The USDA Plant Hardiness Zone (PHZ) map is the standardized tool gardeners use to determine which perennial plants are likely to survive the winter in a specific location. These zones guide the selection of trees, shrubs, and other plants that can tolerate the coldest temperatures of a given area. Because Louisiana stretches from its northern border down to the Gulf of Mexico, the state encompasses a range of climatic conditions and falls into multiple hardiness zones. Understanding the local zone is the first step toward successful long-term gardening and landscaping.
How the USDA Hardiness Zones Are Determined
The USDA Hardiness Zone system is based on the average annual minimum winter temperature. Scientists calculate this average using data recorded over a specific 30-year period, such as the 1991 through 2020 data used for the 2023 map update. This long-term average ensures the resulting zones reflect the typical cold snap for a region, not an unusual event.
The map divides the country into 13 primary zones, with each full zone representing a 10-degree Fahrenheit temperature difference. For example, Zone 8 is 10°F to 20°F, and Zone 9 is 20°F to 30°F. To provide greater precision, each primary zone is further split into “a” and “b” subzones.
Subzones represent a smaller 5-degree Fahrenheit range, with the “a” segment indicating the colder half and the “b” segment indicating the warmer half. This detailed breakdown allows gardeners to fine-tune plant selection, as a small difference in the winter low can determine if a sensitive perennial survives. The PHZ calculation is limited strictly to cold tolerance and does not factor in other growing conditions like precipitation or heat.
Louisiana’s Specific Hardiness Zone Breakdown
Louisiana’s location near the Gulf of Mexico results in a hardiness zone range extending toward subtropical climates. Based on the 2023 USDA map, the state spans zones 8b through 10a. North Louisiana, including cities like Shreveport and Monroe, falls within the cooler Zone 8b (15°F to 20°F).
Moving south, central areas like Baton Rouge and Lafayette are classified as Zone 9a or 9b (20°F to 30°F). The warmest parts are the extreme southern coastal areas and developed urban centers, such as New Orleans, mapped as Zone 10a (30°F to 35°F). This wide distribution means a plant thriving in the north may struggle with southern heat, while a coastal plant may not survive a deep freeze up north.
Using Hardiness Zones for Successful Gardening
The hardiness zone guides the selection of perennial plants, which are species expected to live for more than two years. When purchasing trees, shrubs, or bulbs, gardeners should always check the plant label for its PHZ rating, such as “Hardy to Zone 8.” Choosing a plant rated for your zone or a lower number ensures it can withstand the typical winter cold.
Considering Heat Tolerance
While the PHZ map assesses cold tolerance, Louisiana gardeners must also consider significant summer heat and humidity. The American Horticultural Society (AHS) Heat Zone map offers complementary information. The AHS system rates plants based on the average number of days per year the temperature exceeds 86°F (30°C), the point at which many plants experience heat stress.
For Louisiana’s humid climate, selecting plants compatible with both the USDA Hardiness Zone (winter cold) and the AHS Heat Zone (summer performance) is recommended. This dual consideration ensures plants not only survive the cold but also thrive through the long, hot growing season. Successful landscape designs balance both cold hardiness and heat tolerance.