How to Grow Onions in the USA
How to grow onions in the USA comes down to your own frost dates rather than the calendar — the average last spring frost shifts by roughly two months between USDA zone 3 and zone 10. Start indoors 10-12 weeks before the average last spring frost, or plant sets/transplants 2-4 weeks before the average last spring frost. Give them full sun, space them 3-4 in within row, 12-18 in between rows, and expect 100-120 days to maturity. Your hardiness zone is how you look up your frost dates — it does not by itself tell you how long your growing season is.
When to plant onions in the USA
Find your average last spring frost date, then count backward or forward from it. The table below gives the typical frost window for each of the three US climate bands, but your own local date is what matters — elevation and proximity to water shift it by weeks within a single zone.
| Climate band | Average last spring frost | When to start this crop | Days to maturity |
|---|---|---|---|
|
Cold band USDA zones 3-5 |
mid-May to early June | Start indoors 10-12 weeks before the average last spring frost, or plant sets/transplants 2-4 weeks before the average last spring frost | 100-120 days to maturity |
|
Mild band USDA zones 6-7 |
late March to late April | Start indoors 10-12 weeks before the average last spring frost, or plant sets/transplants 2-4 weeks before the average last spring frost | 100-120 days to maturity |
|
Hot band USDA zones 8-10 |
February to mid-March (frost-free in the far south) | Start indoors 10-12 weeks before the average last spring frost, or plant sets/transplants 2-4 weeks before the average last spring frost | 100-120 days to maturity |
💡 Zone is a lookup, not a schedule
USDA hardiness zones describe average winter minimum temperature. They tell you what survives your winter, not how long your summer is. Use your zone to find your frost dates, then time this crop from those dates.
Soil and site
Well-drained soil, pH 6.0-7.0. Light feeder.
Watering
Even, consistent moisture through bulb formation; taper off as tops begin to fall over.
Spacing and layout
Space onions 3-4 in within row, 12-18 in between rows. Crowding is the most common cause of disappointing yields — plants compete for light and water, and poor airflow between them raises disease pressure sharply.
Companion planting
Carrots, beets, lettuce, tomatoes.
Common problems
Onion maggot and thrips. Day-length is variety-specific — long-day varieties suit northern regions, short-day varieties suit the Deep South and Southwest; the wrong choice for your latitude will not bulb properly.
Two regional patterns are worth knowing. East of the Rockies and along the Gulf, warm humid nights drive fungal disease, so spacing for airflow and watering at soil level matter more than anywhere else. West of roughly the 100th meridian, almost nothing succeeds on rainfall alone, and consistent irrigation is a design decision rather than a fallback.
Turn growing onions into a written plan. The USA Vegetable Garden Planner (printable PDF) has frost-date planning pages, bed layouts, crop rotation, a sowing log and a harvest tracker — 74 printable pages.
Frequently Asked Questions
When should I plant onions in the USA?
Time it from your average last spring frost rather than a fixed date. Start indoors 10-12 weeks before the average last spring frost, or plant sets/transplants 2-4 weeks before the average last spring frost. Look up your frost date using your USDA zone and local extension data, then count from there.
How far apart should onions be planted?
Space them 3-4 in within row, 12-18 in between rows. Closer spacing reduces airflow and raises disease pressure, which matters especially in humid eastern and southern regions.
How long do onions take to grow?
Allow 100-120 days to maturity. Add a couple of weeks for fall plantings, when shorter days slow growth noticeably.
What usually goes wrong with onions?
Onion maggot and thrips. Day-length is variety-specific — long-day varieties suit northern regions, short-day varieties suit the Deep South and Southwest; the wrong choice for your latitude will not bulb properly.
Last updated 25 August 2026
Tools That Help With This Crop
Use these links when the guide moves from reading into sowing, planting, watering, and keeping the crop healthy.
Plan the next step
Use the printable SoilCommander planner for your region to turn this guide into sowing dates, bed layouts and weekly garden tasks.