How to Plant Onion Bulbs

How to plant onion bulbs

Plant onion sets (small bulbs) 1 inch deep with the pointed tip facing up. Space sets 4–6 inches apart in rows 12 inches apart. Plant 2–6 weeks before your last frost date in loose, fertile, well-drained soil.

Onion sets are the easiest way to grow onions in US home gardens. They are small, dried bulbs (grown from seed the previous year) that can be pushed into prepared soil in minutes. They establish faster than transplants and far faster than direct-seeded onions. The entire process from planting to green onion harvest is as short as 4–6 weeks; full-sized bulbs take 100–120 days from planting.

The critical decisions — timing, variety type, and spacing — have more impact on bulb size than anything you do after planting.

Onion sets vs. seeds vs. transplants

Choosing between sets, seeds, and transplants affects your variety options, timing, and workload.

Sets are sold in bags or by the pound at US garden centers from January through April. Standard varieties available as sets include Yellow Sweet Spanish, Copra (yellow), Red Wethersfield, and White Ebenezer. These are reliable producers but represent only a fraction of available onion genetics.

Transplants (bundle of 6–8 inch seedlings) are sold at garden centers in spring and provide a head start. Transplants are slightly more expensive than sets but offer better variety selection and are less prone to bolting than large sets.

Seeds allow access to the full range of specialty and heirloom onions not available as sets. However, onion seeds must be started indoors 10–12 weeks before the last frost date, requiring a grow light setup and more careful management. See our growing onions from seed guide for the full process.

Choosing the right onion type: short-day vs. long-day

Matching onion type to your latitude is the most important variety decision. Onions form bulbs when day length reaches a critical threshold. Long-day types begin bulbing at 14–16 hours of daylight (northern US); short-day types bulb at 10–12 hours (southern US).

If you plant short-day onions in the north: They will bulb up too quickly in spring when days are still short, producing small bulbs before the plants grow large enough.

If you plant long-day onions in the south: They wait for very long summer days (which don’t come to southern latitudes) and may never form proper bulbs, growing as large leafy plants with tiny bulbs.

Sets sold at garden centers in northern states are almost always long-day varieties. In the South (Zones 7–10), look for short-day transplants specifically labeled for southern growing or sold by regional nurseries. Vidalia-type onions are the best-known example of a short-day variety grown commercially in Georgia and surrounding areas.

Soil preparation

Onions need loose, fertile, well-drained soil with a pH of 6.0–7.0 to form large, well-shaped bulbs. Heavy clay, compacted, or poorly drained soil produces small, misshapen onions even with perfect timing and spacing.

Preparation steps:

1. Loosen the soil to 12 inches deep with a fork or tiller
2. Work in 2–3 inches of compost (or well-aged manure) through the top 8 inches
3. If soil pH is below 6.0, add lime 2–4 weeks before planting
4. Rake to a fine texture — large clods prevent proper bulb development
5. Ensure good drainage; onions in soggy soil develop neck rot

Raised beds work exceptionally well for onions because they drain freely and warm up faster in spring. A 4-foot-wide raised bed can produce 60–80 full-sized bulbs in a single planting.

When to plant onion sets

Timing depends on your zone and the onion type you’re growing.

Onions tolerate light frost (28–32°F) once they have emerged. Extended temperatures below 25°F can damage or kill emerged sets, so the planting window is: early enough for good vegetative growth, but not so early that a late hard freeze kills emerged plants.

In Zones 8–10, fall planting is often better than spring planting. Short-day varieties planted in October–November mature in early spring before summer heat causes bolting.

Step-by-step planting

Preparing the bed

1. Use a hoe, trowel, or dibber to mark rows 12 inches apart
2. Make a furrow 1 inch deep along each row
3. Add a light dusting of balanced fertilizer (10-10-10) along the bottom of the furrow, mixed into the soil — do not place fertilizer in direct contact with sets

Planting the sets

1. Sort sets — discard any that are soft, moldy, or extremely large (over 1 inch diameter, prone to bolting)
2. Place sets in the furrow with the pointed tip facing up, root end down
3. Space 4–6 inches apart for full-sized bulbs, or 2 inches apart for scallion harvest
4. Cover with 1 inch of soil and firm gently
5. Water immediately after planting

Mulching

Apply 1–2 inches of straw mulch between rows after planting. Mulch conserves moisture, reduces weed pressure, and moderates soil temperature. Do not mulch over the sets themselves — leave the row area clear to allow normal emergence.

