Plant onions early for bigger summer bulbs

Get onions in the ground in spring and avoid heartbreak at harvest.

Plant as soon as soil is dry enough to work – March and April are prime – said Jim Myers, Baggett‑Frazier Endowed Chair of Vegetable Breeding and Genetics in the Department of Horticulture at Oregon State University.

Most onions grown in Oregon are long-day types. They make green tops until day length reaches about 14 hours, which triggers bulbing. Early planting lets plants size up before bulbing begins – and larger plants make larger bulbs.

“The size of the bulb is dependent on the size of the plant when bulbing begins,” Myers said. “This is why early planting is critical if you want large onions, as well as plenty of water and fertilizer.”

Buy onion sets while firm and dormant. Use larger sets for green onions and smaller sets for bulbing onions. When purchasing transplants, choose plants free of decay or excess drying.

Amend well-drained soil with compost or aged manure. At planting, band a complete fertilizer (for example, 10-10-10) along the row, or use a balanced organic fertilizer.

Planting and spacing

* Green onions: Set bulbs 1½ to 2 inches deep and 1 inch apart. Harvest when tops are 8 to 10 inches tall.

* Bulbing onions (from sets): Plant about ½ inch deep and 4 inches apart in rows 12 to 15 inches apart.

* Transplants: Space 3 to 4 inches apart in rows 12 to 15 inches apart.

Setting onion transplants outside

Harden off transplants for 5–7 days. Plant when soil is workable and not waterlogged. Set plants at, or just slightly above, the original soil line – avoid burying the necks or bending them. Water in gently to settle soil around roots and mulch lightly to conserve moisture.

Water and fertilizer

Fertilize every 2–3 weeks until bulbing starts. Keep soil consistently moist – onions are shallow-rooted and drought triggers early, small bulbs. When tops begin to dry and fall, stop watering so bulbs mature in dry soil. After about half the tops have fallen, wait a week and harvest. Cure bulbs 7–10 days in sun, covering at night.

Regional considerations across Oregon

Western Oregon (Willamette Valley/North Coast)

* Plant earlier in the window; soils warm slowly. Use raised beds if drainage is poor. Mulch to moderate late-spring dry spells.

Varieties that perform well in Oregon

* Yellow: Allium cepa ‘Copra’, ‘New York Early’, ‘Candy’

* Overwintering: A. cepa ‘Walla Walla’

* Red: A. cepa ‘Red Wing’.

* White: A. cepa ‘White Sweet Spanish’, ‘Superstar’

* Green bunching: Allium fistulosum ‘Ishikura’, ‘He-shi-ko’