When to Start Onion Seeds Indoors: 8–12 Weeks Before Planting Out, Dated for Your ZIP

Start onion seeds indoors 8 to 12 weeks before the day you plan to set the transplants out. The set-out day is the real decision. Johnny’s rule sets it 4 weeks before the last frost, which for Minneapolis (median April 23) means sowing January 15–29. University of Minnesota and Nebraska Extension calendars set transplants out in May, which means sowing in February (UMN) or late February to early March (Nebraska). In Georgia, onions are short-day types sown in fall, so a late-winter indoor start is not the usual route.

Four onion schedules: 8–12 weeks indoors, different planting-out days

Four sources give a schedule for raising onion transplants from seed. Their onion-specific figures for time indoors range from 8 to 12 weeks. Where they differ is the day the transplants go out.

UMN’s seed-starting table is organised by sowing month and ends with “early-mid May: harden off all plants”. Its 14–15 weeks therefore reads as early February to a May planting, not as a separate claim that onions need longer under lights. The direct comparison is UMN’s 10–12 weeks on the onion page against Johnny’s 8–10.

Date both rules for your ZIP

The table applies Johnny’s rule, as stored in Seed Window, to three NOAA medians and puts each region’s extension calendar beside it.

As a check, we entered 04/23/2027 in Johnny’s own calculator on 10 November 2026. It returned January 15 – January 29 for starting onions and March 26 for setting them out, the same as Seed Window.

In Minneapolis and Lincoln, the two methods differ by about five weeks. Johnny’s rule plants out on March 26–27; the extension calendars plant out in May, 35 days after March 27 at the earliest (May 1). Because the extension pages also allow more weeks indoors (10–12), the sowing gap is smaller: 3 to 4 weeks (January 15–29 against February 5–26 for the custom crop below). If you sow on Johnny’s January dates and then wait for May conditions, the seedlings spend about 13 to 16 weeks indoors (January 29 to May 1 is 92 days; January 15 to May 7 is 112 days). That is closer to the 14–15 weeks in UMN’s seed-starting table than to the 8–10 weeks in Johnny’s rule.

Choose the planting-out day first, then count back

Nebraska gives a planting cue rather than a fixed date: soil dry enough to work thoroughly and daytime temperatures of 50°F. UMN uses the same 50°F cue, and both pages note that onion transplants tolerate light frosts. Use Johnny’s date as the earliest day you might plant out. If your soil is usually still frozen or wet then, use your own planting-out estimate and count back.

Seed Window can hold both schedules side by side. Add a custom crop with 10–12 weeks indoors, planted out 1 to 2 weeks after the frost date. The “1 to 2 weeks after” is our approximation of “May” and “early May” at these two medians; neither extension page states it as an offset.

Day length decides whether a spring schedule applies

A University of Georgia article by horticulturist George Boyhan (2004) groups onions by the day length at which they bulb: short-day at 11–12 hours, intermediate at 13–14, long-day at 15 or more. It says the onions grown in Georgia are short-day types, sown in fall and harvested in spring. Commercial growers sow in late September and reset plants in November and December; home gardeners can direct-seed in mid-October or buy transplants in November and December. Long-day onions are sown in early spring for a fall harvest. UMN describes Minnesota’s storage onions as long-day types that need 14 or more hours of daylight to bulb. The two sources use different thresholds, but both put storage onions in the long-day group.

Seed Window’s December 16–30 date for Atlanta is Johnny’s spring rule applied mechanically. It matches no onion type UGA describes for Georgia: UGA says long-day onions planted there by mistake should be harvested as green onions, and that short-day and intermediate-day onions are sown in fall. In that climate, follow your state extension’s fall calendar rather than a frost-date calculator.

Reproduce it in Seed Window

1. Open the calculator with ZIP 55401 and click the 50% date, April 23, 2027. Use New plan first if you want to keep your own plan unchanged.
2. In Choose your crops, keep only Onion. The seed chart shows its source and the UMN cross-check column.
3. Open Add your own crop or variety. Enter “Onion (extension timing)”, choose transplant, 10 to 12 weeks indoors, plant out between 1 and 2 weeks after the frost date, then Add to plan.
4. Switch to Timeline to compare the two bars, then pick one before you export. Export the dates to Google Calendar or print them, and keep the hardening-off days before the planting date.

Sources

• Johnny’s Selected Seeds, Seed-Starting Date Calculator (onion row; calculator run with 04/23/2027).
• Marissa Schuh and Cindy Tong, UMN Extension, Growing onions in home gardens (reviewed 2024).
• UMN Extension, Starting seeds indoors, table “When to plant seeds and how long to keep them indoors” (reviewed 2026).
• Sarah Browning, Nebraska Extension in Lancaster County, Growing Onions: Sets, Plants or Seeds?
• George E. Boyhan, University of Georgia, Choose the right Vidalia onion to sow in garden (August 19, 2004).
• NOAA NCEI, U.S. Climate Normals 1991–2020, as served by Seed Window’s ZIP lookup.

All sources read on 10 November 2026.