How to Grow Onions

Start onion seeds indoors about 10–12 weeks before the expected last spring frost. Transplant outdoors as soon as the soil can be worked and temperatures are consistently cool but not severely cold.

Onions tolerate light frost and grow best when established early. Bulbing begins in response to day length, so choose short-day, intermediate-day, or long-day varieties suited to your latitude. Most northern gardens require long-day onions.

Sow seeds about ¼ inch deep and keep the mix evenly moist. Provide bright light after emergence. When seedlings reach several inches tall, their tops may be trimmed lightly to keep them upright and manageable.

Harden seedlings gradually before planting them 1–2 inches deep. Space onions about 3–4 inches apart for mature bulbs or closer for scallions, with rows 12–18 inches apart. Onion sets are easy to plant, but seed-grown transplants offer a wider choice of varieties and often produce larger bulbs.

Choose a site with full sun and loose, well-drained soil enriched with finished compost. Onions have shallow roots and grow poorly when crowded by weeds or planted in compacted ground.

Keep the soil evenly moist while leaves and bulbs are developing. Mulch lightly to conserve moisture and suppress weeds, but avoid burying the bulbs as they enlarge. Maintain balanced fertility early in the season, then avoid pushing fresh leafy growth as the crop approaches maturity.

Choose a site with full sun and loose, well-drained soil enriched with finished compost. Onions have shallow roots and grow poorly when crowded by weeds or planted in compacted ground.

Keep the soil evenly moist while leaves and bulbs are developing. Mulch lightly to conserve moisture and suppress weeds, but avoid burying the bulbs as they enlarge. Maintain balanced fertility early in the season, then avoid pushing fresh leafy growth as the crop approaches maturity.

Onions provide fiber, vitamin C, folate, potassium, and sulfur-containing compounds responsible for their familiar aroma and flavor. Red onions also contain anthocyanins, while many varieties contain the flavonoid quercetin.

Use onions raw, roasted, grilled, pickled, sautéed, or slowly cooked until sweet and deeply browned. The green tops of young onions are also edible and useful as a fresh garnish.

Onion thrips may leave pale streaks or silvery patches on the leaves, while onion maggots can damage roots and bulbs. Rotate onions and related crops, remove cull bulbs and diseased plants, control nearby weeds, and avoid planting onions repeatedly in the same bed.

Good drainage, generous airflow, and watering near the soil help reduce fungal and bacterial diseases. Inspect plants regularly and remove any bulbs that become soft, discolored, or foul-smelling. Thrips are a recognized pest of onions and several related vegetable crops. 

Onions are biennial and insect-pollinated. They usually form bulbs during the first season, survive a period of winter cold, and flower the following year. Different varieties of common onion, Allium cepa, can cross with one another.

Select several healthy, true-to-type bulbs and store them cool, dry, and frost-free through winter where they cannot remain safely in the ground. Replant in spring and allow the round flower heads to mature.

When the seed heads become dry and black seeds begin to show, cut them into paper bags and finish drying under cover. Rub the heads apart, remove the chaff, and store the thoroughly dry seed in a cool, dark place. Onion seed loses vigor faster than many vegetable seeds, so fresh seed is best.

• Choose onion varieties according to local day length.
• Keep beds weed-free while plants are small.
• Harvest scallions from crowded rows as a form of thinning.
• Stop regular watering as the tops begin to fall.
• Cure bulbs thoroughly before storage.