Harvest guide
When to Plant Onions (Spring Timing by Zone)
Plant onions as early as the soil can be worked in spring, about 2 to 4 weeks before your last frost, once soil hits 50 F. In the Deep South, short-day onions go in during fall instead. Match the onion type to your latitude.
Days to maturity
90–110days
Ready when
Tops flop over and yellow
The short answer
Plant onions in early spring, 2 to 4 weeks before your last frost, as soon as the soil can be worked and reaches about 50 F. Onions shrug off light frost, so early is safe. In the Deep South, plant short-day onions in fall instead. Match the onion type to your latitude.
Onions are a cool-season crop, which is why timing feels backwards if you are used to tomatoes. You want them in the ground while it is still chilly, growing leaves in cool weather, so they can bulb up when the days get long. Get the timing and the onion type right, and the rest is mostly watering and weeding. This guide covers when to plant by zone, which type fits your region, and how to tell the soil is ready.
When to plant onions by zone
The rule is the same everywhere: plant as early in spring as the ground can be worked. Onions handle cold well, so the calendar is driven by your soil thawing, not by frost dates. University of Maryland Extension says bulb and green onions can withstand heavy spring frosts, so you are not waiting on warmth.
Colder zones thaw later, so they plant later. The windows below are starting points. Watch your own soil, not the calendar.
| Region / zone | Typical planting window | Method |
|---|---|---|
| Cold (zones 3–5) | Late March – early May | Sets or transplants, once soil thaws |
| Temperate (zones 6–7) | February – April | Sets, transplants, or direct seed |
| Mild South (zones 8–10) | October – November (fall) | Short-day transplants overwinter |
The South is the exception that trips people up. In zones 8 to 10, short-day onions go in during fall, grow through winter, and bulb in spring. Alabama Extension treats onions as a cool-season crop planted for that overwintering window.
Match the onion type to your latitude
This is the step most planting guides skip, and it decides whether you get bulbs at all. Onions bulb based on day length, and the right type depends on how far north or south you garden.
- Long-day onions need 14 to 16 hours of summer daylight to bulb. They are the northern choice, roughly zone 6 and colder.
- Short-day onions bulb at just 10 to 12 hours of daylight. They suit the South, zone 7 and warmer, and are the ones you fall-plant.
- Intermediate (day-neutral) onions bulb around 12 to 14 hours and fit the middle of the country, about zones 5 and 6.
Plant a short-day onion up north and it bulbs too early, before the plant is big, giving you tiny onions. Plant a long-day onion in the Deep South and it never gets enough daylight to bulb at all. University of Minnesota Extension recommends long-day types for the North for exactly this reason.
Pro tip
When you order seed or sets, the catalog labels each variety long-day, short-day, or intermediate. Buy by that label, not by color or name. A "Yellow Sweet Spanish" (long-day) and a "Texas Sweet" (short-day) look similar in the bag but want gardens a thousand miles apart.
How to tell the soil is ready
Two signals tell you it is time, and neither is a date on the calendar.
First, the soil can be worked. Squeeze a handful. If it crumbles, plant. If it packs into a wet ball, wait a few days. Onions rot in cold, soggy ground.
Second, the soil temperature. Onion seed germinates once soil passes about 35 F (per Alabama Extension charts), but it sprouts slowly until it warms up. University of Minnesota Extension times transplants to go out once soil reaches 50 F, which is the practical target for strong early growth.
| Planting stock | When to plant | Target soil temp |
|---|---|---|
| Seed indoors | 8–10 weeks before transplanting | Room temp to start |
| Sets | 2–4 weeks before last frost | Soil workable, ~40 F+ |
| Transplants | 2–4 weeks before last frost | About 50 F |
Because onions take their cold-weather growth and turn it into bulb size later, an early start is an advantage. A bigger plant at the start of summer makes a bigger bulb.
Spring vs fall: which one is yours
For most US gardeners, onions are a spring crop. You plant in late winter or early spring, the plant grows leaves through cool weather, and it bulbs as summer days lengthen. The days to maturity runs roughly 90 to 120 days from transplanting, longer from seed.
Fall planting is the Deep South move. In zones 8 to 10, short-day onions set out in October or November overwinter as small plants, then bulb in spring. They are ready to harvest in late spring, well before the summer heat.
The same allium rhythm shows up in when to plant garlic, which is fall-planted almost everywhere. Onions split by region: South in fall, everyone else in spring.
