Cool-Season vs Warm-Season Vegetables: Timing from the Frost Date
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Every planting calendar sorts crops by how they handle cold. The University of Idaho Extension guide splits transplants into two groups, measured from the average date of last frost:
- Cool-season crops, "like broccoli, onions, and pansies," can be transplanted outside 2 to 5 weeks before that date.
- Warm-season plants, "like tomatoes, peppers, and zinnias," can be moved into the garden about 1 to 3 weeks after it.
Nine crop groups
The same guide's Table 1 gives soil temperature, days to germination and weeks to grow indoors for each group:
| Crop | Ideal soil temperature for germination | Days to germination | Weeks to grow indoors |
|---|---|---|---|
| Broccoli, cauliflower | 65–75 °F | 5–10 | 5–7 |
| Cabbage, kale, collards | 65–75 °F | 5–10 | 5–7 |
| Celery | 65–75 °F | 10–18 | 8–10 |
| Lettuces | 65–70 °F | 7–10 | 3–5 |
| Melons | 80–85 °F | 5–10 | 3–4 |
| Onions, leeks, shallots | 65–70 °F | 10–14 | 4–8 |
| Peppers | 75–85 °F | 10–14 | 6–9 |
| Squash | 70–85 °F | 7–14 | 3–4 |
| Tomatoes | 75–80 °F | 7–14 | 5–9 |
Source: University of Idaho Extension, CIS 1209: Start Seeds Indoors with Success (2014), Table 1
The guide names broccoli and onions as cool-season, and tomatoes and peppers as warm-season. It does not say which group the other crops belong to, so this site calculates move-out dates only for those four. For the rest, the seed starting calculator shows the weeks indoors and leaves the outdoor date to the seed packet.
What the soil temperatures tell you
The guide's soil temperatures are for sprouting seeds. Melons (80 to 85 °F) and peppers (75 to 85 °F) want the warmest soil. Lettuces and onions (65 to 70 °F) want the coolest. A heat mat or a warm spot indoors is how you hit the high end of a range for seeds started inside.
How the two groups differ in practice
Counting from an average last freeze of April 18 (Port Columbus International Airport, Ohio):
| Crop | Group | Move outdoors | Start seeds indoors |
|---|---|---|---|
| Broccoli, cauliflower | Cool-season | Mar 14 to Apr 4 | Jan 24 to Feb 28 |
| Onions, leeks, shallots | Cool-season | Mar 14 to Apr 4 | Jan 17 to Mar 7 |
| Tomatoes | Warm-season | Apr 25 to May 9 | Feb 21 to Apr 4 |
| Peppers | Warm-season | Apr 25 to May 9 | Feb 21 to Mar 28 |
The cool-season move-out window ends two weeks before the last frost date, and the warm-season window starts a week after it: a gap of three weeks between the two groups.
Sources
- University of Idaho Extension, Start Seeds Indoors with Success (CIS 1209), Table 1 and "Getting started."
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, Port Columbus International Airport, Ohio.