TAMPA, Fla. — The last time the USDA Plant Hardiness zone map got an update was in 2012. This year's new map shows nearly half the country warmed into a different climate zone.
According to amateur and professional growers, the warming trend isn't a big surprise.
"We've noticed in our daily ground-level gardening that the temperatures have been changing. We've been here since 2011," Kitty Wallace, Gardening Coordinator for the Tampa Heights Community Garden, told ABC Action News reporter Michael Paluska.
"We used to start our fall gardening in mid-September, and then we backed it up to the end of September. Now, we're going to start our fall gardening in October because it is just really too hot," Wallace said. "This (2012) chart says you can plant broccoli in August, and I was questioning that. So, I said well, let's try this and then plan on doing it later in September. And because the ones that went in six weeks earlier had to struggle through all the heat, they caught up and were the same size by November."
The USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which perennial plants are most likely to thrive at a location. The map is based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones and 5-degree F half zones.
According to the 2023 map, most of Florida went up half a climate zone. Location plays a significant role in what your new zone is. You cansearch your zone by putting in your address or zip code.
In 2012, most of Tampa was zone 9B, now 10A. In coastal Pinellas, zones went from 10A to 10B.
"The presence of oceans or just water bodies in general are very challenging for us to model. Water has a high heat capacity and tends to cool down more slowly than land. And so it tends to be milder along the coast. It does in the interior," Dr. Chris Daly, lead author for the map and Professor in the College of Engineering at the Oregon State University in Corvallis told Paluska.
The USDA map was created in collaboration with the PRISM Climate Group at Oregon State, which Dr. Daly founded and directs.
"Can you put in context what this map means and the fact that the warming trend is shifting farther north?" Paluska asked.
"We have a number of factors at play; the obvious elephant in the room is climate change. But the issue we're not sure about is that the plant hardiness statistic is the coldest temperature of the year, averaged over 30 years. So it's a very extreme value. Are the changes following climate change or not? And I expect that climate change certainly plays a role. But we also have about 60 to 70% more station data, this time, and also some improved mapping methods, which can lead to some zone changes that are not climatically induced."
"Was there anything in your research that shocked you?" Paluska asked.
"I don't think so. I think we're seeing overall across the entire U.S. about a 2.5-degree warming on average, that means about half the country went into a new half zone," Dr. Daly said.
The warming trend has professional eco landscaper and gardener Amanda Streets concerned.
"We've noticed a difference in what we can grow for chill hours for fruit trees, and also how many times a year we have to cover up our plants in the wintertime because it's really cold," Streets, Owner of Living Roots Eco Design, told Paluska. "I remember I used to have to cover up my tomatoes and my peppers and stuff several times a year. And now there are some years I don't cover them at all. We're not getting enough chill hours for some of the fruit trees or the berry plants that need the chill hours."
"Does it worry you?" Paluska asked.
"Does it worry me? A little bit," Streets responded. "I've noticed a trend. What will that look like in 20 or 30 years? I saw some landscape plants that we've been using for a couple of years now melt over the summer during the drought in the heat in August; we had that really hot couple of weeks. And the plants, just a couple, just turned to mush."
Streets said on the flip side, there are positive outcomes to the new zones. A once-imperiled butterfly is making a comeback.
"The Atala butterfly used to be only in south Florida, and they've been found in Pinellas and Hillsborough County and maybe even further north," Streets said.
Streets said the shift north is partly due to more landscapers, such as herself, planting Coontie. According to the Florida Wildflower Foundation, it is "the host plant for the Atala butterfly. The two have a mutualist relationship. The caterpillars eat the plant's leaves, and their abundant fecal droppings fertilize the surrounding soil."
Wallace said they are experimenting with new ways to grow and adapt to the changing zone.
"This is a big deal?" Paluska asked.
"This is a big deal," Wallace said. "We have to be paying attention to planting the most resilient plants because this will continue to change, and it will impact how much food we can grow for ourselves. And really super important thing for everybody to notice is that this is our food; we're talking about changes in the way that our food is grown. And so we are now looking to be more actively involved with saving seeds of our most resilient plants."
