Mobile, Alabama, is primarily located in the USDA Plant Hardiness Zone 9a. This designation indicates which perennial plants are most likely to survive the average annual minimum winter temperature in a specific location.
Mobile’s Hardiness Zone
The designation of Zone 9a for Mobile means the area experiences an average annual extreme minimum temperature between 20°F and 25°F. This range is calculated based on 30 years of climate data, providing a reliable metric for plant survival during the coldest part of the year. The most recent update to the USDA map in 2023 shifted Mobile from its previous Zone 8b classification to the slightly warmer 9a, reflecting a trend toward milder minimum winter temperatures.
Choosing plants rated for Zone 9a or lower ensures they can withstand the winter cold. While the zone indicates the average minimum, gardeners should still be prepared to protect tender plants if temperatures dip toward the lower end of the 20°F range.
The Heat Factor
For a Gulf Coast city like Mobile, the American Horticultural Society (AHS) Heat Zone is an important consideration for plant selection. The AHS Heat Zone measures a plant’s ability to withstand prolonged summer heat, unlike the USDA Hardiness Zone which only addresses cold tolerance. This metric is based on the average number of days per year that the temperature rises above 86°F (30°C), the point where many plants begin to experience heat stress.
Mobile falls into AHS Heat Zone 8 or 9, experiencing between 120 and 180 days annually where temperatures exceed 86°F. This high number of heat days, combined with the region’s high humidity, presents a challenge for many plants. Selecting varieties noted for heat and humidity tolerance is necessary to prevent wilting, stunted growth, and disease issues during the long summer season.
Best Plants for Zone 9a
The combination of mild winters and intense, humid summers allows for a diverse range of plant life to thrive in Mobile. Many classic Southern shrubs and perennials are well-suited to the Zone 9a climate, including fragrant Gardenias and the vibrant, long-blooming Crape Myrtle. Heat-tolerant flowering plants like Lantana and Pentas are also staples.
The mild climate supports many tropical and sub-tropical species, often with minimal winter protection. Citrus trees (such as oranges and lemons) and various types of Palms (including the Foxtail and Lady Palm) lend a tropical feel to the landscape. For vegetables, the long growing season accommodates both warm-season crops like Tomatoes, Peppers, and Okra, and cool-season crops like Broccoli, Kale, and Carrots.
When to Plant in Mobile
Mobile has an extended growing season, allowing for nearly year-round gardening. The average date for the last spring frost falls in early to mid-March, often between March 6 and March 12 for a light freeze. This date signals the safe time to transplant tender, warm-season vegetables like tomatoes and peppers outdoors.
The average date for the first fall frost occurs much later, around late November or early December. This extended period between frosts allows gardeners to achieve multiple harvests. Cool-season vegetables can be planted in late summer or early fall (August or September) to mature during the mild winter months. Light frost can enhance the sweetness of greens like kale and collards.