When to plant onions by city
Start onions indoors 8–10 weeks before your last frost, plant them out 2–4 weeks before your last frost. Here is what that means for 2027 in cities across the US, using each city’s average frost dates (NOAA 1991–2020). Pick the city with the climate most like yours, or enter your ZIP code above.
| City | Last frost | Start indoors | Plant out |
|---|---|---|---|
| Seattle, WA (9a) | Mar 13 | Jan 2–16 | Feb 13–27 |
| Portland, OR (8b) | Feb 25 | Dec 17–31 | Jan 28 – Feb 11 |
| Sacramento, CA (9b) | Jan 12 | Nov 3–17 | Dec 15–29 |
| Phoenix, AZ (10a) | Jan 5 | Oct 27 – Nov 10 | Dec 8–22 |
| Salt Lake City, UT (7b) | Mar 31 | Jan 20 – Feb 3 | Mar 3–17 |
| Denver, CO (6a) | Apr 30 | Feb 19 – Mar 5 | Apr 2–16 |
| Dallas, TX (8b) | Mar 2 | Dec 22 – Jan 5 | Feb 2–16 |
| Minneapolis, MN (5a) | Apr 23 | Feb 12–26 | Mar 26 – Apr 9 |
| Kansas City, MO (6b) | Apr 4 | Jan 24 – Feb 7 | Mar 7–21 |
| Chicago, IL (6b) | Apr 10 | Jan 30 – Feb 13 | Mar 13–27 |
| Columbus, OH (6b) | Apr 20 | Feb 9–23 | Mar 23 – Apr 6 |
| Nashville, TN (7b) | Apr 1 | Jan 21 – Feb 4 | Mar 4–18 |
| Atlanta, GA (8a) | Mar 24 | Jan 13–27 | Feb 24 – Mar 10 |
| Raleigh, NC (8a) | Mar 28 | Jan 17–31 | Feb 28 – Mar 14 |
| Philadelphia, PA (7b) | Mar 30 | Jan 19 – Feb 2 | Mar 2–16 |
| Boston, MA (7a) | Apr 4 | Jan 24 – Feb 7 | Mar 7–21 |
In frost-free parts of Florida, southern California and the Gulf Coast, timing is about heat instead: grow onions from fall through winter.
Onion spacing
In a square-foot bed, plant 9 per square foot. In traditional rows, space plants about 4 inches apart with 12 inches between rows.
Pick long-day varieties in the North, short-day in the South.
How to grow onions
Onions are easy to grow but particular about day length: bulbs form only when days reach a certain length, so the right type for your latitude matters more than anything else. Choose long-day varieties in the northern half of the country, short-day varieties in the South, and intermediate types in between.
Sun and soil
Full sun and loose, well-drained soil rich in organic matter, pH about 6.0–7.0. Roots are shallow and sparse, so keep soil evenly moist, about 1 inch of water a week.
Planting
- Start seeds indoors 8–10 weeks before transplanting, ¼–½ inch deep, or buy transplants or sets.
- Plant 2–4 weeks before the last spring frost when soil is at least 45°F. In mild-winter areas, short-day onions are often planted in fall instead.
- Set transplants or sets about 1 inch deep, 4 inches apart in rows 12 inches apart. Sets larger than about ¾ inch tend to bolt.
Care
- Keep beds weed-free; onions compete poorly.
- Side-dress with nitrogen early in the season while plants are making leaves, and stop once bulbs begin to swell.
- Water steadily during bulbing, then stop when tops begin to fall over so bulbs can cure.
- Mulch lightly to hold moisture, but keep it off the bulbs as they form.
Harvest
Bulbs are ready when most tops have fallen over. Lift them, then cure in a dry, shaded, airy spot for 1–3 weeks until necks are tight and skins papery. Trim tops to about 1 inch. Pungent storage types keep for months in a cool, dry place; sweet onions keep only a few weeks.
Common problems
- Onion thrips: Silvery streaks on leaves in hot, dry weather. Spray plants with a strong stream of water, keep weeds down, and encourage predators.
- Onion maggot: Maggots tunnel into seedlings and bulbs. Rotate alliums, avoid fresh manure, and use row cover in spring.
- Bolting: Large sets, early planting into a cold spell, or the wrong day length. Use small sets and the right type for your region.
- Neck rot in storage: Watery decay inside stored bulbs. Stop watering before harvest and cure thoroughly.
Varieties to try
- 'Walla Walla' — sweet long-day onion, short storage
- 'Copra' — long-day yellow storage onion
- 'Candy' — intermediate-day sweet yellow, widely adapted
- 'Texas Early Grano' — short-day sweet onion for the South
Crop rotation
Onions belong to the onion family (Amaryllidaceae). Onion maggot and white rot persist. Rotate for 3–4 years. Relatives that count as the same family for rotation: garlic, leek, chives, green onion.
Frequently asked questions
When should I plant onions?
What grows well with onions?
How far apart should I plant onions?
How long do onions take to grow?
Can onions handle frost?
Sources
- Frost dates: NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020.
- Hardiness zones: 2023 USDA Plant Hardiness Zone Map, USDA-ARS and PRISM Climate Group, Oregon State University.
- Planting windows and spacing: Vegetable Planting Guide (GardenNotes #720), Colorado State University Extension, and other land-grant extension guides.
- Gardening with Companion Plants (EM128E), Linda Chalker-Scott, Washington State University Extension, 2023
- Onions in the Garden — Utah State University Extension
- Onion, Leek, Shallot & Garlic — Clemson Cooperative Extension (HGIC)