Plant zones, often referred to as “hardiness zones,” are a classification system used to guide the selection of plants based on the climate conditions in which they are most likely to thrive. Developed by the United States Department of Agriculture (USDA), this system is most commonly seen on plant tags and in gardening resources within the United States.
Here’s a breakdown of how these zones work:
- Temperature Ranges: Each zone represents an area with a specific range of average annual minimum winter temperatures. For example, Zone 6 might have temperatures that drop to -10 to 0 degrees Fahrenheit (-23 to -18 degrees Celsius).
- Zone Numbers: The zones are numbered from 1 to 13, with lower numbers indicating colder climates. Each zone is further divided into “a” and “b” subzones to provide more specific guidance. “a” subzones are colder, and “b” subzones are warmer.
- Planting Guidance: Plants are often labeled with the hardiness zone numbers to indicate where they can survive the winter outdoors. If a plant is rated for Zone 6, it should survive the winter in any Zone 6 area or warmer (higher zone number).
Hardiness zones are particularly useful for perennials, trees, and shrubs, which need to survive local winter conditions year after year. However, factors like soil type, humidity, heat, wind, and rainfall can also affect plant survival and health, so hardiness zones are just one tool among many for gardeners.
Zone Definitions
| Zone Number | Temperature Range | Planting Guidance |
| 1 | Below -50°F (-45°C) | Only the hardiest plants can survive; limited to very few species. |
| 2 | -50°F to -40°F (-45°C to -40°C) | Suitable for some hardy trees, shrubs, and perennials. |
| 3 | -40°F to -30°F (-40°C to -34°C) | Can accommodate many deciduous trees, conifers, and perennial plants. |
| 4 | -30°F to -20°F (-34°C to -29°C) | Supports a wide range of hardy perennials, some fruit trees, and evergreens. |
| 5 | -20°F to -10°F (-29°C to -23°C) | Suitable for most fruit trees, many ornamental plants, and broad variety of perennials. |
| 6 | -10°F to 0°F (-23°C to -18°C) | Good for a broad range of flowering trees, shrubs, and perennial garden plants. |
| 7 | 0°F to 10°F (-18°C to -12°C) | Supports many landscape plants, including some varieties of evergreens and hardy citrus. |
| 8 | 10°F to 20°F (-12°C to -7°C) | Can grow robust range of perennials, ornamental grasses, and evergreens. |
| 9 | 20°F to 30°F (-7°C to -1°C) | Suitable for many tropical and subtropical plants, including citrus and avocado trees. |
| 10 | 30°F to 40°F (-1°C to 4°C) | Can accommodate most tropical plants as risk of frost is minimal. |
| 11 | 40°F to 50°F (4°C to 10°C) | Ideal for lush tropical landscapes and year-round gardening. |
| 12 | 50°F to 60°F (10°C to 16°C) | Suitable for very warm, tropical plants; rare frosts occur. |
| 13 | Above 60°F (above 16°C) | Almost exclusively tropical environments, allowing for a wide range of tropical plants year-round. |
Zone Map
Here are some links to plant zone maps for different countries: