Onion

Onion (Allium cepa) is a cool-season bulb vegetable, and the bulb and tops are edible raw or cooked. Home growers can start it from seed, from sets (tiny immature bulbs), or from transplants, and each method suits different seasons and areas. Bulb onions take about 85 to 120 days to mature.

Sowing seed. Sow seed directly outdoors in late March or early April, about ¼ to ½ inch (0.6 to 1.3 cm) deep. Cover lightly with fine soil and keep the rows moist until you see seedlings. Seedlings should emerge in about 2 to 3 weeks. Plant thickly, then thin the plants to 3 to 4 inches (8 to 10 cm) apart in rows 8 to 16 inches (20 to 41 cm) apart, and use the thinnings as green onions. In South Carolina’s Piedmont region, seed is not a recommended method, while fall seeding is used in coastal and central areas.

Indoor starts, sets and transplants. Long- and intermediate-day varieties can be sown indoors 6 to 8 weeks before the last frost date. You can also buy transplants or sow seeds indoors about 8 weeks before planting. Sets can be planted in fall or early spring, and transplants in early spring. Plant sets about 1½ inches (3.8 cm) deep.

Site and soil. Plant in full sun, in moist, well-drained, fertile soil, as soon as the ground can be worked in spring. The ideal soil pH is between 6.2 and 6.5.

Watering. Keep the soil moist, because onions need regular water for growth and bulb size. Stop watering about a week before harvest to help the bulb form its protective skins.

Weeds. Onions have shallow roots and compete poorly with weeds. Pull weeds by hand or cultivate shallowly, and don’t hill soil up around the plants. An organic mulch helps conserve water and reduce weeds.

Feeding. Base fertilizer on a soil test. Excess nitrogen late in the season delays maturity and can make bulbs double, and over-fed onions do not store well.

Bulb mites can occur, and bulb rot can develop in soils that stay too wet. Onions are resistant to damage by deer and rabbits. Onion maggots, which feed on the bulbs, are among the most damaging insect pests of onions.

Onions can bolt, or send up a flower stalk, after a prolonged cold period, which leaves small bulbs with large necks. Larger plants bolt more readily in cold, and sets more than 1 inch (2.5 cm) across are likely to bolt.

Harvest bulb onions when about two-thirds of the dried tops have fallen over. Other advice says to wait until about three-fourths of the tops have fallen. Dig carefully to avoid bruising. Cut the tops about 1 inch (2.5 cm) above the bulb and cure the bulbs for 1 to 2 weeks in a well-ventilated place out of direct sun. Another source says to cure for 2 to 3 weeks. Harvest green onions when the tops are 6 to 8 inches (15 to 20 cm) tall.

Store cured, dry bulbs in mesh bags or crates for good air circulation. Storage at 45 to 55 °F (7 to 13 °C) with 50 to 60 percent humidity is recommended. Other advice gives 32 °F (0 °C) and 65 to 70 percent humidity, for up to 6 to 7 months.

Onions are toxic to dogs and cats. Clinical signs include vomiting, breakdown of red blood cells, blood in the urine, weakness, a high heart rate and panting. Experts advise calling a poison control service or a veterinarian as soon as possible if a pet may have eaten it.