How to Grow Onions from Seed: Complete Guide

By Paul Smith, pemdasgarden.com

Most gardeners grow onions from sets or transplants and never realize what they're missing. Starting onion seeds yourself unlocks dozens of varieties no garden center carries, produces larger bulbs than sets, and costs pennies per plant. The trade-off is patience: onions demand the earliest indoor start of any common vegetable. Give them those 8 to 10 weeks and the rest is straightforward.

Bulb onions (Allium cepa) are biennials grown as annuals. The seed is small and short-lived — always use fresh seed, since onion seed loses viability faster than almost any other vegetable seed, often within a single year.

Start Indoors 8 to 10 Weeks Before Last Frost

Onions need a long runway. Sow indoors 8 to 10 weeks before your average last frost date — for most northern gardeners that means February. Southern gardeners growing short-day types sow in fall for overwintering.

• Depth: Sow 1/4 inch deep in a fine seed-starting mix. Sow thickly — 6 to 12 seeds per cell or a pinch per 4-inch pot — you will separate them at planting.
• Temperature: 70 to 75°F for germination (a heat mat helps), then move to a cooler 60s spot once sprouted.
• Time: Expect sprouts in 7 to 10 days; stragglers can take up to two weeks. Do not discard the tray early.
• Light: 12 to 14 hours of strong light daily. Weak light grows thread-thin seedlings that never size up.

The Haircut Trick: Trim Seedlings to Thicken Them

Here is the technique that separates great onion seedlings from sad ones. When the grass-like tops reach 5 or 6 inches tall, trim them back to 3 or 4 inches with clean scissors — and eat the trimmings as green onions. Repeat whenever they get floppy. Trimming forces the plant to put energy into thicker stems and stronger roots instead of tall, weak blades. Seedlings should be about pencil-thick at transplant time.

KNOW YOUR DAY LENGTH — IT DECIDES YOUR VARIETY

This is the most misunderstood part of onion growing. Bulb formation is triggered by day length, and varieties fall into three groups:

• Long-day (need 14+ hours of daylight to bulb): for northern gardeners, roughly zone 6 and north. White Sweet Spanish is a classic long-day type.
• Short-day (bulb at 10–12 hours): for the South, roughly zone 7 and south, planted in fall.
• Day-neutral/intermediate (12–14 hours): the flexible middle, suited to the middle latitudes.

Growing the wrong type is the #1 reason for marble-sized bulbs. Match the variety to your latitude first; everything else is secondary.

Transplant into a Weed-Free, Fertile Bed

Transplant outdoors 4 to 6 weeks before your last frost — onion seedlings are cold-hardy and actually prefer cool establishment weather. Tease the clump apart gently and plant each seedling 1 inch deep, 4 to 6 inches apart, in rows 12 to 18 inches wide.

Site selection matters enormously: onions are terrible competitors. Plant in the least weedy part of your garden, mulch after establishment, and weed relentlessly. Give them full sun and fertile, well-drained soil worked with compost before planting. Feed with nitrogen every two to three weeks until bulbing begins — then stop, because late nitrogen grows soft bulbs that store poorly.

Sow dates and transplant timing shift with your USDA zone — northern gardeners lean on the indoor start while southern gardeners can fall-sow short-day types directly. Adjust the calendar to your frost dates, not someone else's.

Water, Then Stop Before Harvest

Keep soil consistently moist — about 1 inch per week — through bulbing. Then, when about half the tops have fallen over naturally, stop watering entirely. Lift the bulbs, lay them in a warm, dry, shaded spot, and cure for 2 to 3 weeks until necks are tight and papery. Properly cured onions store for months; rushed ones rot.

Common Mistakes

• Starting too late indoors. Small transplants make small bulbs — those 8 to 10 weeks are non-negotiable.
• Skipping the haircut. Untrimmed seedlings flop over and never develop pencil-thick stems.
• Wrong day-length type for your latitude. Check before you sow, not after harvest disappoints.
• Weedy beds. Onions lose every competition with weeds; mulch and stay ahead of them.
• Watering right up to harvest. Stop when tops fall so bulbs cure instead of rotting.

Timing matters in the garden. Our gardening tools hub has a seed-starting date calculator and a frost date finder to help you plan sowing around your local climate.

Frequently Asked Questions

How long do onion seeds take to germinate?

Usually 7 to 10 days at 70 to 75°F, with stragglers up to two weeks. Use fresh seed — onion seed viability drops sharply after one year.

Can I direct sow onions outdoors?

In long-season climates, yes, but bulbs end up smaller. The indoor head start is what produces full-size bulbs in most of the country.

Why did my onions make tiny bulbs?

The usual suspects: wrong day-length type for your latitude, transplanting too late or too small, weed competition, or drought during bulbing.

Do onions need full sun?

Yes — full sun and fertile soil. Shade plus competition is the recipe for scallion-sized "bulbs."

Keep growing: try fellow alliums in our leek growing guide, browse vegetable seeds, explore more onion seed varieties, or return to the Growing Guides hub.