growers need plants that are heat tolerant since the summers are long and the winters are warm. There is a limited growing season for plants that fall into the cold season, such as pansies and spinach. The best plants to survive the summer heat are those native to the area.

When to Plant: Any time

12

Hardiness Zone 12

Hardiness zone 12 includes parts of Hawaii and Puerto Rico. The average minimum temperature in this zone is 50 degrees Fahrenheit.

Temperatures in Zone 12

  • 12a (50 to 55 °F/10 to 12.8 °C)
  • 12b (55 to 60 °F/12.8 to 15.6 °C)

What plants to grow in Zone 12?

One of the most popular choices for Zone 12 is the hibiscus. This tropical plant is known for its brightly colored flowers, which can add a touch of beauty to any space. Hibiscus plants do best in full sun and well-drained soil, so be sure to keep that in mind when choosing a spot for yours.

When to Plant: Any time

13

Hardiness Zone 13

Hardiness zone 13 is not found in the continental United States, but are located in both Hawaii and Puerto Rico. These two plant hardiness zones are extremely warm, tropical environments that are best suited for plants tolerant of intense heat. With the average minimum winter temperature between 60 and 65 degrees F, Zones 13, the hottest of all the USDA hardiness zones, feature tropical plants and exotic fruits.

Temperatures in Zone 13

  • 13a (60 to 65 °F/15.6 to 18.3 °C)
  • 13b (65 to 70 °F/18.3 to 21.1 °C)

What plants to grow in Zone 13?

Hardiness zone 13 is the highest zone on the hardiness scale and is reserved for only the most heat-tolerant plants. Some of the plants that fall into this category include cactus, agave, and palms. If you’re looking to add some of these heat-loving plants to your garden, be sure to research which ones are best suited for your specific climate.

When to Plant: October to March.

The conditions in your garden can be adjusted for a number of factors, such as the shade, the water drainage system, and the soil quality. There is one thing that you cannot control, and that is the weather. To survive and grow in your area, a plant, vegetable, or tree needs to be able to tolerate the conditions that are present in your region year after year.

This is where the USDA Hardiness Zone Map comes into play. The purpose is to help gardeners of all levels, ranging from professional gardeners to amateurs, to identify which plants in their climate have the greatest chance of thriving so that they do not plant something that will not survive winter or spring frosts in their area.

In addition, the map is a great way to make comparisons between your climate and the ideal climate for the plant that you wish to grow, if you have any particular plant in mind. However, that does not necessarily mean you cannot grow that type of plant in your area. Simply put, it means you have to take extra care to ensure that you address the sensitivities it has. If, for example, a vegetable thrives better in a warm climate, then you should cover that vegetable whenever a frost warning is issued in your area. 

The USDA map can be a useful guideline, but it should not be viewed as a set of strict rules that should be followed. National Gardening Association notes that while this map is a useful resource when it comes to addressing climates of the eastern half of North America, there are a few areas in which it falls short, especially in the west, where there is a greater variety of climates due to the presence of mountain ranges and deserts.

Depending on where you live, there can be different hardiness zones. There can be a heat island effect in a city downtown, which can lead to the zone being warmer than the zone in a suburban area that is less populated. A change in elevation can also have an effect on average temperatures. 

Additionally, you should consider how much sunlight your garden receives during the day, as well as what direction it faces. A big part of your plants’ life cycle is dictated by the way your garden is oriented towards the sun and shaped by its shade.

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