Understanding USDA Hardiness Zones
Why they matter, why they don't.
If you’ve read through my guide on when to start winter sowing, you know that I’m a broken record when it comes to winter sowing within certain windows of time. While knowing your zone can be a helpful starting point, understanding the typical winter weather patterns and characteristics specific to where you live and your last frost date is even more valuable.
What are USDA hardiness zones and what do they tell you?
The U.S. Department of Agriculture takes data from roughly a 30-year period to determine average extreme minimum temperatures across the US and then categorizes this data into 13 zones based on 10 degree differences.
One very specific piece of information this data provides is which plants are hardy to certain zones. In other words, knowing your zone helps you know what plants are perennial where you live, and are therefore most likely to thrive and overwinter.
What this data doesn’t tell you are things like:
How many days you experience that minimum low temperature
What climate or region you are a part of
If you happen to be in a microclimate within your zone
Your first and last frost date
How warm your winters are
How hot your summers are
So what do zones have to do with winter sowing?
Well, honestly, not very much. That being said, the metric of USDA hardiness zones is a helpful starting point, especially for those who are new to gardening and don’t yet have the experience of making connections between how the weather affects what you are growing.
Generalizations
With USDA hardiness zones, we are able to make some helpful generalizations. These generalizations being:
Toward the lower end of the hardiness zones (toward zone 1), winters are colder
Toward the higher end of the hardiness zones (toward zone 13), winters are warmer
These generalizations are why you might hear people refer to “cold zones” and “warm zones”.
Is this a perfectly accurate picture or metric? No—these are generalizations. There will be a lot of factors that this metric does not take into account that can cause divergence within a particular zone—making these generalizations appear irrelevant and even unhelpful. That being said, from these generalizations, we are able to group zones together to determine specific windows for winter sowing that—generally do—work.
While I do point to more generalized guidance, I always want to point you back to the principles behind winter sowing that do more accurately account for some of the nuances within zones.
Winter sowing made simple:
There are two windows for winter sowing:
When the weather in your area becomes consistently cold
The weeks leading up to your last frost date
When the weather in your area becomes consistently cold
This window is for anything requiring cold stratification (like native plants and perennials) followed by cool season vegetables, herbs, and hardy annuals.
This window extends to the start of the second window, and many seeds from this window can still continue to be winter sown at the start of the second window.
The weeks leading up to your last frost date
This window is for your tender annuals; anything that is frost sensitive—including seeds for your warm season vegetable and herb garden.
A third window?
OK… there is a third window. Winter sowing’s best kept secret for those of us in the “warm” zones (those of us with mild to warm winters).
In the fall when temperatures start to cool
This window is more technically a window for “jug” sowing — as it is not quite winter, but we’re still using milk jugs. This window for sowing is meant for the winter garden, including cool season crops and hardy annuals that will get a head start on growth and overwinter for a late winter or spring flowering.
So there you have it. Are zones relevant to winter sowing? Kind of—not really.
Were you helped by this? What questions do you still have about zones or when to start winter sowing? I’d love to hear!