Understanding USDA Plant Hardiness Zones
The USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive in their location. It's based on the average annual minimum winter temperature, divided into 10°F zones.
The Zones at a Glance
| Zone | Min. Temp (°F) | Avg. Last Frost | Avg. First Frost | Growing Season |
|---|---|---|---|---|
| 3 | -40 to -30 | May 28 | Sep 10 | ~105 days |
| 4a | -30 to -25 | May 21 | Sep 15 | ~117 days |
| 4b | -25 to -20 | May 15 | Sep 21 | ~129 days |
| 5a | -20 to -15 | May 7 | Sep 28 | ~144 days |
| 5b | -15 to -10 | May 1 | Oct 1 | ~153 days |
| 6a | -10 to -5 | Apr 21 | Oct 15 | ~177 days |
| 6b | -5 to 0 | Apr 15 | Oct 20 | ~188 days |
| 7a | 0 to 5 | Apr 7 | Oct 29 | ~205 days |
| 7b | 5 to 10 | Mar 28 | Nov 5 | ~222 days |
| 8a | 10 to 15 | Mar 14 | Nov 15 | ~246 days |
| 8b | 15 to 20 | Mar 7 | Nov 22 | ~260 days |
| 9a | 20 to 25 | Feb 28 | Dec 1 | ~275 days |
| 9b | 25 to 30 | Feb 14 | Dec 15 | ~304 days |
| 10a | 30 to 35 | Jan 31 | Dec 31 | ~334 days |
| 10b+ | 35+ | Rare frost | Rare frost | Year-round |
How We Calculate Your Planting Dates
Our planting calendar uses your zip code to determine your USDA zone and average frost dates. From there, we calculate three key dates for each vegetable:
Start Indoors
When to plant seeds in trays or pots indoors, typically 6-12 weeks before your last frost date.
When to plant seeds in trays or pots indoors, typically 6-12 weeks before your last frost date.
Transplant
When to move indoor seedlings outside, usually around or just after your last frost date.
When to move indoor seedlings outside, usually around or just after your last frost date.
Direct Sow
When to plant seeds directly in the garden, based on soil temperature and frost risk.
When to plant seeds directly in the garden, based on soil temperature and frost risk.
Important Notes
- These are averages. Your actual last frost date can vary by 2-3 weeks in any given year. Monitor local weather forecasts.
- Microclimates matter. South-facing walls, urban heat islands, and elevation can shift your effective zone by half a zone or more.
- Soil temperature matters too. Some crops (like corn and beans) need warm soil regardless of air temperature. Use a soil thermometer.
- Extended forecasts are your best friend in spring. When in doubt, wait a week.
📚 Go Deeper with a Regional Guide
Our Harvest Home Guides books include detailed zone-specific advice, companion planting charts, pest & disease guides, month-by-month checklists, and more.
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