How to Grow Onions From Seed

Onions need a long, cool start followed by a warm finish. Start seeds indoors 8 to 10 weeks before your average last frost, or direct-sow as soon as the soil can be worked in early spring. Transplant or thin the young plants out into the garden a few weeks before the last frost, since light frost does not harm them. Match the type to your latitude: short-day onions for the South (planted in fall or very early spring), long-day onions for the North (planted in early spring), and day-neutral onions almost anywhere.

Sow onion seeds a quarter to a half inch deep in a fine, moist seed-starting mix and keep them around 65 to 75F until they sprout in 7 to 12 days. Give seedlings bright light and trim the tops to about 3 inches to build sturdy roots. Harden the plants off, then transplant them into full sun and rich, well-drained soil about 4 to 6 inches apart in rows, setting them an inch deep. Direct-sown onions and scallions can simply be thinned to their final spacing. Water them in and keep the bed weed-free, since onions compete poorly with weeds.

Onions are shallow-rooted and need steady moisture and feeding to size up. Water about an inch a week, keeping the soil evenly moist but never waterlogged, and mulch lightly to hold moisture and smother weeds. Feed with a nitrogen-rich fertilizer every few weeks while the tops are growing, then stop once the bulbs begin to swell and push soil away from the shoulders. For leeks, hill soil up around the stems every few weeks to blanch a long, tender white shank. Keep the bed weeded by hand, as hoeing too deep damages the shallow roots.

Onions are famously good companions: their scent confuses and repels many pests, so they earn a place beside carrots, beets, lettuce, brassicas, and tomatoes. Avoid planting them near beans and peas. These three make especially easy, one-click additions to the onion bed:

Bulbing onions are ready when most of the tops flop over and begin to yellow, usually in mid to late summer. Stop watering, wait a few days, then lift the bulbs on a dry day and cure them in a warm, airy, shaded spot for two to three weeks until the necks are papery and dry. Well-cured storage onions keep for months. Harvest scallions any time once they reach pencil thickness, and lift leeks as needed once the white shanks are thick and tender.

Few crops earn their space like onions. Sweet types shine raw in salads, salsas, and burgers, while storage onions are the backbone of soups, stews, sauces, and roasts all winter. Scallions add a fresh, mild bite to stir-fries and garnishes, pearl onions are perfect for pickling and roasting whole, and leeks bring a gentle, silky sweetness to soups and braises. Home-grown onions reward you with far more flavor and variety than any grocery bag.

• No bulbs, all tops: almost always the wrong day-length type for your latitude. Choose short-day onions in the South, long-day in the North, or day-neutral anywhere.
• Bolting (flower stalks): caused by a cold snap on young plants or oversized transplants. Plant sturdy but not overgrown seedlings and keep growth steady; use bolted onions fresh, as they will not store.
• Small bulbs: usually crowding, weeds, or too little water and feed. Give 4 to 6 inches of space, keep the bed weed-free, and feed nitrogen until bulbing starts.
• Thick necks that will not cure: too much nitrogen late in the season. Stop feeding once bulbs swell, and let the tops fall naturally before harvest.
• Onion maggots or thrips: rotate crops, keep the bed tidy, and plant repellent companions like marigolds and nasturtiums nearby.