New Plant Hardiness Zone Map doesn’t reflect what happens in Texas most winters

Most folks are probably done with talking about the cold, and believe me, I get that. I am, too. I got it last year. And I absolutely, positively, and certainly got it back in 2021 when that monster cold spell leveled our landscapes. (Just above the snow line.)

So, what could there possibly be left to discuss? Actually, it’s something big. I think it’s something far more important. It has to do with plant-buying decisions you will make time and again over the next 15 or 20 years and how a new government publication might set you up to fail. I’m sure they didn’t intend to do so, but I fear it could work out that way for some of us.

The hardiness zone maps

As long as there have been gardeners, people have noticed that some plants can survive extremely cold temperatures while others cannot. Tomatoes die at 32. Apple trees thrive at well below zero.

In a quest to rank plants and predict how likely they were to survive winters in various parts of the country, renowned plant taxonomist Alfred Rehder of the Arnold Arboretum in Boston compiled the first list with eight hardiness zones and assigned the plants that went in them in 1927.

Donald Wyman of the Arnold Arboretum published a new map using weather data of a 40-year period (1895–1935), and the Arboretum subsequently updated it in 1951, 1967, and 1971.

In 1960 the United States Department of Agriculture published its first map based on data from 450 weather stations, but its standards were different from those of the Arnold Arboretum experts, and the latter group’s maps remained the preferred resource for many years.

In 1990, however, the USDA combined its efforts with the U.S. National Arboretum in Washington, D.C. Using data from 14,500 weather stations, their numbers became more and more accurate.

More recently the USDA maps have been reissued in 2012 and 2023. Because they are now using data from the past 30 years instead of going back farther in time, and because those years saw a string of warm winters, the maps have shifted our Texas hardiness zones north by several counties.

Therein lies my concern for us as consumers. It was reflected in the 2012 map, and I have commented on it frequently — that I feel that the 1990 map is the more accurate representation of what really happens in Texas most winters.

We’ve had several cold winters in more recent years, and one of them is quite fresh in your memory (the past week in North Texas).

In November 2023 a new map was released. With some fanfare it encouraged us to enter our ZIP codes to see what lowest temperatures we might expect to encounter each winter. You can do so yourself by going to the website https://planthardiness.ars.usda.gov.

I invite you to enter your own ZIP code. See what it says you might expect to see as your lowest temperature, then think back to see what you experienced within the past seven days.

When I tried 76102 for Fort Worth I was told it would get as cold as 15 to 20 degrees. It notes that that’s a 5-degree increase since 2012.

I checked several ZIP codes of suburban cities in and around Tarrant County and got similar results.

Most of those ZIP codes dropped near or below zero in February 2021, and both were in or near single digits earlier this week.

Point being, when these two newer maps show North Central Texas to be in Hardiness Zone 8b, that just isn’t the case. When you’re buying shade trees and shrubs, they represent long-term investments of substantial amounts of money, and you don’t want to make that fundamental mistake. It is my opinion, based on 55 years of living in this area, that we continue to be in Hardiness Zone 7a, or at the warmest, 7b. If you really want to protect yourself, you’ll make the majority of your shrub and especially tree selections from one zone warmer. You’ll choose woody plants listed as hardy for zones 6a and 6b.

But it doesn’t stop there…

There are a couple of other important factors that need to be mentioned in all of this.

Hardening: Plants that have not been acclimated to light freezes are likely to be hurt much worse by extreme cold. That was manifested in February 2021. Trees and shrubs were geared up and starting to grow.

Flowering plants were budding to bloom. Spring was waiting on the doorstep when that cold smashed in. It could not have come at a worse time, and its damage was severe.

Conversely, when the first freeze in the fall comes early and catches plants still growing after warm, balmy weather, it’s likely to hurt them even though it may be 20 degrees warmer than the average low temperature for the area.

Duration: How long it stays cold is almost as important as the actual minimum temperature. Several days at 22 may do more damage to many plants than just two hours at 15. That was proven back in the winter of 1983-84 when we had almost 300 consecutive hours below freezing. It never got crazy cold, but still there was significant loss from the sustained sub-freezing weather.

Please don’t get me wrong. I use the Plant Hardiness Zone Map and zone ratings all the time. But now I feel like I’m converting Fahrenheit to Celsius and dollars to pounds every time that I do. Where I used to tell people to choose plants for Zone 7, they come back to me reporting that their maps show us to be in Zone 8.

We’ve had enough times of temperatures into single digits since the 2012 map was introduced that I would have expected the 2023 map to have moved the other direction. But, at least for North Central Texas, it appears to this horticulturist that the error has only gotten worse. I guess we’ll all have to wait another 10 or 20 years.

Helpful links

Hang onto these links. They might prove of great value.

The 1990 Plant Hardiness Zone Map: https://planthardiness.ars.usda.gov/system/files/US_Map_1990.jpg

The 2012 Plant Hardiness Zone map: https://planthardiness.ars.usda.gov/system/files/US_Map_2012.jpg

The 2023 Plant Hardiness Zone map: https://planthardiness.ars.usda.gov