Warming temperatures cause changes to north Alabama's plant hardiness zone
Jul. 5—Nothing makes makes many want to dig in the dirt like the rows on rows of colorful potted plants they see in a nursery or home improvement store each year.
But before they dig, they need to know that what used to thrive in north Alabama may not thrive here anymore and what didn't thrive before may now be right at home. Plant hardiness zones in north Alabama have recently changed from 7A to 8A.
The United States Department of Agriculture recently updated its plant hardiness zone map, which tells gardeners what plants will thrive in their particular region. The zone is usually listed on each plant you buy.
Hardiness zones have shifted northward since 2012, according to the U.S. Department of Agriculture. Two factors have caused that: changing weather patterns and additional monitoring stations, said Allyson Shabel, urban regional agent for Morgan County Extension Service. They update the map every 10 years by monitoring the weather at various sites throughout the U.S. to determine the average winter minimum temperatures in each area, Shabel said.
The 2023 map is based on 30-year averages of the lowest annual winter temperatures at specific locations, according to the USDA website.
"If you are a gardener, it is the lows that are more important than the highs just because there is more impact on a plant if it dips to a very low temperature," Shabel said. "The lows have more of a chance of killing a plant than the highs do."
Morgan County farmer Brent Shaw of Shaw Farms south of Calhoun Community College said he believes the plant hardiness zone change is simply reflecting what he has already been seeing.
He sees a pattern in north Alabama's growing season that he didn't see when he started decades ago.
"This is my 44th crop, and we used to drag and wait until mid-April to plant cotton and soybeans and even corn, and now we are at least two weeks earlier than we used to be." He first started noticing the shift about 12 to 15 years ago, he said.
"We're warmer early and our frosts are later," Shaw said. "Our window of planting is wider than it has ever been in the past. It's reality."
Shaw agrees with the change in the hardiness zones.
"We just generally don't see a frost anymore until November or mid-November," he said. "Used to there were years we planted cotton and you may get a frost on it as it came up in April to mid-April and then have frost on it again before we defoliated it in October. You just don't see that type window that we used to have here."
Part of the reason the plant hardiness zone map changed is because there is more data to reflect what changes have already occurred and are occurring, Shabel said.
"There are more stations, so it's kind of more fine-tuned than it used to be with hardiness zone maps," Steedley said.
Temperatures are changing but also the last map they used had many fewer monitoring sites, Shabel said.
The 2023 map incorporates data from 13,412 weather stations compared to the 7,983 that were used for the 2012 map, the USDA said. It also has much more detailed resolution than previous maps.
"You just want to watch out and make sure you are getting something for your climate," Kerry Steedley, regional Extension agent, advised local gardeners.
This additional data had as much to do with the changing zones as anything else, Shabel said.
"I would say it was just more data giving you a more accurate read," she said. "The temperatures have changed, any gardener can tell you that, but people are also paying more attention to it."
The new plant hardiness zone map is generally about one quarter-zone warmer throughout the United States than reported in the 2012 map, said Lee Ellenburg, research engineer and associate state climatologist for the Earth System Science Center at the University of Alabama in Huntsville.
She said plant zone changes are not a reliable method to assess global warming because the plant maps represent a 30-year average of what are essentially extreme weather events (the coldest temperature of the year) and climate change is usually based on trends in overall annual temperatures recorded over 50 to 100 years.
Local weather patterns are different, Shabel said, with more wet periods in winter than we had in the past.
"In recent years, we have been getting more frequent large precipitation events in winter," she said. "Ideally, we would have frequent half-inch rain events spread over each month, but in the past few years, we have been seeing more 1-inch to 2-inch rain events. So, the rain is falling in larger volumes instead of evenly in smaller increments. This leads to more flooding and over-saturation of soils."
She does not mean there is more annual precipitation, only that it is falling in different patterns than in previous years.
"So that kind of weather pattern is impactful on plants and agriculture, too, especially if you are trying to plant things," she said.
For example, it's tough getting a plow into a field that just had 2 inches of rain dumped on it.
The change in the plant hardiness zone opens up new options for area gardeners.
"Because we are now 8A, it does open up a whole new window of plants for the gardeners here to plant," Shabel said. "The minimum winter temperature for 7A is 10 degrees lower than that of 8A, so now our winter temps are higher. With a warmer winter, we can possibly plant some more tropical plants that may not have survived in previous years," she said.
For example, before the zone change, the plant known as elephant ears were marginal for north Alabama — only the hardiest would survive, she said.
Now with the zone change, gardeners might be able to get some of the prettier cultivars — the newer hybrids that are not as winter-hardy — to survive here, she said.
On the flip side of that zone change, certain invasive plant species are more likely to survive here, she said.
"Some invasive species — Chinese tallow tree for one — are more likely to survive here in a zone 8 instead of a zone 7," she said, noting tallow is more an issue in wooded areas.
Changing weather patterns could also change the time you see different insects, Shabel said.
"A lot of longtime gardeners know there is always a certain date when an insect will show up," she said. "Well, that date may be a little sooner in the spring than it used to be going forward."
When insects emerge may be a little out of sync.
Many wildflowers and native plants have specific pollinators, she said.
"Over time, these native insects have synced their life cycle with that of the plant it prefers to pollinate, so that they emerge at the same time the plant is about to bloom," she said. "This benefits the plant because it gets pollinated and the insect because it gets nectar and pollen.
"If wildflowers are blooming early, the pollinators may no longer be able to use it fully to its advantage. This impacts both the pollinators and the plants."
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Nursery
One area nursery said the change in the hardiness zone will have a minimal effect on most gardeners.
Joey Smith of Reseda Nursery & Stone Yard, off U.S. 72 in Madison, said the nursery would not be adjusting its inventory or giving any different advice to planters who frequent its business.
"It's just a change in the climate, so we are not adjusting our inventory because of it. It was really on the line anyway. Some of those 7As and 7Bs have been interchangeable for a long time," Smith said. "On a few plants it may change a little bit, but I don't think it's a whole lot of change."
Smith said he has seen a change in the climate the past three or four years in that we have had some plant-killing winter cold snaps.
He said he simply advises customers to cover their plants with a blanket in the winter.
"We tell them to cover them up now," he said. "If you've got plants that are looking real good and still have foliage, it's a good idea to cover them up for a couple days. You can take it back off and then if you have to — like the last two or three years — take it off and put it back on three or four times. We have to do that here."
— jean.cole@decaturdaily.com or 256-340-2361
