Plant onion bulbs in fall or early spring, pointed end up, 4 to 6 inches apart in loose soil

Onion bulbs go into the ground pointed end up, root side down, at a depth where the tip of the bulb sits just above the soil surface. Space them 4 to 6 inches apart in rows or a scattered pattern—closer spacing gives you smaller onions, wider spacing gives you larger ones. The soil in your raised bed should be loose and well-draining; onions rot in waterlogged ground. Plant in fall (September through November in most regions) for spring harvest, or in early spring (February through April) for summer harvest.

Onion bulbs are not seeds—they are small, dormant onions that will sprout and grow into full-size onions over several months. This makes them faster and more reliable than starting from seed, especially for a first-time grower. The variety you choose determines the color and flavor, and also the timing: some types are bred for fall planting, others for spring.

Key Takeaways

  • Plant bulbs pointed end up with the tip just above soil level, 4 to 6 inches apart, in loose, well-draining soil.
  • Fall planting (September through November) produces onions ready to harvest in late spring; spring planting produces onions in midsummer.
  • Onion bulbs need consistent moisture during growth but will rot if the soil stays soggy, so water when the top inch of soil is dry.
  • Fertilize every three to four weeks with a balanced fertilizer once green shoots appear, or use compost worked into the soil before planting.
  • Harvest when the tops fall over and dry down, usually 90 to 120 days after planting, depending on variety and season.

Prepare your raised bed soil before planting

Onions prefer soil that drains quickly but holds some moisture. Mix your raised bed soil with compost—about one part compost to three parts existing soil—to add nutrients and improve texture. If your bed drains too slowly (water sits on top after rain), add coarse sand or perlite to loosen it. Onions do not need extremely rich soil; overly fertile beds can produce soft bulbs that do not store well.

Check the soil pH if you have a test kit or can send a sample to your local extension office. Onions grow best in soil between 6.0 and 7.0 pH, which is slightly acidic to neutral. Most garden soils fall in this range, so this is not usually a barrier. If you are using fresh potting mix or compost, you are likely fine.

Choose the right planting time for your region

Onion bulbs are sold as either short-day, intermediate-day, or long-day varieties. This refers to the day length that triggers the bulb to stop growing tops and start swelling underground. Short-day varieties (grown in the South) need 10 to 12 hours of daylight and are planted in fall. Long-day varieties (grown in the North) need 14 to 16 hours and are planted in early spring. Intermediate varieties work in the middle of the country and can often be planted in either season.

Check the label or seed catalog to see which type you have. If you live in the upper Midwest, Northeast, or Pacific Northwest, buy long-day varieties and plant in early spring. If you live in the South or Southwest, buy short-day varieties and plant in fall. If you are in between, intermediate varieties give you flexibility. Planting the wrong type for your region will produce small or misshapen bulbs.

Space and depth: the most common mistakes

The most frequent error is planting bulbs too deep. The tip of the bulb should sit just above the soil surface—not buried, not sticking straight up, but with the top quarter-inch or so visible. This allows the shoot to emerge easily and the bulb to swell upward as it grows. If you bury the bulb completely, it has to push through more soil and may rot before it sprouts.

Spacing matters for final size. If you want large onions for slicing, space them 6 inches apart. If you want medium onions or are growing for green onions (scallions), 4 inches is fine. Crowding them closer than 4 inches usually produces small bulbs and increases disease risk because air cannot circulate. Mark your spacing with a stick or string before you plant so you do not guess.

Water and fertilize through the growing season

Onions need consistent moisture, especially during the first four weeks after planting when roots are establishing. Water when the top inch of soil feels dry to your finger. In most climates, this means watering once or twice a week during the growing season, more often in hot weather, less often in cool or rainy periods. Do not let the soil dry out completely, but also do not keep it soggy—soggy soil invites rot and fungal disease.

Once green shoots appear (usually within two to three weeks), begin fertilizing every three to four weeks with a balanced fertilizer like 10-10-10 or 5-5-5. Follow the label rate for your bed size. If you worked compost into the soil before planting, you may not need additional fertilizer, but a light feeding in mid-season helps. Stop fertilizing about four weeks before harvest to avoid soft bulbs.

Recognize when onions are ready to harvest

Onions are ready when the green tops fall over and begin to dry and yellow. This usually happens 90 to 120 days after planting, depending on variety and growing conditions. Do not pull them up as soon as the tops fall—wait another week or two for the outer layers of skin to dry and tighten, which helps them store longer. If you need onions before they naturally fall over, you can harvest them early as green onions, but they will be smaller.

To harvest, loosen the soil around each bulb with a garden fork or trowel and gently pull. Lay them on the soil surface or on a screen in a shaded, airy spot to cure for two to three weeks. Curing dries the outer skin and seals the neck, which extends storage life. Once fully dry, trim the roots and tops to one inch, brush off loose soil, and store in a cool, dry place.

Troubleshoot common onion problems

Bulbs not forming: This usually means the variety was wrong for your day length, or you planted too late in the season. Long-day varieties planted in late spring will not bulb up because day length is already too long. Short-day varieties planted in spring will not bulb up in northern regions. Check the label and plant at the correct time next year.

Soft or mushy bulbs: Overwatering or poor drainage is the cause. Onions need moisture but not waterlogged soil. If your bed drains slowly, add sand or perlite, or plant in a new location with better drainage. Do not fertilize heavily late in the season, as this also produces soft bulbs.

Bulbs splitting or cracking: Inconsistent watering causes this—usually a dry period followed by heavy rain or watering. Water regularly and evenly throughout the season. Mulching with an inch of straw helps keep soil moisture stable.

Pest damage: Onion flies and thrips occasionally appear but rarely cause serious damage in home gardens. If you see small holes in the leaves or stippling, spray with neem oil or insecticidal soap according to label directions. Proper spacing and air circulation prevent most pest problems.

Frequently Asked Questions

Can I plant onion bulbs in spring if I missed fall planting?

Yes, but only if you plant long-day or intermediate varieties in early spring (February through April, depending on your region). Short-day varieties will not bulb up if planted in spring in northern areas. Check your bulb label for the recommended planting window, or contact your local extension office with your zip code and they can tell you which types work for spring planting in your area.

What is the difference between onion sets and onion bulbs?

Onion sets are tiny bulbs (about the size of a marble) that are faster to mature than larger bulbs. Bulbs are larger and take longer but often produce bigger final onions. Both are planted the same way—pointed end up, just below the soil surface. Sets are good if you want onions in 60 to 90 days; bulbs take 90 to 120 days but may give you larger harvest.

Do I need to separate onion bulbs before planting, or can I plant them as they come?

Plant them as they come. Onion bulbs are already individual, dormant onions. Do not separate or break them apart. If a bulb is damaged or soft when you open the bag, discard it, but otherwise plant each one whole, pointed end up.

How deep should I plant onion bulbs if my raised bed is very shallow?

Onions need at least 4 to 6 inches of soil depth to develop properly. If your raised bed is shallower than that, add soil or consider a different crop. Onions planted in very shallow soil often produce small bulbs or fail to bulb at all because there is not enough room for the bulb to expand underground.

Can I grow onions in the same raised bed year after year?

Yes, but rotate the location within the bed or add fresh compost each year to replenish nutrients. Onions are not heavy feeders, but they do deplete some soil nutrients. Avoid planting onions in the exact same spot two years in a row if possible, as this increases disease risk. Adding an inch of compost before planting each season keeps the soil healthy.