Potato Onion Growing Guide

February 19, 2025 Suzanne Fellows

Potato Onion Growing Guide: Planting, Care, and Harvesting Tips

Potato Onions (Allium cepa var. aggregatum), also known as Hill Onions or Mother Onions, are a long-term storage multiplier onion that produces clusters of bulbs. With proper storage, they can last up to 12 months, making them a valuable addition to your garden. Here’s everything you need to know about planting, growing, and harvesting potato onions.

Click here to access the complete Potato Onion Growing Guide (PDF) for detailed information on soil preparation, planting, and long-term care.

Why Grow Potato Onions?

• Long-Term Storage: Can last up to 12 months under the right conditions.
• Larger than Shallots: Produces bigger bulbs with a stronger flavor.
• Cold-Hardy: Ideal for fall planting in mild to moderate climates.
• Multiplying Bulbs: Each bulb produces clusters, increasing your harvest year after year.

Best Growing Conditions for Potato Onions

Soil and Sun Requirements:

• Soil Type: Prefers well-drained, organic-rich soil with a mildly acidic to neutral pH.
• Sunlight: Full sun for maximum growth.
• Watering: Keep soil consistently moist, using drip irrigation to avoid rot.
• Fertilization: Use a 5-10-5 phosphorus-rich fertilizer after planting to promote bulb growth.

Companion Planting:

• Good Companions: Cabbage, carrots, leeks, lettuce, marjoram, parsley, parsnips, roses, and tomatoes.
• Avoid Planting Near: Beans and peas.

How to Plant and Grow Potato Onions

Planting Times:

• Fall Planting: Ideal for mild winters, from October to December. Bulbs will produce larger yields.
• Spring Planting: Best for regions with harsh winters. Plant when the ground is workable.

Planting Depth and Spacing:

• Small Bulbs: Space 4–6 inches apart.
• Large Bulbs: Space 8 inches apart.
• Row Spacing: 12–18 inches apart.
• Depth: Plant bulbs so that ½–1 inch of soil covers them. In colder climates, plant slightly deeper (up to 5 inches of soil covering).

Mulching:

• Fall Planting: Add 4–8 inches of mulch to protect from freezing. Remove most of the mulch in spring.

How to Harvest Potato Onions

When to Harvest:

• Harvest when tops fall over, signaling maturity.
• Stop watering 2 weeks before lifting to allow bulbs to cure.

Harvesting Technique:

• Collect the entire cluster or only a few bulbs for immediate use.
• Always harvest on a dry day to prevent rot.

How to Cure and Store Potato Onions

Curing:

• Cure for 1–2 months in a shaded, well-ventilated area.
• Spread bulbs in a single layer and check regularly for spoiled bulbs.
• After curing, trim tops to 1 inch above bulbs and separate clusters.

Storage Conditions:

• Temperature: Store between 35–40°F or 50–70°F with 60–70% humidity.
• Do Not Refrigerate: Onions may sprout if stored between 40–50°F.
• Storage Method: Use a mesh bag or a single-layer shelf with good air circulation in a root cellar or cool room.
• Shelf Life: Properly stored potato onions can last up to 12 months.

Common Pests and Diseases of Potato Onions

Pests:

• Onion Flies: More common in wet seasons. Use labeled insecticides.
• Onion Thrips: Control with insecticidal soap.
• Flea Beetles: Chew leaves; use sticky traps, row covers like Agribon, or organic insecticides.
• Slugs: Hand-pick or apply slug control products.

Diseases:

• Onion Rot: Prevent with proper crop rotation and well-drained soil.
• Mildew: Use drip irrigation to keep foliage dry and improve air circulation.

Pest Management with Integrated Pest Management (IPM):

• Cultural Controls: Remove plants immediately after harvest to reduce pest breeding grounds.
• Crop Rotation: Rotate onions every 3–5 years to prevent pest and disease buildup.
• Healthy Starts: Always plant disease-free bulbs.