Growing Zones and Frost Dates
TilthIQ uses USDA hardiness zones and local frost date data to help you time your plantings correctly for your specific climate.
What Are USDA Hardiness Zones?
The USDA Plant Hardiness Zone Map divides North America into 13 zones (1 through 13) based on the average annual extreme minimum winter temperature. Zones are further divided into "a" and "b" sub-zones.
- Zone 1 — coldest (interior Alaska, northern Canada)
- Zone 6 to 7 — mid-latitude temperate (much of the continental US)
- Zone 10 to 11 — frost-free subtropical (southern Florida, Hawaii)
Your zone tells you which perennial plants can survive your winters, and it influences your growing windows for annuals.
How TilthIQ Determines Your Zone
When you enter your zip code on your Profile page, TilthIQ automatically:
- Looks up your geographic location
- Determines your USDA hardiness zone
- Retrieves your average last spring frost date and first fall frost date
These dates are displayed on your Profile and used by the Crop Plan wizard when calculating planting and harvest windows.
Frost Dates Explained
- Last Frost Date (Spring) — the average date of the last below-freezing temperature in spring. Warm-season crops (tomatoes, peppers, squash) should not go outside until after this date.
- First Frost Date (Fall) — the average date of the first below-freezing temperature in fall. This is your deadline for harvesting frost-sensitive crops.
These are averages — actual frost dates vary year to year. Always monitor your local forecast in spring and fall.
Cool-Season vs. Warm-Season Crops
| Type | Examples | When to Plant |
|---|---|---|
| Cool-Season | Lettuce, broccoli, peas, spinach, kale | 4 to 6 weeks before last frost; again in late summer for fall harvest |
| Warm-Season | Tomatoes, peppers, cucumbers, squash | After last frost; indoor start 6 to 10 weeks before last frost |
TilthIQ uses this categorization when the Crop Plan wizard suggests planting windows.
Manually Overriding Your Zone
If your microclimate differs significantly from the regional average — common in urban areas, valleys, or elevated sites — you can manually override your zone:
- Go to Account > Profile
- Scroll to the Growing Zone section
- Click Override Zone and select your actual zone from the dropdown
The override applies to all frost date calculations and planting recommendations.
Tip: If you are in a frost pocket or have a south-facing slope, your actual growing conditions may differ from the official zone by a full zone in either direction. When in doubt, track your own frost dates for a few seasons and use the manual override.