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The short answer: to grow onions well, first pick the right day-length type for your latitude — short-day for the South, long-day for the North, day-neutral almost anywhere — then plant in early spring (or fall in the South) in full sun and rich, loose soil. Space bulbs 4 to 6 inches apart, feed nitrogen every few weeks until bulbing starts, keep the shallow-rooted bed weed-free and evenly watered, and harvest when the tops flop over. Get the day-length choice wrong and nothing else you do will produce a large onion.
Onions reward attention to a few specific details more than most vegetables. This guide draws on extension-service growing data and the patterns owners consistently report, described in how we rank. For timing by region see when to plant onions, and if garlic is next on your list, when to plant garlic covers its very different fall schedule.
Step one: choose the right day-length type
This is the decision that makes or breaks an onion crop, and it is the one beginners most often miss. Onions bulb in response to day length, and each type has a threshold:
| Type | Bulbs when daylight reaches | Best for latitude | Example cultivars |
|---|---|---|---|
| Short-day | 10 to 12 hours | South, below ~35°N | Texas Early Grano, Red Burgundy, Georgia Sweet |
| Day-neutral (intermediate) | 12 to 14 hours | Central US, ~32 to 42°N | Candy, Superstar, Red Candy Apple |
| Long-day | 14 to 16 hours | North, above ~37°N | Walla Walla, Yellow Sweet Spanish, Patterson |
Plant a long-day onion in Florida and it never gets enough daylight to bulb, so it stays a scallion. Plant a short-day onion in Maine and it bulbs while still tiny in early summer. Day-neutral types are the safe choice if you are unsure, and they perform across a wide band of the country. When in doubt, buy from a supplier who lists the type and matches it to your region.
Sets, seed, or transplants
You have three ways in, each with trade-offs:
- Sets — small dormant bulbs sold by the bag. Easiest and most forgiving, plant them 1 inch deep, but they bolt more readily and come in few varieties. Choose sets smaller than a dime; larger ones bolt.
- Seed — the widest variety choice and the best-storing bulbs, but slow. Start indoors 10 to 12 weeks before your last frost, sow shallowly, and keep the tops trimmed to about 3 inches to build strong roots.
- Transplants — bunched young seedlings, usually mail-ordered by variety. They give near-seed quality with far less effort and are what many experienced growers use.
Soil and bed preparation
Onions have shallow, fibrous roots that spread only a few inches, so the top layer of the bed does all the work. Prepare it well:
- Choose full sun — 8 hours is ideal. Every leaf built before bulbing becomes a bulb ring.
- Loosen the top 8 inches and work in 1 to 2 inches of compost. Onions struggle in compacted or heavy clay soil.
- Aim for a pH of 6.0 to 6.8. Get a soil test if you have never checked.
- Work in a balanced starter fertilizer or aged manure before planting; onions are heavy feeders from the start.
Planting depth and spacing
Spacing controls final bulb size more directly than almost anything else. Give each plant room:
- Depth: plant sets and transplants about 1 inch deep, no more. Burying onions too deep delays or misshapes the bulb.
- Spacing: 4 inches apart for medium bulbs, 6 inches for large. You can plant at 2 inches and thin every other one for scallions.
- Rows: keep rows 12 to 18 inches apart for airflow and easy weeding.
Firm the soil gently around each plant so the roots make contact but the bulb sits shallow.
Watering and feeding
Onions want steady moisture and steady nitrogen through the leaf-building phase, then a clean break at the end.
| Stage | Water | Feed |
|---|---|---|
| Establishment to leaf growth | 1 inch per week, consistent | Nitrogen every 2 to 3 weeks |
| Bulbing begins (necks thicken) | Keep even; do not let it dry hard | Stop nitrogen once bulbs start swelling |
| Tops start flopping | Stop watering entirely | None |
A high-nitrogen feed early builds the leaves that become the bulb, which is why onions respond well to the same nitrogen-forward blends covered in our vegetable garden fertilizer picks. Cut nitrogen off once bulbing starts, or you get lush tops and thick necks that store poorly.
Weed control matters more with onions
Because onion roots are shallow and their upright leaves cast almost no shade, weeds win easily and steal water and nutrients the onion cannot spare. Weed shallowly and often — deep cultivation slices onion roots. A thin mulch of straw between rows suppresses weeds and holds moisture without smothering the shallow bulbs. Keeping the bed clean is one of the highest-value, lowest-glamour jobs in onion growing.
Common onion pests and diseases
Onions are relatively trouble-free, but a few problems recur and are worth knowing:
| Problem | Signs | Management |
|---|---|---|
| Onion maggot | Wilting, stunted plants; maggots tunneling into bulbs | Rotate crops, use row cover early, avoid planting in the same spot yearly |
| Thrips | Silvery streaks and stippling on leaves | Keep plants watered; a strong spray of water knocks them back |
| Downy mildew | Pale, fuzzy gray patches on leaves in wet weather | Space for airflow, water at the base, rotate |
| Neck rot / white rot | Soft, rotting necks or fluffy white growth at the base | Cure fully before storage; do not replant infected ground |
| Bolting (flower stalk) | A hard flower stem shoots up from the center | Use small sets, avoid cold stress; harvest bolted bulbs first, they store poorly |
Most onion troubles trace back to crowding, overhead watering, or planting in the same bed year after year. A three-year rotation and a clean, airy bed prevent the majority of them. Bolting in particular is a stress response — sudden cold after planting, or oversized sets, is usually the trigger, which is another reason to choose sets smaller than a dime.
Harvesting and curing for storage
Harvest and cure correctly and a good onion keeps for months; skip the cure and it rots in weeks.
- Wait for the tops to flop. When about half the plants have bent over and yellowed, the bulbs are done sizing. Do not bend them over yourself — let it happen naturally.
- Stop watering and let the bed dry for a week or so.
- Lift on a dry day and brush off soil; do not wash the bulbs.
- Cure in a single layer in a warm, dry, airy spot out of direct sun for 2 to 3 weeks, until the necks are papery and fully dry.
- Trim tops to about 1 inch and roots close, then store in mesh bags or braids somewhere cool and dry.
Sweet, short-day onions store the least well (weeks to a couple of months); pungent, long-day storage types like Patterson keep for many months. Match the variety to whether you want to eat fresh or store through winter.
Onions also make good garden neighbors: their scent deters aphids and mites, which is why they appear in our tomato companion planting guide. For the full seasonal calendar and every other crop, visit the seeds and vegetable growing hub, and see how often to water a vegetable garden to dial in irrigation across the whole bed.