Plant onion sets in spring or fall, pointed end up, about 1 inch deep and 4 to 6 inches apart
Onion sets are small, immature onion bulbs you buy from a garden center or seed catalog. They are easier to grow than seeds because they skip the first months of slow growth and go straight to bulbing. In a raised bed, you push each set into loose soil so the tip just shows, space them a few inches apart, and water them in. They need full sun and consistent moisture, and they will be ready to harvest in 8 to 12 weeks depending on the variety and your climate.
The main advantage of sets over seeds is speed and reliability. A set that costs a few cents will give you a full onion in less than three months. The trade-off is that sets offer fewer varieties than seeds do, and they sometimes bolt (flower early) if you plant them in spring and then hit a cold spell. Fall planting avoids this problem in most climates.
Key Takeaways
- Plant onion sets in early spring or early fall, with the pointed end facing up and the root end in the soil.
- Space sets 4 to 6 inches apart in rows, or scatter them across the bed if you prefer a less formal layout.
- Push each set into the soil so only the tip shows above the surface; burying them too deep slows growth and can cause rot.
- Water the bed right after planting and keep the soil consistently moist but not waterlogged for the first two weeks.
- Onion sets need at least six hours of direct sun daily and will mature in 8 to 12 weeks depending on variety and temperature.
Timing: Spring or fall planting, depending on your climate
The best time to plant onion sets depends on where you live. In most of the United States, early spring (as soon as the soil is workable, usually March or April) works well. Plant sets when the soil temperature is around 50°F or higher. If you plant too early in cold, wet soil, the sets may rot before they root.
Fall planting (August through October) is an alternative in mild climates and often produces larger onions because the plants have more time to establish roots before winter. In cold climates where the ground freezes hard, fall-planted sets may heave out of the soil during freeze-thaw cycles, so spring planting is safer. Check what works best for your hardiness zone by asking at a local garden center or checking your county extension office.
Preparing the raised bed soil before planting
Onion sets do not need rich soil, but they do need loose, well-draining soil. If your raised bed is filled with compacted garden soil or clay, loosen it with a garden fork or tiller to at least 8 inches deep. Mix in compost or aged manure if the soil is poor, but do not overdo nitrogen—too much nitrogen makes onions grow leaves instead of bulbs.
If you are starting a new raised bed, fill it with a mix of topsoil and compost in roughly equal parts. Onions prefer a soil pH between 6.0 and 7.0, which is neutral to slightly acidic. If you have never tested your soil, a straightforward kit from a garden center will tell you the pH. Most raised beds with compost-amended soil fall into the right range without adjustment.
Spacing and depth: How to position each set
Push each onion set into the soil so the pointed end faces up and only the tip shows above the surface. The root end (the flat, hairy part) should be fully buried. Space sets 4 to 6 inches apart in all directions. If you are planting multiple rows, leave 12 inches between rows so you can reach in to weed and harvest.
Do not bury sets too deep. If you push them down so the whole bulb is covered, they will take longer to sprout and may rot in wet soil. The tip showing above ground is normal and correct. As the onion grows, the bulb will push up and eventually sit partly above the soil surface, which is fine.
If you prefer a less formal layout, you can scatter sets across the bed at random, as long as each one has 4 to 6 inches of space around it. This works especially well in smaller raised beds where you want to maximize the number of plants.
Watering and moisture after planting
Water the bed thoroughly right after planting so the soil settles around each set and the roots make contact with the soil. For the first two weeks, keep the soil consistently moist—not soggy, but not dry either. Check the top inch of soil; if it feels dry, water again.
Once the sets have sprouted and are growing actively (usually within two to three weeks), you can back off slightly. Onions need about 1 inch of water per week from rain or irrigation. In a raised bed, which drains faster than in-ground soil, you may need to water more often in hot weather. The soil should never dry out completely, but it should not stay waterlogged either. Waterlogged soil causes rot and fungal disease.
Sunlight and temperature needs
Onions need at least six hours of direct sun daily, and eight hours is better. In a raised bed, position it where it gets morning sun, which also helps dry dew and reduce disease. Onions are cold-hardy and can tolerate light frost, so they are not fussy about temperature once they are established. Spring-planted sets may bolt if they experience a prolonged cold spell (below 50°F for several weeks) followed by warm weather, but this is rare and more common in short-day varieties.
If you are in a very hot climate, afternoon shade during the hottest part of summer can prevent the soil from drying out too fast, but do not let shade reduce the total sun below six hours.
Fertilizing and ongoing care
Onion sets do not need much fertilizer if your soil has compost mixed in. If the plants look pale or weak after four weeks, sprinkle a balanced fertilizer (such as 10-10-10) around the base of each plant, following the package rate. Water it in well. Avoid high-nitrogen fertilizers, which push leaf growth at the expense of bulb size.
Weeds compete with onions for water and nutrients, so pull them by hand or hoe gently around the plants every week or two. Be careful not to disturb the onion roots. As the bulbs grow and push up out of the soil, you can lightly hill soil around them if you want to keep them covered, but this is optional.
Harvesting onions from your raised bed
Onions are ready to harvest when the tops fall over and turn brown, usually 8 to 12 weeks after planting depending on the variety and temperature. You can harvest them at any size—pull a small one early if you want a spring onion, or wait until the tops die back for a full-sized storage onion.
To harvest, loosen the soil around the bulb with a garden fork and pull gently. Lay the onions on the soil surface for a few days to dry if the weather is dry, then move them to a cool, dry place to cure for two to three weeks before storing. Cured onions keep for months in a cool basement or pantry.
Frequently Asked Questions
Can I plant onion sets in summer?
Summer planting is not recommended in most climates because the heat and long days trigger bolting (flowering) instead of bulb growth. Stick to spring or fall. In very cool climates, late summer (mid-August) can work, but check with your local extension office first.
What if my onion sets are sprouting before I plant them?
Sets that have already sprouted will still grow, but they may be weaker. Plant them as soon as possible, pointed end up, at the same depth as unsprouted sets. Water them in and keep the soil consistently moist for the first few weeks.
How do I know which onion varieties grow best in my area?
Onion varieties are classified as short-day, intermediate-day, or long-day based on the hours of daylight they need to bulb. Short-day varieties work in the South, long-day in the North, and intermediate-day in the middle. Seed catalogs and garden centers label sets by type, or ask your county extension office which varieties perform best locally.
Do I need to thin onion sets after they sprout?
No. Unlike seeds, sets are already the right size and spacing. If you planted them 4 to 6 inches apart, they will not need thinning. If you planted them closer together by accident, you can pull every other one early as a spring onion and leave the rest to mature.
What causes onion sets to rot in the ground?
Rot usually happens when sets are planted too deep, buried in cold wet soil before they root, or left in waterlogged soil. Plant sets so only the tip shows, wait until soil is at least 50°F, and make sure your raised bed drains well. If rot is a recurring problem, amend the soil with more compost to improve drainage.