Plant onions 4 to 6 inches apart in a raised bed

Space onion sets or transplants 4 to 6 inches apart in rows, with rows themselves 12 inches apart. This spacing gives each bulb room to expand underground without competing for nutrients or moisture. If you are planting from seed, thin seedlings to the same distance once they reach 2 inches tall—this step matters more than most gardeners realize, because crowded seedlings will produce small bulbs no matter how much you water or fertilize later.

The exact spacing depends on the onion variety and your goal. Large storage onions like 'Yellow Sweet Spanish' need the full 6 inches; smaller varieties like 'Walla Walla' or 'Candy' can work at 4 inches. If you want very large bulbs for fresh eating, space them 6 inches apart. If you are growing bunching onions (scallions) for green onion harvests, you can plant them 2 to 3 inches apart and pull every other one as you cook, leaving the rest to mature.

Key Takeaways

  • Space onion sets or transplants 4 to 6 inches apart in rows that are 12 inches apart, depending on the variety and the bulb size you want.
  • Thin onion seedlings to 4 to 6 inches apart once they reach 2 inches tall, because crowded seedlings will not produce full-sized bulbs later.
  • Plant onion sets with the pointed end up and the root end down, pressing them into the soil so the tip just shows above the surface.
  • Bunching onions (scallions) can be planted closer at 2 to 3 inches apart and thinned by harvesting every other plant as you need them.
  • Raised beds warm up faster than ground soil in spring, so you can plant onions 2 to 3 weeks earlier than you would in a traditional garden.

How to plant onion sets at the right depth

Onion sets are small bulbs sold at garden centers, and they are the fastest route to a harvest. Push each set into the soil so the pointed end faces up and the root end (the flat, hairy part) faces down. The tip of the set should sit just barely above the soil surface—if you bury it completely, it may rot; if you leave too much exposed, it will dry out and fail to root.

Press the set down firmly so it makes good contact with the soil below, but do not crush it. The soil should feel snug around the set, not loose. Water when ready after planting so the soil settles and the roots make contact with moisture. In a raised bed, this usually means a thorough soaking that wets the soil 6 inches deep.

Thinning onion seedlings from seed

If you are starting from seed, sow seeds directly into the raised bed in early spring or fall, depending on your climate. Plant seeds about half an inch deep and keep the soil consistently moist until they sprout, which takes 7 to 10 days. Once seedlings reach 2 inches tall, thin them by pinching off or cutting the weaker ones at soil level—do not pull them out, because pulling disturbs the roots of the seedlings you want to keep.

Thin in stages if you are unsure which seedlings are strongest. Remove every other seedling first, leaving them 2 to 3 inches apart. Two weeks later, thin again to your final spacing of 4 to 6 inches. This two-step approach gives you a chance to see which plants are growing fastest and healthiest before you commit to your final layout.

Why spacing matters more in a raised bed

Raised beds have finite soil volume, so crowding onions has a bigger impact than it would in a large garden plot. Each onion draws nutrients and water from a smaller zone, and poor air circulation between plants increases the risk of fungal disease. Onions also need room to push outward as the bulb swells, and tight spacing forces them to grow upward instead, producing tall thin bulbs rather than wide ones.

The raised bed advantage is that you control the soil quality completely. Use a mix of compost, peat moss or coconut coir, and garden soil—this blend drains well while holding moisture and nutrients. Onions prefer slightly acidic soil with a pH between 6.0 and 6.8, and raised beds make it straightforward to adjust the mix before planting rather than trying to amend a large garden area.

Adjusting spacing for your bed size and shape

A standard 4-by-8-foot raised bed holds two rows of onions spaced 12 inches apart, with 4 to 6 inches between plants in each row. This layout gives you 10 to 12 onions per row, or 20 to 24 plants total. If your bed is narrower—say, 2 feet wide—you can fit one row down the center with 6-inch spacing, or two rows 12 inches apart if you do not mind reaching to the middle. Narrower beds are actually easier to manage because you can reach every plant without stepping on the soil.

If you have an odd-shaped bed or want to mix onions with other crops, remember that onions are shallow-rooted and do not shade other plants much. They pair well with lettuce, spinach, or beets planted between the rows. Just make sure the companion plants do not crowd the onions or compete for water during the bulbing stage (usually the last 4 to 6 weeks before harvest).

Common spacing mistakes and how to avoid them

The most common error is planting sets too close together because they look small and seem like they should not take up much space. A set the size of a marble will produce a bulb the size of a tennis ball if given room; crowding it produces a bulb the size of a golf ball. Resist the urge to squeeze in extra plants—you will harvest more total weight from properly spaced onions than from a crowded bed.

The second mistake is forgetting to thin seedlings at all. Onion seeds are tiny and straightforward to oversow, and many gardeners assume the seedlings will sort themselves out. They do not. Crowded seedlings stay thin and weak, and thinning them feels wasteful even though the thinned seedlings are too small to eat. Thin anyway—the remaining plants will reward you with full-sized bulbs.

A third mistake is planting sets upside down or sideways. The pointed end really does need to face up; sets planted sideways or inverted will eventually right themselves, but they waste energy doing it and may rot before they recover. Take the extra second to orient each set correctly.

Timing your planting for your climate

Onions are a cool-season crop and grow best when temperatures are between 55 and 75 degrees Fahrenheit. In most climates, plant onion sets or transplants in early spring as soon as the soil is workable—usually 2 to 3 weeks before your last frost date. Raised beds warm up faster than ground soil, so you can often plant 2 to 3 weeks earlier than gardeners with traditional beds.

In mild climates (zones 8 and warmer), you can also plant onions in fall for a spring harvest. Fall planting actually produces larger bulbs in these areas because the plants have a longer growing season. Plant in late summer or early fall, 8 to 10 weeks before your first frost date, so the plants are established before cold weather arrives.

Frequently Asked Questions

Can I plant onions closer together if I harvest some early as scallions?

Yes. Plant at 3 inches apart and begin harvesting every other plant when they reach 6 to 8 inches tall. This gives the remaining plants more room to develop into full bulbs. You get a scallion harvest in early summer and mature onions in late summer from the same bed.

What happens if I plant onion sets too deep?

Sets planted too deep often rot before they root, especially in cool, wet spring weather. If the tip is buried more than a quarter inch below the surface, the set may not sprout at all. Plant so the tip just barely shows above the soil.

Do I need to space red onions differently than yellow onions?

No. Red, yellow, and white onions all need the same 4 to 6 inch spacing. Spacing depends on bulb size, not color. Large storage varieties need more room than small sweet varieties, regardless of color.

How many onions can I fit in a 4-by-4-foot raised bed?

A 4-by-4 bed holds two rows spaced 12 inches apart, with 4 to 6 inches between plants in each row. That gives you 10 to 12 onions per row, or 20 to 24 plants total, depending on whether you space them at 4 or 6 inches.

Should I fertilize onions more if I space them closer together?

No. Closer spacing does not make up for crowding—it just produces smaller bulbs. Stick to proper spacing and fertilize normally. Onions need moderate nitrogen in spring and more phosphorus and potassium as they bulb in summer.