By: Jacob Kelley, Regional Extension Agent, Commercial Horticulture, jtk0016@auburn.edu |www.MobileCountyMasterGardeners.org
In November 2023, the United States Department of Agriculture (USDA) released an updated Plant Hardiness Zone Map, marking a significant shift in how Gulf Coast residents view their climate. The latest map reflects a full-zone increase for parts of Alabama’s Gulf Coast, raising hopes among some gardeners about planting warmer-climate crops. Many eager to try growing tropical favorites such as avocados, mangos, Valencia oranges, and finger limes are excited by the change. Yet this update also brings complexity. The shift in hardiness zones doesn’t imply that our climate is suitable for tropical cultivation just yet.
The Gulf Coast has been experiencing a gradual warming trend for over a century, largely due to the oceans’ increasing temperatures. Oceans absorb about 90% of the excess heat linked to climate change. This heat contributes to rising sea levels as land and sea ice melt, causing the ocean to expand. Coastal residents already feel the effects of warmer waters through heightened storm activity, disease outbreaks, record-breaking heat, droughts, hurricanes, and more frequent flooding. All these factors are intensified by warming seas and a changing climate.
Alabama’s new classification as part of Zone 9, based on average annual winter temperatures, is a clear sign of this warming trend. But it is crucial to understand that Zone 9 isn’t a green light for tropical planting—at least not without caution.
While average winter temperatures have been climbing, extreme cold snaps are still likely. For instance, in December 2022, Mobile experienced temperatures below 20°F, only to see a high of 75°F later that same month. In March 2023, another freeze nearly wiped out Alabama’s blueberry crop. The USDA Hardiness Zones are based on averages, so sudden freezes or temperature fluctuations remain possible and can be devastating to new, warm-weather plantings.
Fruit-bearing plants face particular challenges as the climate changes. Many deciduous fruits require “chilling hours”—periods between 32°F and 45°F—to trigger flowering and growth. Chilling hours vary among fruit types and cultivars; for example, ‘Honeycrisp’ apples need 800 to 1,000 chilling hours to produce fruit, while Mobile typically sees only half that amount. With winter temperatures rising, chill hour accumulation has decreased, making it harder for some fruits to thrive.
Fluctuating winter temperatures exacerbate the problem, as warm spells in winter can even subtract from chill hour totals. This irregular weather places additional stress on plants, increasing their vulnerability to pests and diseases. For Gulf Coast growers, consistent fruit production is becoming more difficult.
The USDA’s updated map is a reminder of our warming climate and rising average temperatures. But it’s equally a reminder that, despite warming trends, Gulf Coast residents should still expect occasional winter frosts. Those interested in growing plants suited to warmer zones should proceed with care, recognizing that severe weather events may continue to disrupt their efforts.