Seasonal Guides
Frost Dates by Zone: When to Plant Tomatoes This Spring
Tomatoes are the most-asked-about plant in our chat logs, and the #1 question is always the same: "when can I put them out?"
The honest answer is: it depends on your zone, your microclimate, and the 10-day forecast. Here's how to think about it.
The rule of thumb
Wait until 2 weeks after your average last frost date, when nighttime soil temperatures are reliably above 55°F (13°C).
Tomatoes are tropical. They don't just dislike frost — they sulk in cold soil. A tomato transplanted into 50°F soil in early May will be smaller in July than the same plant put out three weeks later into warm soil.
Average last frost dates by USDA zone
These are regional averages. Your microclimate (south-facing slope, urban heat island, frost pocket) can shift these by 1–2 weeks.
| USDA Zone | Avg. last frost | Earliest safe transplant |
|---|---|---|
| 3 | May 15 – June 1 | June 1 – June 15 |
| 4 | May 10 – May 25 | May 25 – June 5 |
| 5 | April 30 – May 15 | May 15 – May 30 |
| 6 | April 15 – April 30 | May 1 – May 15 |
| 7 | April 1 – April 15 | April 15 – April 30 |
| 8 | March 15 – March 30 | March 30 – April 15 |
| 9 | February 15 – March 1 | March 1 – March 15 |
| 10 | Frost rare | Year-round (avoid summer heat) |
The 3 signals that say "go"
Don't rely on the calendar alone. Wait for all three:
- Soil temperature ≥ 55°F at 4 inches deep, measured in the morning. A cheap soil thermometer pays for itself in one season.
- Nighttime lows ≥ 50°F in the 10-day forecast.
- No frost in the extended outlook. Check your NOAA local forecast, not just the app on your phone.
If even one of these is off, wait a week.
Hardening off — the step everyone skips
A tomato grown indoors or in a greenhouse has never felt wind, direct sun, or temperature swings. Putting it straight into the garden is a shock that sets it back weeks.
The 7-day hardening-off schedule:
- Days 1–2: 1–2 hours outside in shade, no wind.
- Days 3–4: 3–4 hours outside, partial sun.
- Days 5–6: Most of the day outside, full sun.
- Day 7: Overnight outdoors (if no frost).
Skipping this is the single most common reason transplants stall in June.
What to do if a late frost is forecast
Already planted and a 32°F night sneaks in? You have options:
- Row cover (Reemay or similar) — adds 4–8°F of protection.
- Upside-down 5-gallon bucket over each plant, removed by 9am the next morning.
- Old bedsheets draped over hoops or stakes — never directly touching leaves.
- Christmas lights (incandescent, not LED) under a cover add several degrees.
Don't use plastic touching the leaves — it'll freeze them where it contacts.
How GrowLog AI handles this
When you tell GrowLog AI your zip code, it knows your average last frost, pulls your local 10-day forecast, and warns you if you're about to make a planting mistake. We log every transplant date with the soil temp and forecast at the time — so next spring, when you ask "when did I plant tomatoes last year, and how did they do?" the answer is one message away.
Plant patient, plant warm, and your tomatoes will thank you in August.