How to Grow Onions

Onions are a garden staple that takes patience but rewards you with storage crops that last for months. They're grown from seeds, transplants, or sets (small dormant bulbs) and take 3–5 months from planting to harvest.

The key factor in onion success is day length. 'Long-day' varieties (for northern gardeners, above 35–38°N latitude) need 14–16 hours of daylight to bulb up. 'Short-day' varieties (southern gardeners) bulb with 10–12 hours. Choosing the wrong type for your location is the most common onion failure.

Growing Tips

Start from transplants or seeds rather than sets for the best size and variety selection.
Plant shallowly — just the roots should be underground. Bulbs form above ground level.
Stop watering when the tops start falling over — let the bulbs cure naturally in the ground for 2–3 weeks.
After pulling, cure in a warm, airy place (80–85°F) for 2–4 weeks before storage.
Long-day varieties for northern gardeners: 'Copra,' 'Walla Walla.' Short-day for southern gardeners: 'Texas 1015,' 'Yellow Granex' (Vidalia type).

Companion Planting

Onion-family plants are traditionally paired with carrots, and some research supports that onion volatile compounds may reduce carrot fly egg-laying. As heavy feeders requiring well-tilled soil, onions grow best alongside crops with compatible soil preparation requirements. The planner helps you group crops with similar soil and fertility needs in the same bed.

FAQ

Why do my onions stay small?

The most common reason is wrong variety choice — using short-day onions in the north or vice versa. Also, onions planted too late, crowded too closely, or lacking nitrogen won't size up properly.

When are onions ready to harvest?

Harvest when half to three-quarters of the tops have fallen over naturally. Pull them, let them cure in the garden (if dry weather) or in a warm, airy location for 2–3 weeks until the outer skin crinkles and the neck dries completely.

How long do homegrown onions keep?

Well-cured onions keep 4–6 months in cool (35–50°F), dry, dark storage. Sweet varieties keep only 4–6 weeks. Pungent storage onions keep longest.

Can I grow onions from last year's onion?

If an onion sprouts, plant it — you'll get green onion tops for eating. However, it won't form a new large bulb; it will bolt and produce seed. Save the seed if you want, but don't expect a large storage onion.

What are onion sets vs. transplants vs. seeds?

Sets are small dried bulbs — easiest and fastest but limited variety choice. Transplants are seedlings grown in greenhouses — best sizing and variety choice. Seeds are cheapest but require 10–12 weeks of indoor growing.