In early spring, many gardeners are thinking about what they’ll grow in their vegetable and flower gardens. According to the USDA, 80 million American gardeners and growers use its Plant Hardiness Zone Map as a guide for planning and planting.
Last November it was updated from the 2012 version using data collected at weather stations from 1991 to 2020. About half the country moved into the next warmest half-zone classification.
To learn more about it, The Allegheny Front’s Kara Holsopple spoke with Glen Bupp, a commercial horticulture educator for Penn State Extension. Bupp works with vegetable and small fruit growers, as well as landscapers.
LISTEN to the conversation
[This interview has been edited for length and clarity.]
Kara Holsopple: How does the USDA Plant Hardiness Map work? How are the zones determined?
Glen Bupp: They are based on the average low temperatures [over 30 years] in the winter, and they’re in ten-degree increments. So the difference between 6 and 7 would be ten degrees. But within those zones, there’s a and b which is separated by five degrees.
And they are helpful, especially for gardeners and people in horticulture, to determine the best plant fits for plants that are going to be acclimated to different temperature zones. But the map also helps determine when plants are going to bud out and growing degree days for pests.
Many insects emerge based on the amount of temperature that has accumulated through the season. So obviously, in areas where it’s colder, it takes longer for accumulated temperatures to accumulate, so insects in those areas may emerge later, whereas in areas where it’s warmer, they’re going to come out earlier.
Some insects also have different amounts of generations per year based on how long those warm temperatures last. So obviously, that has implications for areas when, if they become, on average, warmer, you may see changes in these insect populations.
Kara Holsopple: How do you use the USDA Plant Hardiness Zone map?
Glen Bupp: It’s a very good tool, especially when talking with folks about plant selection and when they have questions about whether or not a plant is going to be well adapted to a specific area. That’s one good way to use it.
Another would be for planting dates. It can be very informative for when we expect our last and first frost to be, which is useful for knowing if there’s enough time for a plant to be harvested, or if you’re going to put a plant in the ground, what kind of protection it may need if you’re kind of hedging your bets prior to a frost.
Kara Holsopple: An updated map was released late last year. What are some of the changes in Pennsylvania?
Glen Bupp: I deal mostly with western Pennsylvania, and so when you type in zip codes for western Pennsylvania, it shows you what your USDA growing zone is. It also will show you what the changes are from that 2012 map.
On average, it’s like a 3 to 4-degree change across different zip codes in western Pennsylvania. So that might take you from a 5a to a 5b. It might even leave you in 5a. That doesn’t mean that in your zone you still won’t get that possibly record-breaking cold. It just means that those averages have adjusted three degrees warmer.
That change, to me, doesn’t mean that we’re generally warmer. But what that does mean is that we have more wild variations, especially in our transitional season time. It doesn’t mean that we can start planting palm trees, right? But it does mean that we need to be careful with these temperature swings – kind of what it feels like we’re experiencing now where it’s really warm and then really cold. It’s kind of indicative that you’re going to get more of that.
Kara Holsopple: That’s what happened last year when we had that crazy cold snap at Christmas time and a bunch of my plants that had overwintered for a lot of years died.
Glen Bupp: You see a lot of people whose plants start to break dormancy early, and then a cold front or a cold snap can come in and ruin that new growth because new growth is sensitive to the cold.
This can happen with apple growers, for instance. You get these flower buds that come out and then the cold whacks the flowers off. And that’s when you get a bad apple crop, and can be determined this early in the season.
But the same thing is true with ornamental shrubs that might break dormancy early, and we get a cold snap that comes in, or a late frost, and all that foliage just kind of almost burns off. It turns brown and drops.
Kara Holsopple: How can home gardeners, or people who have farms navigate changes in either their plant hardiness zone or increased temperatures?
Glen Bupp: You can be thoughtful about what you plant for one. The best advice is to plant things that are well-adapted to our area. I always like to promote native plants, for one. But just because it’s native doesn’t mean it’s necessarily the right native. You want to select plants that have wide ranges in their USDA growing zone adaptability.
So, for instance, if you’re in a USDA growing Zone 6, choosing something with a range between 3 and 9 will probably be a pretty safe bet. Whereas if you’re on the edge of that, you know, selecting a plant that maybe is a 3 to 6a, you might have problems with that. And you can think of it from the other perspective as well.
So not pushing those zones, but also selecting plants that are kind of generalist in their range. And diversity is good, too. Diversity, to me, is the same as resilience. So if you can put more diversity in your landscape, you’re going to be resilient to variable changes in the weather.
But if you do have plant specimens that you’re protective of and they break that dormancy early, you might consider covering them with a cloth-type material. Avoid plastic, because plastic will not trap in that heat as well and can lead to more issues. And bringing your plants in too. If they’re containerized plants, don’t be afraid to just put them in the garage for a short period to get them through some of the variable temperatures.
Kara Holsopple: My home garden is in zone 6b, which is the same as the 2012 map. And so the average lowest annual winter temperatures are between -5 and 0°F. I have never been able to overwinter lavender in my front yard. I’m trying it in different places, but I’m just going to try it in a pot, I think.
Glen Bupp: That’s I guess a nod to picking regionally adapted types of plants. So obviously lavender likes more arid type conditions. Along with these changes in temperatures, we can probably also expect changes in different climate patterns to bring either more or less wetness. In some cases, there is more or less cloud cover.
You see issues with Colorado blue spruce all over the place. Even though that has probably a growing zone that we fit within, it’s well adapted to drier conditions. The East Coast is quite wet. So there are quite a few fungal problems that come in with the humid weather.
That’s why it’s not just North American natives, but picking something that’s even more regionally adapted on top of the wide growing zone adaptations is going to provide at least an easier gardening experience.
Kara Holsopple: What are some of the common questions you get from gardeners or farmers about planting zones?
Glen Bupp: What zone are we in, for one. Knowing what the USDA growing zone tool is and what it actually means. For instance, I had a question today from a farmer asking me if they should uncover their strawberries because the strawberries are starting to emerge under the mulch that they’ve put them under. But we’re going to get a lot of cold coming in next week. So you get a lot more types of questions like that that hit on the reason why the changes to the map have been made.
