Growing Onions

This is the most important decision in onion growing. If you are north of the 37th parallel (roughly a line from San Francisco to Richmond, Virginia), grow long-day varieties. South of the 35th parallel (roughly Albuquerque to Memphis), grow short-day varieties. In between, grow intermediate-day types or experiment with both.

Within each day-length category, choose varieties based on your priority: sweetness for fresh eating (Walla Walla, Vidalia), storage life (Copra, Patterson, Redwing), or size (Ailsa Craig can exceed 2 lbs). Sweet varieties typically store poorly (1–3 months), while pungent storage varieties keep 6–9 months.

Seeds offer the widest variety selection and lowest cost ($3–5 per packet = hundreds of onions) but require the longest growing time. Start seeds indoors 8–10 weeks before your transplant date. Onion seedlings are slow-growing and look like green threads for weeks - patience is essential.

Sets are small dormant bulbs available at garden centers. They are the easiest to plant (just push into soil) and establish quickly but offer limited variety selection and are more prone to bolting (premature flowering). Sets are best for green onions and quick harvests.

Transplants are young seedling bundles, typically sold in bunches of 50–75. They offer good variety selection, establish faster than seeds, and have lower bolting rates than sets. Many specialty onion growers (especially in the South) prefer transplants.

Onions need loose, fertile, well-drained soil. They are shallow-rooted - most roots occupy the top 6 inches - so raised beds work well if your soil is heavy. Work in 2–3 inches of compost and a balanced organic fertilizer. Onions are heavy feeders, especially for nitrogen and phosphorus.

Till or loosen soil to 8 inches deep and remove rocks and debris that can deform bulbs. The ideal pH is 6.0–7.0. Add sulfur to lower pH or lime to raise it based on a soil test. Onions prefer loose, loamy soil - in clay, add sharp sand or perlite to improve drainage and prevent compaction around developing bulbs.

For long-day varieties in the North, transplant seedlings or plant sets as early as 4–6 weeks before the last frost date. Onions tolerate light frosts and actually benefit from early cold exposure. Plant sets 1 inch deep, tip up. Plant transplants at the same depth they were growing, trimming tops to 4 inches if floppy.

Space plants 4–6 inches apart for full-size bulbs, or 2 inches apart if you plan to thin and harvest some as green onions. Row spacing of 12–16 inches allows for cultivation. For intensive raised bed production, a 4-inch grid pattern works well.

For short-day varieties in the South, plant transplants or sets in October through January depending on your specific climate. These overwinter as small plants and bulb up in spring as days lengthen.

Onions are heavy feeders. Side-dress with nitrogen every 2–3 weeks from planting until bulbing begins (when necks start to soften and tops begin to fall). Use fish emulsion, blood meal, or ammonium sulfate. Stop nitrogen once bulbing starts - excess nitrogen at this stage produces thick necks and poor storage.

Provide 1–1.5 inches of water per week. Consistent moisture is critical during bulb expansion (the 4–6 weeks before harvest). Irregular watering causes growth rings, splitting, and doubled bulbs. Drip irrigation is ideal - overhead watering promotes foliar diseases like downy mildew and botrytis.

Keep beds meticulously weeded. Onions are terrible competitors against weeds due to their narrow, upright foliage. Mulch carefully after plants are 6+ inches tall to suppress weeds and conserve moisture.

Onions are ready to harvest when 50–80% of the tops have fallen over naturally. Do not bend tops over manually - this old wives' tale actually damages the neck and introduces rot organisms. When tops fall, the neck cells collapse naturally, sealing the bulb for storage.

Pull onions on a dry, sunny day. If soil is tight, loosen with a fork first. Lay bulbs on the soil surface to field-cure for 1–2 days if weather permits (no rain). Then move to a covered area with good air circulation for full curing.

Proper curing is the key to long-term storage. Spread onions in a single layer on wire racks, screens, or hang in mesh bags in a warm (75–85°F), dry, well-ventilated area. Cure for 2–4 weeks until the outer 2–3 skins are papery, the neck is completely dry, and roots are wiry.

Trim roots close and cut tops to 1 inch (or leave for braiding). Sort bulbs - any with thick necks, soft spots, or damage should be used first, not stored. Store cured onions at 32–40°F with 65–70% humidity for maximum longevity. Well-cured storage varieties (Copra, Patterson) can last 8–10 months.

Never store onions with potatoes - potatoes emit ethylene gas that causes onions to sprout, and onions produce compounds that accelerate potato sprouting. Store them in separate, well-ventilated areas.