How to Grow Potato Onions: A Growing Guide
Potato Onions, (Allium cepa var. aggregatum) also called Hill Onion, Mother Onion, or Pregnant Onions, are a long-term storage multiplier onion that can be grown in the fall or spring (depending on where you live). Potato onions produce clusters of onions similar to shallots with some important differences. They are typically larger than shallots with a stronger flavor and store much longer, up to 12 months if conditions are right. Potato onions are long day, so not appropriate for southern Texas or Florida.
Soil: Potato Onions grow in a wide range of soils, but prefer mildly acidic to neutral soils that are well drained and rich in organic matter.
Spacing: Space rows 12–18” apart, sow every 4–6” for small bulbs and 8” apart for larger bulbs.
- Incompatibility — beans, peas
- Companions — cabbage, carrots, leeks, lettuce, marjoram, parsley, parsnip, roses, tomatoes
Water Requirements: Keep potato onions well watered, if soil dries out then growth will stop. It's best to use drip irrigation rather than overhead watering.
Fertilization: After planting, feed with a fertilizer that is higher in phosporus, a 5-10-5 mix is a good choice.
Weeding: Keep well weeded, does not compete well with weeds.
Plant in full sun.Very small bulbs may not multiply the first year, but will the second season.
Planting & Growing
Potato onions can be planted in the spring or fall depending on where you live and how harsh the winters are. Fall planting can give bigger yields but if the winter is very harsh, some bulbs may be lost.
Spring Planting: If your winters are very harsh, plant potato onions in spring when ground is workable. Plant bulbs deep enough so about ½–1” of soil covers the bulb. In colder winter areas, plant potato onions a little deeper, so about 2–5” of soil covers the bulb. Mulch heavily (4–8”) making sure to remove most of the mulch in the spring.
Fall Planting: If winters are mild to moderate you can plant potato onions in the fall from October to December. Plant bulbs deep enough so about ½–1” of soil covers the bulb.
As the potato onions are getting close to harvest the tops will fall over. When you see the tops start to fall over, cut off the water and make sure all of the mulch is removed (about 2 weeks before lifting). Once about half of the tops have fallen over, the potato onions are ready to lift. You can harvest the whole cluster or take only a few for immediate use. Make sure to harvest on a dry day.
To cure, bring the bulbs into a shaded, dry warm area with good air circulation. Spread them out in a single layer and let them cure for about 1–2 months. During the curing perion make sure to monitor for any spoiled bulbs and remove them. After curing, cut the tops off about 1” above the bulb and separate the onion clusters.
After curing store between 35–40°F or 50–70°F with about 60–70% humidity. Onions may sprout if stored between 40–50°F, so do not put them in the refrigerator. Best to store in a mesh bag or on a shelf (in a single layer) with good air circulation and placed in a root cellar, cool room, or garage. Potato onions will keep for up to 12 months if conditions are right.
Common Pests & Diseases
Onion Fly: Usually a problem during very wet growing seasons. Control with a product labeled for onion flies (or maggots).
Onion Thrips: Can control with insecticidal soap.
Slugs: Control by either hand picking them off or use a product labeled to control slugs.
Pest Control – IPM
Integrated Pest Management (IPM) is an effective and environmentally sensitive approach to pest management that starts with the least invasive or impactful control methods and considers the use of chemical controls as the last step. For more information watch our Integrated Pest Management video.
Important to practice good cultural controls for pest management of potato onions. Cultural controls such as removing plants after harvest (to avoid leaving food for insects to continue to multiply on), use healthy bulbs to plant, practice crop rotation (e.g. do not plant onions in same area for 3–5 years).
Heirloom: Heirloom seeds come from open-pollinated plants that pass on similar characteristics and traits from the parent plant to the next generation plant. Heirloom vegetables are old-time varieties generally which have been in production since before WWII, and have been saved and handed down through multiple generations.
Hybrid: a cross between two or more unrelated plant varieties. The two different varieties are cross bred, resulting in a seed that carries one or more favorable traits (increased yield, uniformity, color, disease resistance.) Hybrid seeds are not GMO, as they are manually cross-bred, not genetically modified in a lab. Hybrid seed is often sterile or does not reproduce true to the parent plant. Therefore, never save the seed from hybrids.
Open Pollinated: generally refers to seeds that will “breed true”. When the plants of an open-pollinated variety self-pollinate, or are pollinated by another representative of the same variety, the resulting seeds will produce plants roughly identical to their parents. Genetic traits may differ only slightly due to variations created by local conditions.
GMO: Genetically Modified Organisms were genetically modified in a laboratory where DNA genes are extracted and mixed with other unrelated plants to improve characteristics. Saved seed will not always be viable and may be trademarked to prevent unauthorized use.