Onion

Growing Conditions

Onions are day-length sensitive — bulb formation begins when day length reaches a certain threshold. Long-day varieties (14+ hours) are suited to northern gardens; short-day varieties (10–12 hours) to southern gardens; intermediate-day varieties to the middle latitudes. Planting the wrong type for your latitude results in poor bulb formation. Zone 5 growers need long-day varieties.

Onions prefer full sun, fertile well-draining soil, and consistent moisture throughout the growing season. They are heavy feeders.

Cold climates: Start from transplants or sets, planting out 4–6 weeks before last frost. Onions tolerate frost once established.

Warm climates: Plant short-day varieties in fall for spring harvest.

Sowing & Planting

Most home gardeners start from transplants (bare-root bundles) or sets (small dried bulbs) rather than seed — both are faster and more reliable than direct seeding. Plant transplants or sets 1 inch deep, 4–6 inches apart, in rows 12 inches apart. Sets are the easiest starting point for beginners.

For seed starting: sow indoors 10–12 weeks before transplanting, ¼ inch deep. Trim tops to 3 inches before transplanting to reduce moisture loss.

Care & Maintenance

Onions are heavy feeders — amend soil with compost before planting and side-dress with nitrogen-rich fertilizer every 3–4 weeks through midsummer. Water consistently — 1 inch per week. Stop watering when tops begin to fall over naturally. Keep beds weeded; onions compete poorly with weeds.

Common Problems

Onion thrips cause silvery streaking on leaves — use insecticidal soap. Downy mildew in wet conditions causes pale, wilting tops. Neck rot in storage results from harvesting before tops have dried fully — cure thoroughly. Bolting produces a flower stalk and reduces bulb quality — occurs when plants experience cold stress early in the season; use transplants rather than sets to minimize risk.

Harvest

Harvest when 50–75% of tops have fallen over naturally. Don’t wait for all tops to fall — bulbs can split or rot if left too long after maturity. Loosen soil with a fork and pull. Lay bulbs in a single layer in a warm, dry, well-ventilated location to cure for 2–4 weeks until outer skins are papery and necks are completely dry.

Properly cured storage onions keep 6–12 months. Sweet onions (Walla Walla, Vidalia) have high water content and store only 1–2 months.

Varieties

Long-day varieties for northern gardens: Copra (excellent storage), Patterson, Ailsa Craig (large, mild). Short-day for southern gardens: Granex, Georgia Sweet. Intermediate-day: Candy, Super Star. For Zone 5 home gardens focused on storage, Copra is one of the most reliable choices available.