Calculator inputs
Formula used
The USDA zone starts with average annual extreme minimum temperature. Each full zone spans 10 °F, while each subzone spans 5 °F. Microclimate choices then shift the result by half-zone steps.
Subzone index = floor((minimum temperature in °F + 60) ÷ 5). The calculator converts Celsius values before applying this formula. Frost dates remain broad planning estimates, not weather forecasts.
How to use
Enter your location label and extreme minimum temperature. Add several yearly minimums for a stronger long-term estimate. Select site conditions that meaningfully alter winter exposure.
Add one or more plants to compare their ranges. Review frost dates, risk warnings, and seasonal guidance. Confirm important planting decisions with trusted local climate records.
Example data
| Example | Minimum | Site condition | Approximate result |
|---|---|---|---|
| Cold inland garden | -18 °F | Exposed wind | Zone 5b, adjusted colder |
| Sheltered city courtyard | 8 °F | Walls and urban warmth | Zone 7b, adjusted warmer |
| Mild coastal garden | 24 °F | Coastal moderation | Zone 9a, adjusted warmer |
Hardiness zone temperature table
| Zone | Temperature range | General interpretation |
|---|---|---|
| 1 | -60 °F to -50 °F | Extremely cold |
| 2 | -50 °F to -40 °F | Extremely cold |
| 3 | -40 °F to -30 °F | Extremely cold |
| 4 | -30 °F to -20 °F | Cold temperate |
| 5 | -20 °F to -10 °F | Cold temperate |
| 6 | -10 °F to 0 °F | Cold temperate |
| 7 | 0 °F to 10 °F | Moderate to mild |
| 8 | 10 °F to 20 °F | Moderate to mild |
| 9 | 20 °F to 30 °F | Moderate to mild |
| 10 | 30 °F to 40 °F | Warm winter climate |
| 11 | 40 °F to 50 °F | Warm winter climate |
| 12 | 50 °F to 60 °F | Warm winter climate |
| 13 | 60 °F to 70 °F | Warm winter climate |
Microclimates and plant survival
Walls, paving, buildings, and water can retain useful warmth. Valleys and exposed slopes may increase damaging cold. Small temperature differences can change marginal plant outcomes.
Hardiness zones describe winter cold tolerance only. Soil drainage, heat, humidity, and rainfall still matter. Healthy plants usually tolerate stress better than weakened plants.
Winter protection guidance
Mulch roots after the ground begins cooling. Wrap vulnerable stems and block drying winter winds. Move tender containers into protected frost-free spaces before freezes.
Choose sheltered positions for plants near their limits. Avoid heavy feeding late in the growing season. New soft growth often suffers the greatest damage.
Frequently asked questions
What is a plant hardiness zone?
It groups locations by average annual extreme minimum temperature. Gardeners use zones to compare winter survival potential. Zones do not describe every growing condition.
What does the letter “a” or “b” mean?
Each full zone contains two five-degree subzones. The “a” half is colder than “b”. Subzones support more precise plant comparisons.
Can a postal code determine my exact zone?
Postal codes can cover several elevations and microclimates. This page therefore requires temperature data for calculation. Official regional maps provide finer geographic boundaries.
Why does the calculator ask for yearly records?
Several years reduce dependence on one unusual winter. Average records better represent a long-term hardiness zone. The coldest option supports conservative planning.
Can microclimates change a full zone?
Strong local effects sometimes shift practical conditions noticeably. Sheltered walls may warm plants during cold nights. Frost pockets can create the opposite result.
Are frost dates guaranteed?
No, frost dates are broad seasonal estimates. Actual freezes vary from year to year. Always check a current local weather forecast.
Does a higher zone always mean better growing?
No, some plants need winter chilling. Warm zones may also bring heat stress. Plant requirements should match the complete climate.
How should container plants be treated?
Container roots cool faster than ground-grown roots. Treat pots as less hardy during severe winter. Insulate or move vulnerable containers when needed.
What if my plant is one zone too tender?
Choose shelter, mulch, and excellent drainage. Consider growing it in a movable container. Accept that extreme winters may still cause loss.
Does soil type affect winter survival?
Yes, wet winter soil can damage roots. Free-draining soil often improves cold tolerance. Soil condition does not change the official zone.
Are international systems directly interchangeable?
No, different systems use different climate factors. The calculator provides useful temperature-based comparisons only. Consult official national guidance for final decisions.