When and How to Plant Onions for Big, Healthy Bulbs

Introduction: Lessons Learned from Early Onion Plantings

The first time I planted onions, I tucked them in too shallow and way too close together. By midsummer, I had a mess of thin, twisted bulbs that looked more like knotted string than food for the table. That’s when I learned onions aren’t forgiving if you skip the basics. The timing, the depth, and even the kind of onion you choose all make the difference between spindly disappointments and fat, storage-worthy bulbs.

If you’ve struggled with onions before, don’t write them off. Get the planting right, and they’ll reward you with some of the most satisfying harvests you’ll ever pull from the soil.

Understanding Onion Types and Day-Length Sensitivity

Onions are picky about how much daylight they need to form bulbs. Choose the wrong type for your region, and you’ll either get no bulbs at all or plants that bolt before they size up.

• Short-day onions need only 10–12 hours of daylight. Best for the South (Zones 7–10). Plant them in the fall for a spring harvest.

• Intermediate-day onions need about 12–14 hours. Perfect for middle regions (Zones 6–7). They balance well between the extremes.

• Long-day onions require 14–16 hours of daylight. These are for the northern states (Zones 4–6) where long summer days give them the stretch they need.

Quick chart:

• South (below 35° latitude) → short-day onions

• Middle (35°–40° latitude) → intermediate-day onions

• North (above 40° latitude) → long-day onions

Choosing the right onion for your latitude is step one if you want bulbs worth bragging about.

Soil Temperature and Timing for Onion Success

Onions like cool roots and steady growth. Get the soil temperature right, and they’ll take off.

• Seed germination: 50–75°F soil temp, with the sweet spot near 70°F.

• Bulb growth: Best between 55–75°F.

Planting windows by USDA Zone:

• Zones 8–10: Plant short-day onions in fall (October–December).

• Zones 6–7: Plant intermediate-day onions in late winter or very early spring.

• Zones 4–5: Start long-day onions indoors 8–10 weeks before the last frost, then transplant when soil is workable.

If you’re in a colder climate, raising seedlings indoors under lights gives you a strong head start. Warmer climates can rely more on direct planting of sets or fall-planted transplants.

Planting Depth and Spacing

Onions don’t like being crowded, and they’ll punish you for planting them wrong. Sets and seedlings each have their own sweet spot:

• Onion sets (small dried bulbs): Plant 1 inch deep, pointed end up. Space 4 inches apart in rows 12–18 inches apart.

• Seedlings or transplants: Set them so that the white part of the stem is just under the soil, leaving the green shoots visible. Space them 4–6 inches apart.

Grandma’s tip: “Plant ‘em deep enough to cover the white, shallow enough to see the green.”

That little reminder has kept me from burying onions too deep or leaving them high and dry.

Mulching for Weed Control and Moisture

Onions don’t compete well with weeds. A weedy bed means stunted bulbs. That’s where mulch comes in.

• Best options: straw, shredded leaves, or grass clippings.

• How to apply: spread a light layer (1–2 inches) once seedlings are established. Keep it loose so stems aren’t smothered.

Mulch also locks in soil moisture, which onions depend on during bulb formation. Dry soil leads to split or tough bulbs.

Pro Tips and Common Mistakes to Avoid

Avoid these pitfalls if you want onions worth storing:

• Overcrowding. Give each plant breathing room. Tight spacing equals tiny bulbs.

• Too much nitrogen. Fresh manure or heavy feedings push leafy greens at the expense of bulb growth.

• Inconsistent watering. Fluctuations cause split bulbs. Keep soil consistently moist but never soggy.

• Letting weeds win. Even small weeds rob onions of light and nutrients. Stay on top of it early.

Faith Touch

Onions take months of steady growth before you see results. You plant them in faith, keep them watered and weeded, and only later do the bulbs swell beneath the soil. It’s a reminder of patience and trust.

“Therefore be patient, brethren, until the coming of the Lord. See how the farmer waits for the precious fruit of the earth, waiting patiently for it until it receives the early and latter rain.” — James 5:7 (NKJV)

Just like with onions, the harvest we long for takes time—and God’s timing is always right.

Closing: Encouragement and Next Steps

Planting onions isn’t complicated, but it does demand attention to the basics. Choose the right variety for your latitude, plant at the right depth, keep them weed-free, and you’ll pull up bulbs that store for months and flavor every meal.

If you’re just getting started, try a small patch this season. Make notes on which type works best in your soil and climate, then expand from there.

📥 Next Step: Grab my Onion Planting Chart — a printable guide with day-length types, planting times by USDA Zone, and spacing rules — so you can pin it in your garden shed and never guess again.