calculatemygarden
Units

When to Start Onion Seeds

Dates to start onions indoors, harden off and plant out for your ZIP code or zone.

Tomato and pepper seedlings in trays under an LED grow light

Onion planting dates by zone

At the average last frost date for each USDA half-zone (the median across its ZIP codes, NOAA 1991 to 2020 normals). Enter your ZIP code below for your own dates.

ZoneLast frostStart indoorsTransplant
3aMay 28Apr 2 to Apr 16Apr 30 to May 14
3bMay 17Mar 22 to Apr 5Apr 19 to May 3
4aMay 13Mar 18 to Apr 1Apr 15 to Apr 29
4bMay 8Mar 13 to Mar 27Apr 10 to Apr 24
5aMay 4Mar 9 to Mar 23Apr 6 to Apr 20
5bMay 1Mar 6 to Mar 20Apr 3 to Apr 17
6aApr 29Mar 4 to Mar 18Apr 1 to Apr 15
6bApr 23Feb 26 to Mar 12Mar 26 to Apr 9
7aApr 16Feb 19 to Mar 5Mar 19 to Apr 2
7bApr 6Feb 9 to Feb 23Mar 9 to Mar 23
8aMar 29Feb 1 to Feb 15Mar 1 to Mar 15
8bMar 18Jan 21 to Feb 4Feb 18 to Mar 4
9aMar 3Jan 6 to Jan 20Feb 3 to Feb 17
9bFeb 2Dec 8 to Dec 22Jan 5 to Jan 19
10aJan 16Nov 21 to Dec 5Dec 19 to Jan 2
10brareby seasonby season
Your location

Zone medians. Your ZIP code gives the dates of your nearest weather station.

Frost risk

Half of years still get a frost after the spring date, and half get the first fall frost before the fall date.

Frost dates at three risk levels
AverageSaferSafest
Last spring frostApr 23Apr 30May 9
First fall frostOct 22Oct 16Oct 7
Crops (1 selected)

Start your first seeds indoors

Feb 26onion

Average frost dates: last Apr 23, first Oct 22 (zone 6b).
Season
Spring seed starting schedule
CropStart indoorsHarden offTransplantNotes
OnionHardyFeb 26 to Mar 12Mar 12 to Mar 25Mar 26 to Apr 9Indoors 6 to 8 wk before last frost. Transplant 2 to 4 wk before last frost. Sprouts in 7 to 12 days, best at 50 to 95°F soil. Pot up at the first true leaves.

How to start seeds indoors

Light
Give seedlings 14 to 18 hours of light a day on a timer, with fluorescent or LED fixtures 2 to 6 in above the leaves. Raise the lights as the plants grow.
Clemson HGIC: 16 to 18 hours, tubes 2 to 4 in above. UMD Extension: 14 to 16 hours, 2 to 6 in.
Seed starting mix
Use a sterile, soilless mix, never garden soil. To mix your own, combine equal parts by volume of fine pine bark, sphagnum peat moss and perlite.
NC State Extension Gardener Handbook; Clemson HGIC.
Sowing and warmth
Sow two or three seeds per cell and keep the strongest seedling. A mix temperature of 65 to 75°F germinates most vegetables; after they sprout, 55 to 60°F nights and 65 to 70°F days keep them stocky.
UMD Extension.
Potting up
Move seedlings into larger pots shortly after the first true leaves appear, while they are small and recover quickly.
UMD Extension; Clemson HGIC (one or two sets of true leaves).
Hardening off
Start two to three weeks before transplanting, once plants are about 3 in tall: set them outdoors by day, bring them in before sundown, and give them more direct sun each day. Skip days below 45°F or with strong wind.
Clemson HGIC: 2 to 3 weeks. NC State: at least 2 weeks, not below 45°F.

When to start onion seeds

Start onion seeds indoors 6 to 8 weeks before your last spring frost and set the transplants out 2 to 4 weeks before it. Onions are hardy, so unlike tomatoes and peppers they go into the garden while frost is still possible. Those timings come from Utah State University Extension and Johnny's Selected Seeds, and the zone table and planner above count from them.

Sow indoors = last frost − 8 to 6 weeks · Transplant = last frost − 4 to 2 weeks

Example dates for three zones

At the median last frost date for each half-zone, from NOAA's 1991 to 2020 normals:

ZoneLast frostStart indoorsHarden offTransplant
5bMay 1Mar 6 to Mar 20Mar 20 to Apr 2Apr 3 to Apr 17
7aApr 16Feb 19 to Mar 5Mar 5 to Mar 18Mar 19 to Apr 2
9aMar 3Jan 6 to Jan 20Jan 20 to Feb 2Feb 3 to Feb 17

Because the transplant date comes before the frost, the whole schedule runs early: in zone 7a the seed goes in during February and the plants go out in March. Hardening off gets the two weeks between the last sowing day and the first transplant day.

Germination

Onion seed is unusually tolerant of cold. Oregon State University Extension gives 35°F as the minimum soil temperature and 50 to 95°F as the best range; expect seedlings in 7 to 12 days. Sow 1/4 to 1/2 inch deep.

Match the variety to your latitude

The sowing date is the same for every onion, but the variety is not. Bulbing is triggered by day length: short-day onions suit the South, intermediate and long-day onions suit higher latitudes. Check the day length group on the seed packet before you sow.

How long onions take

Colorado State University Extension counts about 110 days from seed to harvest. In zone 5b, transplants set out from Apr 3 still have well over 150 days before the Oct 10 first frost, so the season is long enough everywhere the table covers.

Next steps