Onions take 16 to 24 weeks to grow, depending on whether you start from seeds or sets and which onion type you plant. Seed-sown onions started in January or February need up to 24 weeks, while sets planted out in March or April are ready in as little as 16 weeks, says Morris Hankinson, director of Hopes Groves Nurseries. The crop is ready to harvest when the leaves yellow and flop over and the bulbs feel firm above the soil. Planting time, location, drainage, and steady moisture all decide whether those weeks end in a full harvest or a row of small bulbs.
Because the season runs half a year for some crops, the planting date sets the harvest date, and getting the calendar right matters more than most garden chores. The same reasoning applies outside the garden: shed rental terms and durability decisions hinge on how long a structure must last, and an onion planting window is a duration decision in the same spirit.
How Long Onions Take to Grow
The full timeline runs 16 to 24 weeks from planting to harvest. Seeds take the longest because the plant spends its first weeks as a small seedling, while sets skip that stage. Sets are small, dormant bulbs grown the previous season, and they give the crop a head start of several weeks. Transplants, which are seedlings grown indoors and moved out, land between the two.
Long projects reward careful planning, whether it is timber home planning that stretches over a decade or an onion crop that occupies a garden bed for nearly half a year. The principle is the same: the longer the timeline, the more the early decisions matter.
Seeds, sets, and transplants compared
| Starting method | When to plant | Weeks to harvest |
|---|---|---|
| Seeds sown indoors | January to February | 20 to 24 |
| Direct-sown seeds | Early spring, soil workable | 20 to 24 |
| Sets planted out | March to April | 12 to 16 |
| Transplants | After the last frost | 12 to 16 |
Day length drives the schedule
Bulbing onions are sorted into three types by daylight requirement. Short-day onions need 10 to 12 hours of daylight, intermediate-day onions 12 to 14 hours, and long-day onions 14 to 16 hours. The plant starts forming a bulb only when the day length crosses its threshold, so planting the wrong type for your latitude delays or stalls bulbing entirely.
What happens during bulbing
Before bulbing, the onion puts energy into leaves. When daylight crosses the threshold, growth shifts to the bulb, and the leaves gradually yellow and fall over as the bulb matures. That transition is why the harvest sign is in the tops, not the clock.
Short-Day, Intermediate-Day, and Long-Day Onions
Match the onion type to your daylight hours and zone, because the categories exist for a reason. Southern gardens get short winter days and warm temperatures, northern gardens get long summer days, and the middle of the country sits between.
| Type | Daylight needed | Best zones | Typical planting | Harvest window |
|---|---|---|---|---|
| Short-day | 10 to 12 hours | 7 and warmer | Fall | Late spring |
| Intermediate-day | 12 to 14 hours | 5 to 6 | Early spring | Midsummer |
| Long-day | 14 to 16 hours | 6 and cooler | Spring | Late summer |
Short-day onions for the South
Short-day onions suit zones 7 and warmer, where winters are mild. They are usually planted in fall and harvested in late spring, before summer heat arrives. In these regions, a spring-planted long-day onion never sees the daylight hours it needs.
Intermediate-day onions for the middle of the country
Intermediate-day onions need 12 to 14 hours of daylight and grow best in zones 5 to 6, the band that runs across the middle of the United States. They are planted in early spring and harvested in midsummer.
Long-day onions for the North
Long-day onions need 14 to 16 hours of daylight and do best in northern zones 6 and cooler. They are planted in spring as soon as the soil can be worked and mature in late summer. For a step-by-step planting walkthrough, Bob Vila’s how to grow onions guide covers bed preparation, spacing, and watering in more detail.
The Onion Calendar: From Sowing to Harvest
Work backward from your expected harvest date to set every planting step. The sequence below assumes a typical spring planting in a temperate zone.
- Choose the day-length type that matches your zone
- Decide between seeds, sets, or transplants
- Sow seeds indoors 8 to 10 weeks before the last frost date
- Harden off seedlings and transplant 3 to 4 weeks before the last frost
- Plant sets as soon as the soil is workable in early spring
- Note the days to maturity printed on the seed packet
- Harvest when at least a third of the tops have yellowed and flopped
Starting seeds indoors
Seed starting gives the widest variety choice. Sow seeds 8 to 10 weeks before the last frost in a seed tray with good drainage, keep the soil around 60 to 70 degrees Fahrenheit, and keep the seedlings under bright light so they stay stocky instead of leggy.
Planting sets in spring
Sets go straight into the garden bed, planted about 1/2 to 1 inch deep and 4 to 6 inches apart. They tolerate cool soil, so they can go in as soon as the ground thaws and dries enough to work. Schedules are easier to keep when they are written down, whether you are tracking log home build timelines with contractors or noting planting dates in a garden journal.
Soil, Sun, and Spacing
Onions need full sun, at least six hours a day, and loose, well-draining soil with a pH between 6.0 and 6.8. Soggy soil rots bulbs, and heavy clay that stays wet is the most common cause of failure. Raised beds solve most drainage problems before they start.
Preparing the bed
Loosen the soil 8 to 10 inches deep and mix in compost a few weeks before planting. Avoid fresh manure, because excess nitrogen grows tall, leafy tops at the expense of bulbs. A soil test tells you whether phosphorus and potassium need attention before planting.
Spacing and depth
Space sets 4 to 6 inches apart in rows about 12 inches apart, and press them in so the tip sits just below the surface. Seeds go about 1/4 inch deep. Crowded onions compete for light and water and end up small.
A garden bed is a small structure, and the same support logic that governs a floor system applies: builders weigh floor joists versus rafter ties based on the loads they carry, and gardeners match bed depth and drainage to the water load the soil will hold.
Watering and Moisture Management
Onions need steady moisture, about 1 inch of water per week, and the soil should never dry out completely during bulb formation. At the same time, standing water invites rot, so drainage and watering work together.
Watering through the season
Water deeply and less often rather than a light sprinkle daily, which encourages shallow roots. Mulch with straw or grass clippings to hold moisture and suppress weeds, since onion roots are shallow and cannot outcompete weeds.
When to stop watering
As the tops begin to yellow and fall, taper off watering. Dry soil at the end of the season helps the bulbs cure and store longer. Moisture management is a discipline shared with building science, where spa room moisture control depends on ventilation and vapor barriers to keep humidity in check.
Harvest Signs and Storage
The crop is ready when the leaves yellow and flop over and the bulbs feel firm above the soil line. Check for all three signs before lifting the whole row:
- Tops yellow and flop over at the neck
- Bulbs feel firm and push up above the soil line
- The neck feels soft and bends easily
Curing and storing
Lift the bulbs on a dry day, leave the tops attached, and cure them in a warm, dry, airy spot out of direct sun for two to three weeks until the necks feel papery. After curing, trim the roots and cut the tops back to about an inch. Store cured onions in a cool, dry, dark place with air movement; mesh bags or braided strings work well. Check stored bulbs regularly and pull out any that soften, because one soft bulb can spoil a whole batch.
The onion season runs 16 to 24 weeks, and the harvest is set by decisions made months earlier: the right day-length type, the right starting method, and a bed that drains. Gardeners who map the calendar before planting get the same payoff as builders who settle the loads and schedule first, the way engineers do when they engineer a 19 foot floor span for a long-span floor. Plant on time, keep the moisture steady, and the flop of the tops tells you when the weeks are up.