What are the USDA zone designations and what do they mean to you as a gardener?
The USDA Plant Hardiness Zone maps have been developed based on AVERAGE maximum cold temperatures in the winter to help people determine which plants have the best chance of surviving in their area.
But, the Hardiness Zones are pretty generalized and don’t take into consideration micro-climates – the variations caused by such things heat islands produced by buildings, additional cooling from lake/pond evaporation, localized wind tunnels, or windbreaks created by buildings or large, dense vegetation, yet these are all factors that can have a significant impact on your yard and garden.
Used alone, the zones are an excellent starting point for determining what plants will grow in a specific area, but, they do not tell the whole story.
When choosing plants to put into your yard or garden there are several additional factors that need to be taken into consideration.
At the top of my list for other factors to consider is the AVERAGE number of frost-free months (or weeks) in a specific area. This is often listed as the AVERAGE growing season.
The next thing to consider is the AVERAGE length of the growing season for the specific varieties you are considering, but even this is not an absolute answer because there are ways to extend the growing season when cold is a problem.
The third factor to consider is your soil. Is it sandy, silty, clay, loam? What’s the pH? Have you tested it so you know what kind of soil you have? (see https://eclecticmusings.blog/2020/06/18/the-dirt-on-dirt/) Again, this is not an absolute because soil can be amended to make it more compatible with the plants you want to grow.
Now, back to the USDA zones.
You may have noticed that I have emphasized AVERAGE. An average is nothing more than compiled data, including large and small measurements, in this case high and low temperatures that don’t match the average, but still effect it. Where I live, the average low temperature for September is 35o F but last September, my low temperature in the middle of September was -10o F! That’s a HUGE difference, and all of my plants suffered from it since I wasn’t expecting that low temperature and had not sufficiently prepared my plants, plus, the trees and shrubs nearly all still had their leaves, not yet having gone dormant for the winter. What’s really puzzling to me though is that when I looked back through the national weather records, they say the low temperature here was really 35, not the -10 both of my outdoor thermometerS registered. Others in the area also recorded temperatures within 2-3 degrees higher or lower than I recorded, and similar plant damage. Hmmmmm…. If anyone has an explanation for that, please let me know.
There are several ways to find out what zone you live in.
If there is a garden store nearby, they can probably give you accurate information.
You can consult a map such as this one from the USDA
Or even better still, check out your specific state
You can also go to their website and enter your zip code, but remember, this is generalized and it’s AVERAGE information. https://planthardiness.ars.usda.gov/PHZMWeb/Default.aspx
In the rural area where I live, zip codes tend to cover a lot of ground with significant potential for temperature variation within the zip code boundaries. I’ve learned from close recorded observations that temperatures where I live generally run 3-5 degrees colder in winter and 3-5 degrees warmer in summer than they do in town.
In summary, know your zone, know how your average temperatures vary from your zone, know the zone range for the plants you want, know what/when your average growing season is, and know your soil.
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