When to Plant Onions in Iowa
The backbone of the kitchen garden. Choose short-day, intermediate, or long-day varieties based on your latitude.
The Short Answer
Iowa Frost Dates
Your planting dates depend on which part of Iowa you're in. Here are the frost date ranges by region:
| Region | Zones | Last Frost (Spring) | First Frost (Fall) |
|---|---|---|---|
| Northern Iowa | 4b, 5a | May 5 - May 20 | Sep 20 - Oct 5 |
| Central Iowa | 5a, 5b | Apr 25 - May 10 | Oct 1 - Oct 15 |
| Southern Iowa | 5b | Apr 20 - May 5 | Oct 5 - Oct 20 |
Onions Planting Schedule for Iowa
Northern Iowa (Zones 4b, 5a)
Average last frost: May 5 - May 20 · Average first frost: Sep 20 - Oct 5
Central Iowa (Zones 5a, 5b)
Average last frost: Apr 25 - May 10 · Average first frost: Oct 1 - Oct 15
Southern Iowa (Zones 5b)
Average last frost: Apr 20 - May 5 · Average first frost: Oct 5 - Oct 20
Growing Onions in Iowa
Onions in Iowa's Climate
You're in long-day onion territory — plant varieties that bulb when days exceed 14-16 hours. This means Copra, Walla Walla, and Yellow Globe Danvers. Plant sets or transplants from mid-April. The critical distinction: short-day onions from southern catalogs will NOT form bulbs in your zone because your long summer days trigger bulbing too early, before the plant is large enough.
Soil Considerations for Iowa
Outstanding black prairie soils. Some of the richest agricultural land in the world. Minimal amendment needed. Loose, well-drained soil is especially important for onions since the edible portion grows underground. If your soil is heavy clay, consider raised beds.
Iowa Climate & Growing Season
Continental climate. Cold winters, warm humid summers. Good growing season with adequate rainfall. Onions can handle frost well, which is an advantage in Iowa's climate. You can push planting dates earlier in spring and extend into fall.
Growing season length varies across Iowa: Northern Iowa (4b, 5a) has a last frost around May 5 - May 20, while Southern Iowa (5b) sees frost end around Apr 20 - May 5. This difference matters for onions — but onions handle frost well, so the timing difference is less critical.
Growing Tips
Day length triggers bulbing. Northern gardeners need long-day varieties. Southern gardeners need short-day varieties.
Companion Planting
Plant onions alongside these companions for better growth:
Keep onions away from:
The Bottom Line
Last reviewed: March 29, 2026