Sweet Onion Sets

Days to Maturity: 105-110

An indispensable garden plant, sweet onion is a bulb vegetable. Sweet onions are large and have a slightly flattened globe shape, thin skin, and thick, golden flesh. Having less sulfur, their flavor is milder and sweeter than the more pungent, eye-stinging varieties. They can be eaten raw, but they are excellent caramelized, roasted, or battered and fried. This variety stores well. Onions are a good source of folate and vitamin C.

This plant is a member of the amaryllis family (Amaryllidaceae). Typically grown as an annual, the sweet onion is an herbaceous biennial plant with an upright habit of linear, cylindrical, hollow, glaucous leaves that arise in a basal rosette and are typically bluish green in color. The plant has a medium growth rate. Onions are great companion plants in the garden. They naturally deter certain garden pests, like aphids, Japanese beetles, sugar beet-flea beetles, deer, and rabbits. Sweet onions can also improve the flavor of some of their companions, while some companions, like chamomile and summer savory, can improve the onion’s flavor. Scattering them around the garden helps to produce healthier harvests.

Think of sweet onions as a leaf crop, not a root crop. They prefer full sun and loose, fertile, moist, well-drained soil with a pH of 6.2 to 6.5. A lower or higher pH may result in poor bulb quality or small yields. An intermediate-day or day-neutral variety, the most adaptable for any region, sweet onions require around 12 to 14 hours of sun. The more sun and space between plants, the bigger the bulb. Onions have shallow root systems and need consistent moisture. Onions are heavy feeders. Adding manure or compost to the soil several weeks before planting (or the winter before) and applying fertilizer when planting and again when plants have 5 to 6 leaves increases plant size. And since each leaf represents one ring of the onion bulb, more leaves equal more rings, resulting in bigger bulbs. Keep beds free of weeds, which will compete with the onions for water and nutrients. To avoid disease, practice crop rotation.

A cool-season vegetable, one of the first to be planted in the garden, sweet onion sets can be direct sown into the garden as soon as the soil is workable in early spring, weeks before the last frost-free date, for a summer harvest. But they can also be started indoors and transplanted, spring and fall. Planting sets in midsummer will produce a fall or spring harvest. It needs cold temperatures to germinate and grow. Plant shallowly and early but protect plants from heavy frost with row covers.

Harvest sweet onions when two-thirds of the leaves fall over and turn brown. Leave them lying in the garden for 1 to 2 days to dry, if weather permits; continue drying them for 2 to 3 weeks in a cool, dry, sunny location; and once completely dried, store them in a dark, dry environment in mesh bags or crates with good air circulation.

Onion sets are small onion bulbs grown in the previous season. The partially formed bulbs are around ¾ inch in diameter, making them easier to plant than seed. They develop into full-size bulbs quickly after planting, in approximately 3½ months. Sets tend to produce stronger, more vigorous plants compared to seed, ensuring larger yields.