How Deep to Plant Onion Bulbs: Complete Planting Guide

Plant onion bulbs 1-2 inches (2.5-5 cm) deep with the pointed end facing upward for optimal growth. This depth ensures proper root development while providing enough soil coverage to protect bulbs from temperature fluctuations. Space bulbs 4-6 inches apart in rows 12-18 inches apart for healthy bulb formation and maximum yield.

Why Planting Depth Matters for Onion Success

Getting the planting depth right for onion bulbs directly impacts your harvest quality and quantity. Planting too shallow exposes bulbs to temperature swings and drying out, while planting too deep restricts growth and delays emergence. The ideal 1-2 inch depth creates the perfect balance for root establishment and bulb development. Research from the University of Minnesota Extension confirms this depth range produces the largest, most uniform bulbs across multiple onion varieties.

Onion Planting Depth Guide by Variety

Onion Type	Recommended Depth	Spacing Between Bulbs	Row Spacing
Yellow Storage Onions	1-2 inches (2.5-5 cm)	4-6 inches (10-15 cm)	12-18 inches (30-45 cm)
Red Onions	1-1.5 inches (2.5-4 cm)	4 inches (10 cm)	12-15 inches (30-38 cm)
White Onions	1.5-2 inches (4-5 cm)	5-6 inches (13-15 cm)	15-18 inches (38-45 cm)
Green Onions (Scallions)	0.5-1 inch (1.25-2.5 cm)	2-3 inches (5-7.5 cm)	10-12 inches (25-30 cm)

Step-by-Step Onion Planting Instructions

Soil Preparation (Before Planting)

Prepare your garden bed 2-3 weeks before planting by incorporating 2-3 inches of compost into the top 6-8 inches of soil. Onions thrive in well-draining soil with a pH between 6.0-7.5. The Oregon State University Extension Service recommends conducting a soil test to determine if additional phosphorus is needed, as onions require adequate phosphorus for strong root development.

Planting Technique

1. Wait until soil temperature reaches at least 50°F (10°C) before planting
2. Create shallow trenches at your desired row spacing
3. Place bulbs with pointed end up, flat root end down
4. Space bulbs according to variety-specific recommendations
5. Cover with 1-2 inches of soil (less for green onions)
6. Gently firm soil over bulbs without compacting
7. Water thoroughly after planting

Adjusting Depth for Different Conditions

Your exact planting depth should vary based on specific garden conditions. In heavier clay soils, plant slightly shallower (1 inch) to prevent bulbs from sitting in moisture too long. In sandy soils that dry out quickly, plant toward the deeper end (2 inches) to maintain consistent moisture around developing roots. During unusually cold springs, adding an extra half inch of soil provides additional insulation for emerging shoots.

Onion Growth Timeline: What to Expect After Planting

Understanding the growth stages helps you identify potential problems early:

• Days 1-7: Roots begin developing downward while the shoot pushes upward
• Days 7-14: First green shoots emerge from soil surface
• Weeks 3-6: Active leaf growth with minimal bulb development
• Weeks 7-12: Bulb formation begins as days lengthen
• Weeks 12-16: Bulb swelling phase with rapid growth
• Weeks 16-20: Maturation phase when tops begin falling over

Common Planting Mistakes That Reduce Your Harvest

Gardeners frequently make these errors when planting onion bulbs:

• Planting too deep: Bulbs struggle to emerge, resulting in weak plants and smaller yields
• Planting upside down: Roots grow upward while shoots grow downward, wasting energy
• Insufficient spacing: Crowded bulbs compete for nutrients, producing undersized onions
• Planting too early: Cold soil temperatures cause bulbs to bolt (produce flowers)
• Ignoring soil preparation: Poor drainage leads to rot, while nutrient-deficient soil limits growth

Troubleshooting Poor Onion Growth

If your onions aren't performing as expected, check these common issues:

• Yellowing leaves: Could indicate planting too deep or nitrogen deficiency
• Small bulbs: Often caused by overcrowding, poor soil fertility, or incorrect day-length varieties
• Bolting (flowering): Usually happens when exposed to extended cold after planting
• Rotting bulbs: Typically results from heavy, poorly draining soil or excessive moisture

Harvesting and Storage Tips

Harvest onions when tops naturally fall over and begin drying. Cure bulbs in a warm, dry, well-ventilated area for 2-3 weeks before storage. Store cured onions in mesh bags or crates in a cool (32-45°F), dark, dry location with good air circulation. Properly stored onions can last 4-6 months depending on variety.

Can I plant onion bulbs deeper than 2 inches for larger bulbs?

No, planting deeper than 2 inches typically produces smaller bulbs. Research from Cornell University shows onions planted at 3 inches depth had 30% smaller bulb size compared to those planted at 1.5 inches. The deeper planting restricts the plant's energy for bulb formation.

What happens if I plant onion bulbs upside down?

Planting onion bulbs upside down forces the plant to redirect energy to correct growth direction, resulting in weaker plants and smaller bulbs. The roots will grow upward while the shoot grows downward initially, wasting valuable energy during critical establishment phase. Always position bulbs with the pointed end up and flat root end down.

How deep should I plant onion sets versus onion seeds?

Onion sets (small bulbs) should be planted 1-2 inches deep as described in this guide. Onion seeds require much shallower planting at only 0.25-0.5 inches deep. Planting seeds too deep prevents proper germination. Seedlings should be thinned to proper spacing once they reach 4-6 inches tall.

Should I cover onion bulbs with mulch after planting?

Yes, applying 2-3 inches of straw or shredded leaves after planting helps maintain consistent soil moisture and temperature. Mulch prevents the soil from crusting over, which can restrict emerging shoots. Avoid heavy mulch that might retain too much moisture and cause rot in heavy soils.