In December 2023, the USDA updated their Plant Hardiness Zone Map based on data from the past 30 years and many of us are now in new zones. I have gone from Zone 6a to 6b. So, what does that mean?
According to the USDA, the Plant Hardiness Zone Map is based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones ranging from zone 1 (coldest) to zone 13 (warmest). Each zone is divided into half zones designated as ‘a’ or ‘b’. For example, 6a and 6b are 5-degree F increments representing the colder and warmer halves of zone 6, respectively. These designations serve as convenient labels and shorthand for communicating and comparing the extreme winter temperatures within the United States and Puerto Rico. Zone numbers are typically listed with the descriptions of perennial plants in catalogs and other sales information produced by commercial nurseries and plant suppliers.
The sequence of colors assigned to the zones mimics the chromatic spectrum produced by a prism (i.e., red, orange, yellow, green, blue, indigo, and violet), providing a graphical representation of the plant hardiness zones. The overlay of colored zones on the map provides a convenient tool for understanding and comparing plant cold-hardiness across the United States and can facilitate the selection of appropriate perennial plants based on their observed performance in other regions of the country.
Notice I only said perennial plants. The Plant Hardiness Zone Maps serve as general guides for growing perennials. They are based on the average lowest temperatures, not the lowest temperature ever. Zones in this updated edition of the map are based on 1991-2020 weather data. This does not represent the coldest it has ever been or ever will be in an area, but it is the average lowest winter temperatures for a given location for this particular 30-year span.
The previous map was updated in 2012 and covered the 30-year period 1976-2005. The previous update occurred in 1990 and the first version was designed in 1960. To give you an example of how things have changed, in that first version, my area of Missouri was listed as zone 6a. In 1990, we were re-designated as zone 5b. In the 2012 version, we were pushed back into zone 6a. And, with this new map, we have now been re-designated as zone 6b. Which means the average low temperature in winter in my area over the past 30 years is 5 to 10 degrees F warmer than it was before.
Now, this change may not seem significant since I’m still considered zone 6, just the warmer edge of zone 6. But I’ve already heard from some of you that you’ve moved from the warm edge of one zone into the next zone up. Does this mean you should immediately start putting perennials in the ground that are for that warmer zone?
If you’re growing perennial fruits or vegetables or decorative plants that are riding the line of the coldest temperatures in your zone and an extreme cold snap hits, colder than usual for your area, and it lasts for even just a full day or two, perennials that have happily grown in your garden for years could suddenly be lost if they’re susceptible to those colder temperatures. We as gardeners need understand that these past weather records cannot provide a guaranteed forecast for future variations in our weather. So, if I’m in zone 6b and I plant a fruit tree that says it’s hardy to zone 6 and we have an extraordinarily cold winter where temps are regularly dipping lower than usual, that tree may not make it especially if it’s not well established. If I wanted to hedge my bets, I’d plant a variety that’s hardy down to zone 5 to give myself some wiggle room. Mother Nature is unpredictable. So if you suddenly find yourself in a warmer zone than you were before, err on the side of caution when it comes to planting perennials.
We also have to remember that there are so many other factors that go into the success or failure of perennial plants. The frequency and speed of wind, our soil type, the level of soil moisture, humidity, and pollution are all factors. Just because something says it’s hardy in your zone does not automatically mean it’s a great choice. You may need to do a little more research.
- Light: Plants need to be planted where they’ll get the proper amount of light during each part of the season. For example, plants that require partial shade and you’ve got it planted under a tree it may do fabulously well for most of the year. But if you live in an area that gets lots of sunshine in the winter and suddenly the tree that plant has been relying on for shade has dropped its leaves, well, that plant may now be getting too much sun and very inconvenient time of the year. This needs to be factored in.
- Soil moisture: Plants have different requirements for soil moisture and sometimes this requirements change with the season. If the soil is too moist or too dry in the fall as they enter dormancy, some plants can suffer from moisture stress and be damaged.
- Air Temperature: Plants grow best within a range of optimal temperatures, high to low. That range might be pretty wide for some varieties and some species but really narrow for others. Does that plant’s temperature range match the range for your area? Zone 8 in North Carolina can be very different from Zone 8 in Texas the same way that Zone 5 in Wisconsin can be different from Zone 5 in Colorado.
- Duration of exposure to cold: Many plants that can survive a short period of exposure to cold may not tolerate longer periods of cold weather. Just like some of our annuals can that can handle freezing temperatures for two hours but not twelve, many perennials have the same tolerance issues.
- Humidity: High relative humidity limits cold damage by reducing moisture loss from the plant. If you’re area is usually pretty humid, your plants may be able to withstand colder temperatures whereas cold injury can be more severe if you’re in an area where the humidity is usually low.
So, what does this mean for the annual plants in our gardens and landscape? Does this seemingly warmer winter weather have any affect on those? Likely not. Frost-sensitive plants are simply that, frost sensitive. The date that frost appears may or may not be any later than you’ve been experiencing. Any of you who’ve been gardening for more than a few years and especially those that keep a garden journal know whether or not your area has been getting warmer or not. I know that, on average, our frost dates have been within about a week or two of usual but the temperatures immediately following that first frost have been getting warmer and staying warmer for longer into the late fall. That has changed how and when I have my frost protection on hand but it’s not changing what I plant or when I plan for those crops to be mature.
Your friend in the garden,
References and Resources:
2023 USDA Plant Hardiness Zone Map | USDA Plant Hardiness Zone Map