USDA Hardiness Zones, First & Last Frost Dates, and Your Growing Season
If you’ve ever started the process of planning a garden, chances are you’ve hopped on to Pinterest or Google for a “garden plan” and found something titled “Zone 7 Gardening Plan” or “Zone 8 Planting Schedule.” You then do a little Internet search to find what zone you live in, find the appropriate Pinterest Planning sheet, and assume that it won’t be far off of what you need.
Sorry, it’s probably wrong.
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Okay, maybe that’s a bit harsh. If you live east of the Rocky Mountains, it’s probably pretty close to right. West of the Rockies? Probably wrong.
Let me explain:
First, you need to know what “Zones” refer to.
The USDA has designated different zones for hardiness throughout the United States. Zones are determined by the average annual extreme minimum temperature, which just means how cold your area typically gets.
There are 13 zones, and each zone has a range of 10 degrees Fahrenheit. Zone 1 has an average low temperature of between -50 and -60 degrees, while Zone 13 has an average low of 60 to 70 degrees.
I personally live in a zone 6, which means we usually see a low temperature of between 0 and -10 over the course of a winter.
What does this mean - and what does it NOT mean - for your garden?
Which USDA Hardiness Zone you live in will tell you which plants will survive (or NOT survive) over winter. This means that knowing your zone will help you decide which perennial plants will work (the ones that come back every year) and which annuals you can plant in the fall.
Most vegetable varieties and many cut flower varieties fall under the “annuals” category, and if you’re just starting a garden you likely will not be starting seeds in the fall, and will plant in the spring instead.
This means that USDA zones will have NO IMPACT on your first garden plan or planting schedule. If what you are growing does not need to survive the winter, what zone you grow in DOES NOT MATTER!
Then what do you need to know to plan a garden?
The most important information to have when planning a garden is your first and last frost dates. They’re exactly what they sound like: the last day you can expect it to freeze in the winter (the start of your season) and the first day it freezes again in the fall (the end of your season).
The last frost date is when you can safely plant most seeds in the ground, or transplant plant starts, since they have little to no risk of freezing and dying off. Some plants can withstand a light frost (28*), but many common beginner garden annuals like tomatoes, peppers, and zinnias need to be planted after any danger of frost.
The first frost date in the fall marks the end of your growing season. When it freezes, those annuals that cannot survive a frost will die off. The length of time between your last frost date of the spring and the first frost date of the fall (measured in days) is the the length of your growing season. Knowing this number will tell you which plants have enough time to finish growing and produce fruit or flowers. Those in a short growing season will not be able to grow certain varieties in enough time by planting seeds in the ground, and will need to either choose a different variety or start seeds indoors.
Here’s how this plays out:
My growing season is approximately May 31 (last frost) to September 15 (first frost), giving me a growing season of just over 100 days. I can plant zinnias directly in the ground, but since they take 70 days to mature, I will only be able to pick flowers for 30 days before they die. Some tomato varieties mature in 60 days, which could be planted in my zone, but some take 100 days, which means they would need to be started indoors to be able to get any fruit in my area.
On the other hand, my mom has a growing season of May 1 (last frost) to October 1 (first frost), which is a growing season of 155 days. She can plant almost any standard variety of garden crop at the beginning of the season and have fruit before it freezes.
The surprising factor here?
We live only 3 hours apart and grow in the same USDA zone.
I live in an area with higher elevation, a drier climate, and a wider daily temperature swing, all which have a much greater impact on my growing conditions than the USDA zone does. I plant different varieties and at different times than my mom despite being in the same zone.
Why are there so many garden plans based on zones then?
Frost dates and zones match up really well where the other geographic factors also line up well. East of the Rocky Mountains, areas in the same zones share a lot of these environmental factors, making the frost dates similar as well. If you look at the USDA Hardiness Zone map, the zones fall in nice linear zones across the country. There’s not a lot of variation across these areas.
West of the Rockies is.. well, the Wild Wild West. The zones are actually difficult to find in some areas, and can change several times within a smaller radius. Microclimates are everywhere, elevation changes and weather patterns are way less consistent, and your “zone” can look wildly different from someone else’s, even if they share the same number.
ALL OF THIS TO SAY:
Zones are important if you are planting perennials and overwintering annuals, and can be useful for comparison if you live in the Eastern U.S.
They are NOT important if you are planting spring annuals and live in the Western U.S. You are served much better if you take a look at your climate, and find your first and last frost dates to determine when and what to plant!
You can easily find your frost dates by typing in your zip code at (DAVES GARDEN LINK).
