One thing that gardeners should know when they are planting trees, shrubs, and vegetable gardens is what Plant Hardiness Zone they are located in.
For those in the United States, this would be the USDA Plant Hardiness Zone.
A good map you can use is:
USDA Plant Hardiness Zone Map, 2012. Agricultural Research Service, U.S. Department of Agriculture. Accessed from https://planthardiness.ars.usda.gov/.
Even though this is something gardeners should know, it cannot be the only piece of information to rely on when choosing which plants to grow in your area. There are other things you must know and understand when it comes to determining if your plants will survive your climate.
What does the USDA Plant Hardiness Zone map tell you?
According to https://planthardiness.ars.usda.gov/, “The 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location.”
This is a pretty great map. You simply put in your address and match the color on the map with the chart on the right. You can’t get too much simpler than that, except, maybe if they identified the zone within the search result that pops up so you don’t have to match colors that are so similar to the adjacent ones on the chart. Some people may have a hard time doing this because the colors are so close in appearance to each other.
Each Zone is separated into 10 degree increments and represent the average minimum winter temperature.
This is a good thing to know so you don’t plant trees, shrubs, or vegetable plants that may not survive when it gets too cold for them.
What I dislike about these maps
The description given on their website for a USDA Plant Hardiness Zone is a bit misleading. They say “which plants are most likely to thrive at a location.” The keyword, “Thrive“. The Zone does not tell you which plants will thrive at a location, especially since the Zone designation only considers the average minimum temperature. There is much more than what the Zone tells you that you need to know for a plant to thrive.
The Hardiness Zones only consider the average minimum temperature for that area.
They do not take into account the amount of average rain, average yearly cloud cover or sunny days, soil types, humidity, average high temperature, wind, and so on.
With all these other variables, there is no way that the Zone could tell you which plants will thrive in that area.
The other thing I dislike about this designation is that many people rely on it to determine if a plant will survive and do well in their area. Why wouldn’t they? It is one of the key items labeled on most plant tags.
Parts of Washington and California are in the same Zone 8b
What?? Yup. That surprised me too. But remember…the USDA Plant Hardiness Zone is only for the average minimum temperature for that area.
Both my mom and sister moved to Washington and I was surprised when I found out that they are in the same USDA Hardiness Zone we are in…Zone 8b.
We both occasionally get snow, but our climates are so very different from each other.
Auburn, Washington, where my sister lives, tends to be a more wet and cloudy climate for a good portion of the year, especially during the winter, compared to Apple Valley. The summers tend to have nicer weather for them.
You don’t get this kind of greenery in the desert:
In Auburn, they average about 162.5 days of rain fall and get approximately 17.87 inches of precipitation a year according to https://www.weather-us.com. They may have received much more than that already this year, but it could be an odd year for them.
The deserts here in Southern California are typically sunny and dry with very minimal rain. Fancy that, it’s a desert!
Less wet areas are great for cactus:
Here in Apple Valley, we average about 46.2 days of rain fall and get approximately 4.65 inches of precipitation a year according to https://www.weather-us.com. Even that seems high for us these days as we have hardly received any this year or past years. Maybe two or three days, tops.
The data statistics below came from the two links above.
They get more snow than we do, but we get it typically at least once a year. Their average low is 32.5 degrees F, while ours is 38.3 degrees F. This is why the USDA Plant Hardiness Zone says we are in the same zone. We are both in the 30-39 degree F range; both ends of the range, but still the same zone (Zone 8b, 15 to 20 degrees F).
What about the average high temperature? Well, in Auburn, their warmest month is August with an average high of 76.5 degrees F. Apple Valley’s is July with an average high of 92.8 degrees F. It is not uncommon for us to reach 110 degrees F on any given day during the summer. That is a huge difference in summer heat!
Humidity is another big difference between these two cities. Auburn’s relative humidity averages between 69-86% and Apple Valley averages between 26-51%.
We can grow cactus here in Apple Valley. Most of the time, the cactus would not be on drip irrigation because if they get too much water, they could puff up and split or rot because they are getting too much water. If they were in Auburn, they would just rot from the excessive amount of water they would receive just from the rain.
There are certain plants that just do not do well will higher amounts of water. Likewise, others require more water to survive.
My sister has a very difficult time growing watermelons, but they do great here in Apple Valley. I am assuming this has to do with the amount of full sun we get and possibly the higher summer temperatures. She can also grow lettuce and tomatoes at the same time, something we typically can’t do here because it gets too hot for the lettuce during the time tomatoes would do good.
If not the USDA Plant Hardiness Zone, what should you be looking at?
We are not going to discuss climate change here.
The point I want to make with this article is that you cannot rely strictly on the Plant Hardiness Zone designation to figure out if a plant will do great in your area.
You need to look at other aspects in addition to the Zone, such as:
- How much water does the plant require? This is sometimes included on the plant tag.
- Does it require a lot of water to survive? If so, then it may not be water-wise in the desert.
- Will the plant die from too much water?
- Does it handle high summer heat? Usually not on the plant tag.
- Full sun, partial shade, full shade? This is sometimes included on the plant tag.
- What is the length of your growing season and how does it affect the plant?
- What is your average humidity?
- Are you in a micro climate?
Plant Hardiness Zone Maps are just one piece of the puzzle in determining which plants are suitable for your area. Do not only rely on the Zone designation on its own. Look at all the other aspects listed above and then make the determination for that plant.
Of course, you could always change the climate by planting indoors and provide heating, cooling, water misters, shade, heat lamps, heat pads, humidifiers, and so on, to achieve the desired artificial climate, but that is not the point of this article.
Besides the USDA Plant Hardiness Zone maps, which only tell you how plants will survive in the cold, there are also Heat Zone maps and Sunset Climate Zone maps. The difficult part with these other maps is that most plant companies do not yet list them on their plant tags, giving the buyer nothing to go by.
- Cold Winter Temperatures – Use USDA Plant Hardiness Zone Map
- Heat tolerance – Use Plant Heat Zone Map
- Precipitation, wind, humidity, heat and cold tolerance, and other climate factors – Use Sunset Climate Zone Map
Some commercial growers are now including information on their plant tags to give you an idea on their heat tolerance zone. Maybe one day, everyone will get on board with the Sunset Climate Zone map as well.
I hope this has shed a little light on what the USDA Plant Hardiness Zone is and what it is used for. Don’t go out there and assume you can buy a plant that says it is in your Zone and expect it to thrive in your area.
Rick Simper has various hobbies, including woodworking, metalworking, composting, and gardening. He is an Architect full time and uses these hobbies to get away from the busy week of Architecture.