How to Grow Perennial Potato Onions

An endless supply of onions that store forever and are easy to grow? Enter perennial potato onions. These are some of our favorite alliums out there. They are so easy to grow and will store for up to 18 months. They grow similarly to shallots and other multiplier onions - but can stand in for other onions in recipes in a way that shallots simply cannot. And yes, they are perennial! They can be replanted year after year for a continuous supply. 

Their easy and adaptable growth habit makes them super doable for those of you practicing organic gardening. They have limited pests and tend to hold up to stress better than other onions. These onions are winter hardy as well! In most places, these onions will survive the winter no problem - but if yours are particularly harsh, just a thick layer of mulch will do to keep these guys alive! 

We love these onions for their approachability. How it works is that you plant one bulb, and many bulbs are produced from that single planting. They’re great for the gardener that is somewhat intimidated by traditional onions or who have struggled with them in the past. Let’s be honest, starting onions from seed is not for the faint of heart. But if all you need is that delicious onion flavor but want to put in less work? These are perfect for you. 

Where to Grow Perennial Potato Onions

Potato onions are widely adapted to many climates. They are most notably known for their ability to withstand freezing temperatures. They can be grown in any zone so long as they are properly planted and mulched where necessary. We recommend growing these onions in growing zones four through nine. 

How to Grow Perennial Potato Onions

Perennial potato onions are traditionally planted in the fall and are overwintered to give the bulbs a head start before spring. This will result in better bulb development and larger yields. The bulbs should be planted three to four weeks before your first frost in the fall - around the same time as garlic. 

Soil and Sun

Like most root crops, these onions will prefer well-draining soil that is high in organic matter. They will need full sun in order to grow to maturity but can handle some shade. Ahead of planting, consider amending the soil with a phosphorus and/or calcium rich fertilizer, such as Bone Meal. 

Spacing

When planting, larger bulbs should be planted six to eight inches apart while smaller bulbs should be planted four to six inches apart. Plant each bulb ½ to one inch deep and pat lightly on top of the soil to ensure adequate contact. 

Planting Tips

Growing perennial potato onions is easy but following these simple tips will help ensure your success! 

• Don’t break apart any clusters that exist until you’re ready to plant them. 

• Consider the direction in which you’re planting them. The root end should be facing down and the pointy, sprouting end should be facing up. 

• Planting your onions too deep can affect their ability to cure properly for long-term storage. 

Caring for Perennial Potato Onions

Caring for these perennial onions is about as easy as planting them - very. But these tips should help ensure their survival through the winter, their flavor, and ability to thrive. 

Watering

Potato onions are overall pretty tolerant. They do not need special watering attention except during prolonged dry spells. That being said they will benefit from about an inch of water per week during the growing season. Overwatering - or extended stays in overly wet and boggy soil - can cause the bulbs to rot. 

Mulching

Potato onions are very winter hardy but if your winters are particularly brutal, we’d definitely recommend mulching as “insurance” for your crop. Depending on your winters, a six inch to one foot layer of organic straw, leaf mulch, or grass clippings will keep your bulbs cozy! 

Seasonal Care

As soon as the soil is workable in spring, thin or remove the mulch over your potato onions, if applicable. Depending on your growing zone and your planting time, you may find that your bulbs will sprout at different times - either before winter or after. Either is totally acceptable. As your green tops start to sprout, remove any remaining mulch. Potato onions are pretty self-sustaining! 

Fertilization

Nitrogen should not be applied during bulb development (when the green shoots pop up). This may happen before or after winter when planting in fall. Because of this, we recommend applying fertilizer or amending your garden bed ahead of planting. As soon as the soil is workable in the spring, you can apply a 0-10-10 or similar to your potato onions to encourage root development. 

Harvesting Your Perennial Potato Onions 

And now it’s time to enjoy the bounty! Harvesting potato onions is straightforward but follow these steps to ensure you’re getting the most out of your crop in size and also in storage. 

When to Harvest

When planted in the fall, potato onions will typically be ready for harvest in late spring or early summer. You’ll know when it’s time to harvest when the tops turn brown and flop over. This is your sign! While technically they can be left all summer to then regrow in the fall, there may be issues if weeds are left to overtake these dormant roots. For eating and replanting, it is best to harvest before the heat of summer and replant your seed onions in the fall. 

How to Harvest

Take a shovel or digging fork and gently insert it into the soil about six to eight inches away from the clump of onions. Angle it in so you can gently lift the soil, and the onions, out of the ground. At this point, removing them from the soil should be easy enough to do without damaging them. Gently brush off any excess soil or clumps and separate the bulbs from each other. 

Curing

Once you’ve harvested your onions, you’ll want to get them ready to cure. The curing process ensures that they’ll be able to store for as long as possible. Trim the remaining browned tops to about ½ inch and brush off any dirt as well as peel off any loose skin.

To begin the curing process, lay your bulbs out in a single flat layer on a flat surface. Ideally they’d be dried on screens but that’s not entirely necessary. Leave them in the shade to cure and protect them from moisture if there is a risk of rain in the forecast. Cure them for two to four weeks. 

Storage

To maximize their long-term storage ability for up to 18 months, keep them in a cool, dry place with good air circulation. A room with a fan will do just fine. Take care to sort through them every once in a while to remove any that have spoiled. This will help protect the others. 

Harvest and Storage Tips

• While ideally you’ll be harvesting once all of the tops are brown, if there are still some bulbs with green on the top, leave the tops intact to dry out the rest of the way before trimming them to ½ inch. 

• Potato onions can also be braided! Briefly dip the tops in water to braid your onions like you would garlic. This can be an effective way to store them. 

Pro Tips for Growing Perennial Potato Onions

• Try planting in raised beds if you have heavy clay soil.

• The largest bulbs will produce many small onions while the smallest bulbs will produce fewer large onions. For seed stock, it’s best to save the smallest onions to replant, but in order to grow a little seed every year, set aside a large onion or two so that you can expand the number of small bulbs you can plant. 

• Yellow potato onions will very rarely flower but if they do, this flower should be cut back to send energy back into bulb development rather than aerial parts.  

Get Growing!

This onion is easy to grow, an excellent addition to any perennial garden, and delicious as well. It’s time to get growing! Perennial potato onions can be pre-ordered from Sow True Seed and will ship right at planting time in October. Also check out other multiplier onions like shallots and Egyptian walking onions.

	Article Written by: Hannah Gibbons
	About the Author: Hannah Gibbons, an employee at Sow True Seed since 2020, has nearly a decade of experience in the agricultural industry. Their passion for environmental education and regenerative agriculture has been the cornerstone of their work, aimed at making gardening accessible to all.