Sweet Onion

Sweet onions are short-day varieties of the common onion, which set bulbs when they get about 12 hours of daylight. They generally have a mild, sweet flavor and are best eaten fresh, but they do not store especially well. Onions are biennials that are usually treated as annuals in the garden.

Sowing. Short-day onions grown from seed can be started indoors from mid-September through mid-October, about 6 to 8 weeks before transplanting. Sow seed about ¼ inch to ½ inch (0.6 to 1.3 cm) deep and cover it lightly with fine soil. Seedlings usually emerge 2 to 3 weeks after planting. Some growers sow bulb onion seed in early fall.

Thinning and light. Once seedlings emerge, thin them to about 3 to 4 inches (8 to 10 cm) apart. Plant in full sun, meaning at least 6 hours of direct sunlight a day. Plants reach about 1 to 1½ feet (30 to 46 cm) tall and 6 inches to 1 foot (15 to 30 cm) wide.

Watering. Onions need regular watering, so keep the soil moist. Water matters most after the bulbs start to enlarge.

Weeding and soil. Onions have shallow roots and compete poorly with weeds, so hand-pull weeds or cultivate shallowly. Don’t hill up soil around the plants, since this can encourage stem rot. Onions grow best in fertile, well-drained soil with plenty of organic matter, with an ideal pH between 6.2 and 6.5.

Pests and disease. Bulb mites and bulb rot in wet soil can occur. Onion maggots and onion thrips are the most damaging insect pests. White rot is a listed problem for onions.

Bolting. Onions can send up a flower stalk if they are exposed to a long cold spell after a good growing period, which leads to small bulbs. Planting too early can also cause bolting.

When to pick. Harvest bulb onions when about two-thirds of the dried tops have fallen over. Another guide suggests waiting until about three-fourths of the tops have fallen. Dig them carefully to avoid bruising.

Curing. Let the bulbs dry or cure for 1 to 2 weeks with the tops intact, out of direct sun. Cut the tops about 1 inch (2.5 cm) above the bulb before storing. Green onions can be harvested when the tops are about 6 inches (15 cm) tall.

Keep cured bulbs in shallow boxes or mesh bags in a cool, well-ventilated place. Ideal conditions are about 45 to 55 °F (7 to 13 °C) with 50 to 60 percent humidity. Onions kept in ventilated bags or boxes in a cool basement or garage can last 2 months or more.

Onions are toxic to cats and dogs. A plant database entry lists low-severity poison characteristics for the plant, but describes the bulb and tops as edible raw or cooked.