When to plant / Onions
When to plant onions
Plant onions 2 to 4 weeks before the last spring frost. Onions are a hardy crop in Colorado State University Extension's planting guide, and that is the guide's rule for the class. Across the 39 U.S. cities on this site the window opens between Jan 12 (Sacramento, CA) and Apr 15 (Concord, NH), because each city's own NOAA last-frost date is different.
The planting rule for onions, in plain words
Colorado State's Vegetable Planting Guide sorts vegetables into four frost classes and gives each class one timing rule relative to the average last spring frost. Onions sit in the hardy class, whose rule reads: As early as 2-4 weeks before the date of the average last spring frost.
On this site that becomes -4 to -2 weeks from the station's 50% probability last 32°F date, the same arithmetic every city calendar uses.
This page uses the guide's row for dry onions from sets, the form most gardeners plant. Onions from seed and green (bunching) onions share the same hardy class and the same spring window; dry onions from seed take the same 110 days to harvest, while green onions are picked at 60.
Soil temperature for onions
Minimum soil temperature for onions in the cited sources: 35°F (minimum germination soil temperature for onions (green, dry seed, sets)). A frost date cannot see the soil, so take a morning reading before trusting the window; the soil temperature guide covers every crop.
Onions: planting dates for 39 U.S. cities
The rule above applied to each station's own NOAA normals, earliest first. City links open that station's full calendar.
| City | Last spring frost (32°F) | Plant sets | Last fall planting |
|---|---|---|---|
| Sacramento, CA | Feb 9 | Jan 12 – Jan 26 | Aug 29 |
| Houston, TX | Feb 18 | Jan 21 – Feb 4 | Sep 3 |
| Austin, TX | Feb 19 | Jan 22 – Feb 5 | Sep 1 |
| San Antonio, TX | Mar 1 | Feb 1 – Feb 15 | Aug 27 |
| Dallas–Fort Worth, TX | Mar 8 | Feb 8 – Feb 22 | Aug 21 |
| Seattle, WA | Mar 8 | Feb 8 – Feb 22 | Aug 17 |
| Portland, OR | Mar 16 | Feb 16 – Mar 2 | Aug 13 |
| Atlanta, GA | Mar 20 | Feb 20 – Mar 6 | Aug 17 |
| Birmingham, AL | Mar 27 | Feb 27 – Mar 13 | Aug 8 |
| Oklahoma City, OK | Mar 27 | Feb 27 – Mar 13 | Aug 5 |
| Washington, DC | Mar 28 | Feb 28 – Mar 14 | Aug 13 |
| New York City | Apr 1 | Mar 4 – Mar 18 | Aug 14 |
| Philadelphia, PA | Apr 2 | Mar 5 – Mar 19 | Aug 6 |
| Louisville, KY | Apr 3 | Mar 6 – Mar 20 | Jul 31 |
| Raleigh, NC | Apr 3 | Mar 6 – Mar 20 | Jul 31 |
| St. Louis, MO | Apr 3 | Mar 6 – Mar 20 | Jul 30 |
| Charlotte, NC | Apr 4 | Mar 7 – Mar 21 | Jul 30 |
| Richmond, VA | Apr 4 | Mar 7 – Mar 21 | Jul 31 |
| Boston, MA | Apr 6 | Mar 9 – Mar 23 | Aug 4 |
| Nashville, TN | Apr 6 | Mar 9 – Mar 23 | Jul 29 |
| Kansas City, MO | Apr 12 | Mar 15 – Mar 29 | Jul 19 |
| Columbus, OH | Apr 16 | Mar 19 – Apr 2 | Jul 23 |
| Indianapolis, IN | Apr 16 | Mar 19 – Apr 2 | Jul 18 |
| Des Moines, IA | Apr 17 | Mar 20 – Apr 3 | Jul 13 |
| Detroit, MI | Apr 20 | Mar 23 – Apr 6 | Jul 20 |
| Omaha, NE | Apr 21 | Mar 24 – Apr 7 | Jul 7 |
| Salt Lake City, UT | Apr 21 | Mar 24 – Apr 7 | Jul 20 |
| Chicago, IL | Apr 22 | Mar 25 – Apr 8 | Jul 15 |
| Cleveland, OH | Apr 22 | Mar 25 – Apr 8 | Jul 27 |
| Hartford, CT | Apr 24 | Mar 27 – Apr 10 | Jul 9 |
| Pittsburgh, PA | Apr 24 | Mar 27 – Apr 10 | Jul 16 |
| Minneapolis, MN | Apr 25 | Mar 28 – Apr 11 | Jul 6 |
| Albany, NY | Apr 28 | Mar 31 – Apr 14 | Jul 6 |
| Grand Rapids, MI | Apr 28 | Mar 31 – Apr 14 | Jul 12 |
| Portland, ME | May 1 | Apr 3 – Apr 17 | Jul 6 |
| Madison, WI | May 2 | Apr 4 – Apr 18 | Jul 1 |
| Burlington, VT | May 4 | Apr 6 – Apr 20 | Jul 4 |
| Denver, CO | May 6 | Apr 8 – Apr 22 | Jul 2 |
| Concord, NH | May 13 | Apr 15 – Apr 29 | Jun 26 |
Fall column: 110 days to harvest counted back from the first fall 32°F date, plus a 14-day allowance for the hardy class that is this site's own assumption (Purdue HO-66-W method, methodology).
