You’ve probably seen it on a seed packet or a plant tag, “Hardy to Zone 5b” or “Perennial above Zone 7a” .
What are these zones and why are they important?
The USDA established Hardiness Zones, based on the lowest average temperature of an area, as a sort of shortcut for people to know what plants would be suitable for their climate. The Zone map is updated periodically to adjust for changes in climate; the latest update was in 2012. Each zone designation represents a ten degree temperature difference, the lower the number of the zone, the colder it is. The zones are further divided by A and B which represents a five degree difference. Zone 6a will be about five degrees colder than Zone 6b on average.
Knowing what zone you are in helps you make good decisions about what to grow. A gardener in zone 5a would have a hard time growing banana plants or citrus trees without some kind of modifications.
The zones are linked to zip codes. To find your zone, simply go to Garden.org and enter your zip code.
You’ll probably notice, each hardiness zone covers a very broad areas, so within each zone there can be a large amount of variation. According to my zip code, I live in Zone 8b, but I know the post office for my town (the basis of the zip code therefore the zone designation) is 3,491feet above sea level, while my house is only 1270. Thanks to that difference in elevation, the temperature at my house is almost always at least ten degrees warmer than it is in town. That means I’m actually in Zone 9b. The elevation of the rest of the town continues to climb on the other side of the post office, so those folks are more likely to be in Zone 8a or even 7b.
Your official hardiness zone is just a starting place, you’ll need to make adjustments based on the topography of your location. Many places have microclimates, areas that are significantly different from the rest of the property. For instance, we have a slope that is warmer at the top and colder at the bottom where the cold air settles. I can expect things planted at the top of the slope to grow and ripen faster than things at the bottom. I plant my cooler temperature plants at the foot of the slope and my heat-loving ones at the top. Structures can also create microclimates, the north side of the house or a wall will be cooler and more shaded than the south side.
You can modify your zone with season extenders like cloches, hoop houses, high tunnels, cold frames or greenhouses.
More Dates and Numbers to Know
While we’re on the subject of climate, there are a few more numbers other than the hardiness zone, you need to know about yours:
- Average First Frost Date
- Average Last Frost Date
- Average Growing Season (number of days)
- Average Annual Precipitation (how much rain / snow does your area get)
- Average High Summer Temperature
- Average High Winter Temperature
You can find most of these dates online by searching “average temperatures (your location)”. You can find your growing season at garden.org. When you put in your zip code to get your Hardiness Zone, it will also tell you the length of your growing season. Mine is 255 days.
You’ll also want to know some dates that are specific to your homestead, you can find these by keeping records of your daily high / low temperature:
- The First Below Freezing Date
- Your Cool Season (32-50 degrees F)
- When does it start (first 50 degree day)
- How long does it last (count days until your first 32 degree day)
- Your Ideal Growing Season (50 – 85 degrees most vegetables stop growing when the days get above 85 degrees F. Get the most out of your ideal growing season)
- Your Hot Season (above 85 degrees)
- First day
- Last day
I can’t stress enough how important it is to have a system (and use it!) for recording important data about your homestead from season to season and year to year. I don’t know how many times I’ve thought “I’ll remember this (date, event, etc.), I don’t need to write it down” and then promptly forgotten it. Don’t be me! Keep a journal, make a spread sheet, write it on a the calendar (then keep the calendar), take a picture with your phone then overlay some text on it and save it to an album. Use whatever system you like, just use it.
One More Thing to Measure and Record
The temperature of your soil is just as important as the air temperature. Seeds germinate at specific soil temperatures and plants grow (or not) based on the temperature of the soil. You’ll need a soil thermometer for this
Make chart and record the soil temperature as the air temperature starts to change in your area. For instance, take the soil temperature in January and record it. When your days start to warm up, take it again. When you hit your Ideal Growing Season, record the soil temperature and again when you hit your Hot Season. If you count the number of days in between you’ll have the length of your Ideal Growing Season, you can find ways to cool the soil and extend the season.
When the days start to cool off again, take the soil temperature again and continue recording it so you know when your soil drops below the germination range. At this point, you know that any seeds you sow will sit dormant until the soil warms up again.
This may sound like a lot to know, but once you have your data recorded, you’ll have a tool that will help you know what to plant and when. It’s also a good base reading to track how your climate changes from year to year.