In November 2023, the USDA released updated plant hardiness zones. The new map shows that many areas have become warmer in the past decade, with some areas moving up one or two zones.

Comparison between 2012 and 2023 maps, focusin on Illinois.
The hardiness map for Illinois. Left side: 2012. Right side: 2023.

When starting a garden, understanding the growing climate in your area is an important first step. There are two key pieces of information to know about your local growing climate: your hardiness zone and frost dates. While sometimes represented together, hardiness zones and frost dates are separate concepts. Read on to learn what they are and why they matter for a successful garden.

What are Plant Hardiness Zones?

USDA plant hardiness zone map
View the interactive map to determine your plant hardiness zone. Image Source.

In North America, plant hardiness zones commonly refer to the US Department of Agriculture (USDA) Plant Hardiness Zones. Canada has developed its own hardiness zone system, but for practical purposes the USDA zones are commonly used. There are many plant hardiness zone systems in use around the world.

The USDA Plant Hardiness Zone Map divides the USA and North America into zones based on average annual minimum temperatures. The lower the zone number, the lower the average annual minimum temperature in that area. There is a 10°F difference in the average annual minimum temperature between each zone. Zones are further subdivided into 5°F segments denoted by “a” or “b”.

Hardiness zones are a guideline for determining which perennial plants can survive winter in your area. When buying perennials, the tag typically indicates which zone(s) that plant is hardy in.

USDA hardiness zones don’t consider the myriad of other factors that affect how a plant will perform in a particular area. They also don’t account for “microclimates”- local variations on growing conditions within a zone. Hardiness zones are a starting point, but experience will hone your instincts for which plants will flourish in your garden.

Frost Dates: Why Do They Matter?

If you garden in a temperate climate, it is important to understand your first and last frost dates. The last frost date is the last day in spring when a frost occurs (i.e., when air temperatures reach 32°F (0°C) or lower). The first frost date is the first day in fall when a frost occurs. Frost dates can be estimated by using average historical frost dates from local weather stations.

Knowing your average last frost date helps you decide when to start planting your frost-sensitive crops outdoors. Deciding when to plant also depends on how willing you are to risk a late frost! Again, with experience you’ll be able to dial in the best time to plant in your area.

The number of days between your last and first frost date is the length of your growing season. Depending on the length of your growing season, plants that take longer to mature may need a head start indoors.

Planter does the work of calculating this for you! First, set-up your location in the app settings. For each plant, Planter provides a Growing Calendar that is customized to your location. The Growing Calendar shows which plants can be started outdoors, which should be started indoors and transplanted, and when:

Planter Growing Calendar screenshot