3 frost-free stations are left out. Not in the table? The planner takes any pair of frost dates.
Three cities, one rule
- Sacramento, California — last spring frost Feb 9, so plant sets Jan 12 – Jan 26. 297 frost-free days in all.
- Kansas City, Missouri — last spring frost Apr 12, so plant sets Mar 15 – Mar 29. 194 frost-free days in all.
- Concord, New Hampshire — last spring frost May 13, so plant sets Apr 15 – Apr 29. 140 frost-free days in all.
Questions about planting onions
When should I plant onions outside?
Onions are a hardy crop, so Colorado State Extension's rule is 2 to 4 weeks before the last spring frost. In Sacramento, CA that is Jan 12 – Jan 26; in Concord, NH it is Apr 15 – Apr 29. Find your city in the table above, or enter your own frost date in the planner.
Can I start onions indoors?
Onions are planted directly in the garden on this site's calendars, following the Colorado State guide, so no indoor date is computed. The outdoor window for Kansas City, MO is Mar 15 – Mar 29.
What soil temperature do onions need?
The minimum germination soil temperature in Colorado State Extension's guide is 35°F. The frost window is the earliest the crop is safe from freezing; the soil reading tells you whether it will actually grow. Measure at 4 inches in the morning before planting.
How long do onions take to harvest?
Colorado State's typical days to harvest for this row is 110 days. Compare that to your city's frost-free days in the table: Sacramento, CA has 297, Concord, NH has 140.
Can I plant onions for a fall harvest?
39 of the 39 cities in the table show a separate fall planting date for onions, found by counting the days to harvest back from the first fall frost. Where the row says no separate fall sowing, the spring window already covers the season.
Related planting dates
- When to plant garlic — 1 to 2 weeks after the first fall frost
- When to plant potatoes — up to 2 weeks before the last spring frost
- When to plant tomatoes — 2 to 4 weeks after the last spring frost
- All crops · calendars by city · cool-season vs warm-season crops
Sources for this page
| Source | Used for | Read |
|---|---|---|
| NOAA/NCEI U.S. Climate Normals 1991–2020 | 50% probability last spring and first fall 32°F and 28°F dates for each station | 2026-10-16 |
| Colorado State University Extension, CMG GardenNotes #720 Vegetable Planting Guide | spring class rules, transplant age and days to harvest, via crops.json | 2026-10-16 |
| Colorado State University Extension, Vegetable Planting Guide (germination soil temperature table) | figures as quoted in this site's soil-temperature guide (/blog/soil-temperature-for-planting-vegetables/); the page could not be re-fetched on 2026-10-23 and the 2026-10-17 reading stands; the same minimum germination figures (tomato 50, pepper 60, potato 45, onion 35, cucumber and summer squash 60°F, read at 8 a.m. at 4 inches) appear in the soil temperature table of CMG GardenNotes #720, re-read 2026-10-23 | 2026-10-17 |
Every date in the table is arithmetic on the station's NOAA normal and the offset stated by the source; the quoted sentences are the only figures on this page that are not computed. Normals are 30-year averages and describe the station, not your yard. Rules checked 2026-10-23; see the methodology page.