Hardiness Zone Map adjusted to account for warming trend

By Allison Marlow
Special to Gulf Coast Media
Posted 12/1/23

Have you noticed flowers or vegetables in your garden continuing to thrive later in the season without the help of a heat lamp or covers on cool nights? It's not a fluke. Across the continental …

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Hardiness Zone Map adjusted to account for warming trend


Have you noticed flowers or vegetables in your garden continuing to thrive later in the season without the help of a heat lamp or covers on cool nights?

It's not a fluke.

Across the continental United States, the lowest likely winter temperature overall is 2.5 degrees warmer than it was a decade ago. It's a shift reflected in the recently released plant hardiness zone map, a must-have for gardeners and growers.

The map, culled by the U.S. Department of Agriculture, is used to help determine what plants will thrive the best in a region. It was updated for the first time in a decade this month and showed a trend that has become similar across disciplines and studies – warming temperatures.

There is a 5-degree difference in many places, in fact.

"It's scary from the climate change point of view," said Carmen Flammini, regional extension agent for the Alabama Cooperative Extension System. She specializes in horticulture issues in Baldwin, Butler, Escambia and Conecuh counties.

"This map is mainly to help with planting and harvesting dates, but when you talk about it temperature-wise, this will really be a problem if every 30 years it increases 5 degrees higher," she said. "We don't know that will happen, but this makes you think about that."

Still, she stressed, "It is a tool for growers and harvesting, not necessarily a picture of climate change."

The map is crafted when analysts take 30 years of temperature data and determine the average lowest temperature.

Flammini said it is important to keep in mind that these are averages, not fixed numbers. For example, much of Baldwin County now rests in zone 9a with an annual low of 20 to 25 degrees.

"It doesn't mean next year we won't have a crazy minus-10 day. That can still happen," Flammini said. "This is not a prediction of the future."

Across Baldwin County, many average low temperatures increased by 5 degrees on the new map. The small number can make a big difference for gardeners.

In fact, Jack LeCroy, regional extension agent for Clarke, Mobile, Monroe and Washington counties, calls the change "exciting."

While much of the non-gardening world was oblivious, LeCroy says his email and phone were "blowing up" when the new map was announced.

Five degrees means the growing season in Alabama may extend well into December, allowing plants to produce more fruits and vegetables than in years past.

It may also cause growers to sow earlier in the year to avoid the onslaught of summer heat. Both are good news for growers.

Warmer winters could also allow more sun-loving plants and flowers to survive longer in Alabama backyards or with less protection over the winter months.

Warmer winters, however, don't necessarily mean heat-loving plants will fare well.

LeCroy said tomatoes, which are a tropical plant, stop producing fruit when the thermometer climbs over 90 degrees.

"Fruits and vegetables are kind of like people. Once it reaches over a certain temperature, they are just trying to survive and they kind of shut down and slow down," LeCroy said.

And as it turns out, the extra 5 degrees is equal parts exciting and worrying.

LeCroy says the extra 5 degrees in summer months could prove exhausting for many plants, even those that love the heat. Invasive plants may begin to encroach upon areas where the cold temperatures once kept them at bay.

And insects that prefer the warmth may begin to creep into new territories bringing disease and destruction with them.

"That to me is the interesting side of things," he said. "Plant research takes many, many years though, so we won't know until we kind of see the years go by."

So, what do backyard gardeners and homeowners do now?

First, don't rip up your plants or your landscape plans, LeCroy said.

Do check the map before buying new plants.

"If you have a plant that is on the very end of the zone, you're getting to the limits of where it can grow," he said. "In my garden, I like to pick plants that are kind of in the middle of the zone so I know they are going to be able to handle the extreme on either end, hot or cold."

He said to also pay attention to what is happening in your own yard, not just predictions for the overall region. Even individual yards can be subject to microclimates dictated by the soil, water and plants in that small area.

"This year, not only did we have a crazy drought, we got hit with that late freeze back in March and then had the excessive heat," LeCroy said. "There's definitely been some weather patterns just this year that have made it difficult.

"I think a lot of gardeners are a little exhausted just from this year alone."