🧅 When to Plant Onions

Onions are one of the most timing-sensitive vegetables in the garden because bulb formation is driven by day length, not just temperature. Get the right variety for your latitude and the timing right for your zone, and onions are surprisingly low-maintenance from transplant to harvest.

Variety Selection

Day-length type is the single most important decision you make before planting onions. Choosing the wrong type is the most common reason bulbs stay small no matter how well you water and fertilize.

Short-day varieties begin bulbing when days reach 10-12 hours of light. They are the right pick for zones 7 through 11, where early summer days are shorter than in the north. Yellow Granex (the Vidalia type), Red Creole, and Burgundy are widely available short-day options.

Long-day varieties need 14-16 hours of daylight to form bulbs, making them the natural choice for zones 6 and north where midsummer days are longest. Walla Walla, Sweet Spanish, and Copra (an excellent long-keeping storage type) are well-proven long-day picks.

Day-neutral (intermediate) varieties bulb at 12-14 hours and work across zones 5-8. Candy is widely available and reliable across that range.

A simple rule of thumb: if you live north of roughly the 36th parallel (the Memphis-to-Raleigh line), choose long-day. South of it, choose short-day. Right on the border, go day-neutral.

Spacing, Sun, and Soil

Onions want full sun (at least 6 hours; 8 or more grows the biggest bulbs) and loose, well-drained soil worked 8-10 inches deep. Heavy clay prevents bulb expansion. Work in compost to loosen it. Target a soil pH of 6.0-6.8.

Space transplants and sets 4-6 inches apart in rows 12-18 inches apart. Crowding limits bulb size more than almost anything else. If you are growing for scallions, you can halve the spacing and harvest them young before competition becomes an issue.

Onions are heavy nitrogen feeders during their leafy growth phase. Side-dress with a nitrogen-rich fertilizer 3-4 weeks after transplanting and again when plants are pencil-thick. Stop fertilizing once bulbs begin to push up out of the soil, or you will end up with thick necks and bulbs that rot faster in storage.

Watering

Consistent moisture through the bulbing phase is what separates firm, full-sized onions from small, tight ones. Aim for about 1 inch of water per week, combining rain and irrigation. Sandy soils dry faster, so check more often during warm stretches.

As tops begin to yellow and fall over, taper off watering. Wet soil at this stage softens necks and invites rot in storage. Stop watering entirely once most tops have fallen and harvest within 7-14 days.

Top Growing Tips

• Match day-length type to your latitude before buying: long-day for zone 6 and north, short-day for zone 7 and south, day-neutral for the border zones
• Sets give the quickest start; transplants give full variety selection; direct seeding takes longest but costs least per plant
• Weed consistently in the first four to six weeks: onions compete poorly with weeds and yield suffers when they share space
• Pinch off any flower stalk the moment you see it; a bolted onion will not size up further and does not store well
• Cure harvested bulbs in a warm, well-ventilated spot for 2-4 weeks before moving them to long-term storage

Companion Planting

Good companions: carrots, beets, lettuce, tomatoes, and chamomile. Onions and carrots are a classic pairing: onions help deter carrot fly while carrots deter onion fly. Chamomile planted at the edge of a bed is said to improve flavor in neighboring alliums.

Avoid planting near: beans, peas, and asparagus. Alliums release compounds that inhibit legume growth, and asparagus planted near onions tends to decline over time.

Common Problems

Thrips are the most common onion pest: tiny, straw-colored insects that rasp leaf surfaces and leave silver streaking. Check the innermost leaves regularly. Insecticidal soap or neem oil knocks them back; heavy infestations during dry weather can stall bulb development noticeably.

Bolting typically follows a cold spell after seedlings are established. Once a flower stalk forms, the bulb stops sizing up. Pinch it off and use those bulbs first. Prevent bolting by timing transplants so they go out after soil temperatures stabilize above 50°F.

Downy mildew shows up as pale, violet-tinged fuzzy growth on leaves during cool, wet weather. Improve air circulation by spacing plants adequately and switch from overhead watering to drip or furrow irrigation. Copper-based fungicides can slow spread if caught early.

Neck rot in storage appears as a soft, sunken area near the neck and is almost entirely preventable at harvest: cure bulbs fully until the neck is dry and papery before putting them away.

Harvest Timeline

Onions grown from sets mature in 90-120 days. Direct-sown seed takes 100-175 days total, which is why northern zones starting seed indoors in January or February still reach a summer harvest.

The harvest cue is visual rather than calendar-based. When 50-75% of the tops have naturally fallen over, lift a test bulb: the neck should feel tight and dry, with the outer skin beginning to turn papery. If it passes, harvest the row. If not, wait another week.

Once harvested, cure onions in a single layer in a shaded, well-ventilated spot for 2-4 weeks. Trim the roots and cut tops to about 1 inch above the neck. Storage varieties like Copra keep 3-6 months in a cool, dry location; sweet varieties last only 1-2 months and should be used first.