Spacing: full bulbs vs. green onions

Spacing is the easiest way to get different harvests from the same planting.

• 4–6 inch spacing: Full-sized bulbs at maturity. At harvest, pull the entire plant when the tops fall over.
• 2–3 inch spacing: Plant densely, then thin as they grow. Pull every other plant at the green onion stage (6–8 inches tall) for immediate use. The remaining plants have room to form full bulbs.

The dense-and-thin method produces two harvests from the same space: fresh scallions in May and full bulbs in July (Zone 5–6 timing).

Care after planting

Watering: Onions need consistent moisture — about 1 inch per week — during the growing season. They are not drought-tolerant and produce smaller bulbs without adequate water. As harvest approaches (tops begin to fall over), reduce watering to allow bulbs to cure slightly in the ground.

Fertilizing: Apply a nitrogen fertilizer (ammonium sulfate or blood meal) every 3–4 weeks from emergence through midsummer. Stop fertilizing when bulbs begin to form (visible swelling at the base of the leaves). Late-season nitrogen encourages leafy growth and reduces storage life.

Weeding: Onions have shallow roots and compete poorly with weeds. Hand-weed or hoe shallowly — do not cultivate more than 1 inch deep near rows, as root damage stunts bulb development. Regular shallow cultivation every 7–10 days prevents weed establishment better than deep periodic weeding.

Thinning: If you planted 2–3 inches apart for a two-stage harvest, pull scallions when they reach 6–8 inches in height. Use them immediately or refrigerate for up to a week.

Companion planting with onions

Onions are classic companion planting plants in US vegetable gardens. The sulfur compounds in their leaves and roots repel several common garden pests.

Good companions:

• Carrots — onion scent deters carrot fly; carrot scent deters onion fly
• Lettuce — onions use different root zones
• Tomatoes — onion companion effects are mild but positive in most studies
• Roses — onion sulfur reportedly reduces black spot incidence

Poor companions:

• Peas and beans — onions suppress legume growth (avoid planting adjacent)
• Other alliums — garlic, leeks, chives compete directly and share pests

For a full companion plant reference, see our companion planting chart and our specific guide to what to plant with onions.

Harvesting and curing

Harvest onions when 50–75% of the tops have fallen over naturally. Do not wait for all tops to fall — this risks disease entering through open necks.

Curing process:

1. Pull onions on a dry day
2. Lay flat in a warm (80–90°F), dry, well-ventilated location out of direct sun — a garage, covered porch, or shed works well
3. Cure for 2–4 weeks until the outer skin is papery dry and the neck is completely dry
4. Trim off the dried tops to 1 inch above the bulb
5. Store in mesh bags or open crates at 35–45°F and low humidity

Properly cured onions store for 3–8 months depending on variety. High-sulfur varieties (most yellow storage onions) last longest; sweet varieties like Walla Walla store only 2–4 weeks after harvest.

See our full guide to when to harvest onions for timing details by variety type.

Common problems with onion sets

Bolting (flower stalk formation): Triggered by large set size, cold stress after emergence, or planting too early. Use marble-sized sets (under 3/4 inch diameter). Remove flower stalks immediately if they appear — they will not stop bulb formation if caught early.

Neck rot: Soft, water-soaked areas at the neck of curing onions. Caused by Botrytis fungi in warm, humid conditions. Maintain good air circulation during curing. Remove affected onions from storage immediately.

Thrips: Silvery streaking on leaves. Spray with insecticidal soap. Thrips populations increase in hot, dry conditions.

Onion maggot: The can you plant onions and potatoes together guide discusses crop rotation strategies that reduce maggot pressure — rotating the onion bed location every year is the single most effective preventive measure.

The National Onion Association maintains a variety guide covering the most commonly grown US onion types by region.