Common mistake
The two errors that cost a crop are both about type and timing. Planting the wrong day-length type for your latitude is the big one, since a short-day onion up north or a long-day onion down south barely bulbs. The second is planting too late in spring, which gives the plant too little cool weather to grow leaves before it has to bulb, so the onions stay small. Buy the right type, and get it in as early as the ground allows.
Get the spacing right before you plant
Spacing decides bulb size as much as timing does. Onions sized too close compete and stay small, so give each one room from the start. The guide to how far apart to plant onions lays out the in-row and between-row numbers, and the plant spacing calculator counts how many fit your bed.
Try it — Plant Spacing Calculator
Full calculatorExtra to cover losses (10% is typical).
You can plant
32plants
- Per row
- 8
- Rows
- 4
- Buy (incl. spare)
- 36 plants
Plan the bed before you order, because onion sets and transplants come in bunches, and it is easy to buy more than the bed holds.
Your next step
Plant onions as early as the ground can be worked in spring, 2 to 4 weeks before your last frost, once the soil reaches about 50 F. In the Deep South, plant short-day onions in fall instead. The one decision that matters most is matching the type to your latitude.
Laying out the bed? Open the plant spacing calculator and set onions so every bulb has room to size up.
Common questions
What month is best to plant onions?
In most of the US, plant onions in early spring as soon as the ground can be worked, which lands between February and April depending on your zone. University of Minnesota Extension sets onion transplants out once soil reaches 50 F. In the Deep South, short-day onions go in during fall instead, usually October through November.
Can you plant onions too early in spring?
Onions are cold-hardy, so early is rarely the problem. University of Maryland Extension says bulb and green onions can withstand heavy spring frosts. The bigger risk is planting into cold, soggy soil that does not drain. Wait until the ground can be worked and is not waterlogged, then plant.
Should I plant long-day or short-day onions?
Match the type to your latitude. Long-day onions need 14 to 16 hours of summer daylight and suit the northern US, roughly zone 6 and colder. Short-day onions bulb at 10 to 12 hours and suit the South, zone 7 and warmer. Intermediate or day-neutral types fit the middle, around zones 5 and 6.
When do you plant onions in the South?
In the Deep South and Gulf Coast (zones 8 to 10), plant short-day onion transplants in fall, usually October through November. They grow through winter and bulb in spring as days lengthen, ready to harvest in late spring. Cooler southern areas can also plant in late winter.
Can I grow onions from seed, sets, or transplants?
All three work. Sets (small dormant bulbs) and transplants go out 2 to 4 weeks before the last frost. For seed, University of Minnesota Extension says start indoors about 8 to 10 weeks before transplanting outdoors. Sets are the easiest for beginners, transplants give the most reliable big bulbs.
Sources
Agronomic claims in this guide are checked against these primary sources.
- Growing onions and other alliums — University of Minnesota Extension
- Onions — University of Maryland Extension
- The Alabama Vegetable Garden - Onions — Alabama Cooperative Extension System
- Onion day-length types and planting — Harvest to Table
Keep reading
When to Harvest Onions (Signs They're Ready)
Storage onions are ready when about half to most of the tops have flopped over and the necks soften, usually around 90 to 110 days. Here are the signs, plus how to lift, cure, and store them.
Read →When to Plant Garlic (Fall Timing by Zone)
Plant garlic in fall, about 3 to 6 weeks before the ground freezes, so roots set before winter. That lands mid-September to November across most US zones. Plant cloves 2 inches deep, pointy end up, 4 to 6 inches apart.
Read →When to Plant Tulip Bulbs (by USDA Zone)
Plant tulip bulbs in fall once soil at 6 inches deep cools to 50-60°F, usually September to November depending on your zone. Warm zones need 6-10 weeks of pre-chilling first.
Read →When to Plant Zucchini (Frost + Soil Temp Timing)
Plant zucchini after your last spring frost, once the soil hits at least 60 F (ideally 65 to 70 F). Direct-sow seeds 1/2 to 1 inch deep, or set out transplants started 2 to 4 weeks earlier. Warm zones get a second fall crop.
Read →When to Plant Tomatoes (Frost + Soil Temp by Zone)
Set tomato transplants out 1 to 2 weeks after your last spring frost, once soil hits at least 60 F. Start seeds indoors 6 to 8 weeks before that frost date. Cold soil stalls them, so wait for warmth.
Read →When to Plant Swiss Chard (Spring and Fall Timing)
Plant swiss chard 2 to 4 weeks before your last spring frost, once the soil hits 40°F. For a full fall crop, sow about 9 to 14 weeks before the first fall frost. Seeds go half an inch to an inch deep.
Read →