July 5 – Nothing makes people want to dig in the earth more than the rows and rows of colorful potted plants they see every year at a nursery or hardware store.
But before they dig, they should know that what used to thrive in North Alabama may not thrive here, and what didn’t thrive before may now feel right at home. North Alabama’s plant hardiness zones were recently changed from 7A to 8A.
The U.S. Department of Agriculture recently updated its plant hardiness zone map, which tells gardeners which plants will thrive in their region. The zone is usually listed on each plant you buy.
According to the U.S. Department of Agriculture, the hardiness zones have shifted north since 2012. That’s due to two factors: 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 weather at various locations across the U.S. to determine the average minimum winter temperatures in each area, Shabel said.
According to the U.S. Department of Agriculture (USDA) website, the 2023 map is based on 30-year averages of the lowest annual winter temperatures at specific locations.
“If you’re a gardener, the low temperatures are more important than the high temperatures because there’s more impact on a plant if it drops to a very low temperature,” Shabel said. “The low temperatures are more likely to kill a plant than the high temperatures.”
Brent Shaw, a Morgan County farmer who works at Shaw Farms south of Calhoun Community College, says he thinks the change in plant hardiness zones is simply a reflection of what he’s already seen.
He sees a pattern in the growing season in north Alabama that he didn’t see decades ago.
“This is my 44th harvest and before we had to wait until mid-April to plant cotton, soybeans and even corn. Now we’re done at least two weeks earlier than we used to be.” He first started noticing the change about 12 to 15 years ago, he said.
“We’re warmer early and our frost is later,” Shaw said. “Our planting window is wider than it’s ever been. That’s the reality.”
Shaw agrees with the change in hardiness zones.
“We generally don’t see any frost until November or mid-November,” he said. “There used to be years when we planted cotton and you could get a frost when it came up in April to mid-April and then frost again before we defoliated it in October. You just don’t see that type of window that we used to have here.”
According to Shabel, the plant hardiness zone map has changed in part because there is more data showing what changes have already occurred and what changes are still happening.
“There are more stations, so the hardiness zone maps are more accurate than they used to be,” Steedley said.
Temperatures are changing, but the last map they used also showed far fewer measurement points, Shabel said.
The 2023 map includes data from 13,412 weather stations, up from 7,983 used in the 2012 map, the USDA said. The map also has a much more detailed resolution than previous maps.
“You just have to pay attention and make sure you buy something that is right for your climate,” Kerry Steedley, regional Extension agent, advised local gardeners.
This additional data had as much to do with the changing zones as it did with anything else, Shabel said.
“I would say it’s just more data that gives you a more accurate reading,” she said. “Temperatures have changed, any gardener can tell you that, but people are also paying more attention to it.”
According to Lee Ellenburg, a research engineer and associate professor of climatology at the Earth System Science Center at the University of Alabama in Huntsville, the new map of plant hardiness zones for the United States is generally about a quarter of a zone warmer than the 2012 map.
She said changes in plant zones are not a reliable method for assessing 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 total annual temperatures recorded over 50 to 100 years.
According to Shabel, local weather patterns are different, with more wet periods in winter than in the past.
“In recent years, we’ve had more large precipitation events in the winter,” she said. “Ideally, we’d have more half-inch rain events spread out over each month, but in the last few years, we’ve seen more 1-inch to 2-inch rain events. So the rain is falling in larger volumes instead of evenly in smaller amounts. This is leading to more flooding and oversaturation of the soil.”
She does not mean that there is more precipitation each year, but only that precipitation falls according to different patterns than in previous years.
“Those kinds of weather patterns also affect plants and agriculture, especially if you want to plant things,” she said.
For example, it is difficult to get a snowplow into a field that has just had 2 inches of rain fall.
The change in the hardiness zone of plants offers new opportunities for gardeners in the area.
“Because we’re now at 8A, it opens 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 8A, so now our winter temperatures are higher. With a warmer winter, we may be able to plant some more tropical plants that may not have survived in previous years,” she said.
Before the zone change, for example, the plant known as elephant ears was marginal in northern Alabama: Only the hardiest specimens would survive, she said.
Now that the zone change has taken place, gardeners may be able to keep some of the fancier cultivars — the newer hybrids that aren’t as hardy — here, she said.
On the other hand, certain invasive plant species are more likely to survive here, she said.
“Some invasive species, like the Chinese tallow tree, are more likely to survive here in Zone 8 than in Zone 7,” she said, noting that tallow is a bigger problem in forested areas.
Changing weather patterns can also affect when you see different insects, Shabel said.
“A lot of seasoned gardeners know that there’s always a certain date that an insect shows up,” she said. “Well, that date may be a little bit earlier in the spring than it used to be.”
The insects may not emerge completely in sync.
Many wildflowers and native plants have specific pollinators, she said.
“Over time, these native insects have synchronized their life cycle with that of the plant they prefer to pollinate, so that they emerge at the same time as the plant is about to flower,” she said. “This is beneficial to the plant because it gets pollinated and to the insect because it gets nectar and pollen.
“If wildflowers bloom early, pollinators may not be able to fully utilize it. This has consequences for both the pollinators and the plants.”
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According to a local nursery, the change in hardiness zone will have minimal impact on most gardeners.
Joey Smith of Reseda Nursery & Stone Yard, on U.S. 72 in Madison, said the nursery would not be adjusting its inventory or providing different advice to plant growers who frequent the nursery.
“It’s just a change in the climate, so we’re not adjusting our inventory for it. It was already on the cards. Some of those 7As and 7Bs have been interchangeable for a long time,” Smith said. “A few plants might see a little change, but I don’t think it’s a whole lot.”
Smith said he has seen a change in the climate over the last three or four years, in that we have had cold spells in the winter that are damaging to our plants.
He says he simply advises customers to cover their plants with a blanket during the winter.
“We tell them to cover them now,” he said. “If you have plants that look good and still have leaves, it’s a good idea to cover them for a couple of days. You can take it off then, and if you have to — like the last two or three years — take it off and put it back on three or four times. That’s what we have to do here.”
— jean.cole@decaturdaily.com or 256-340-2361
