Planting Bulbs
Planting Potato Onion Bulbs
Add two inches (5 cm) of compost into top six inches (15 cm) of soil, or irrigate with compost tea prior to planting. Plant in rich, well-drained soil in full sun approximately 8-12 inches (20-30 cm) apart. I plant my large bulbs eight inches apart in all directions; smaller bulbs can be 4-6 inches (10-15 cm) apart because they generally produce a single large bulb instead of a nest.
Soil: pH 6.0 - 7.0
Potato Onions need a lot of direct sunshine, nutrients and adequate moisture to develop the largest and best-tasting bulbs.
Looking inside potato onion bulbs.
Spacing of large potato onion bulbs while planting, through harvest.
Spacing of medium potato onion bulbs while planting, through harvest.
Spacing of "small" potato onion bulbs while planting, through harvest.
When to Plant
Planting during Spring - warm and/or wet climates
Plant at soil level so half of the bulb exposed.
Day length during the growing cycle is crucial, so get them in the ground as early as possible, by mid-April at the latest. If you can't get them in by mid-April, fall planting may be a better option.
If you live in a wet climate, spring planting may be the best option to prevent rot.
Fall - cold climate, or planting red or white varieties:
I suggest planting red/white during fall simply because they do not store well.
Fall planting can trigger flowering, but it's not guaranteed.
Fall planted onions tend to mature earlier - as much as a month - than spring planted. I harvest my fall-planted bulb mid-July, which means I can get another crop planted in the same space - such as squash, lettuce, chard, pak choi - and harvested during the same season.
No matter the color of potato onion you're growing, plant it so the top of the bulb is barely covered with soil. This protects the bulb from frost. In zone 5B, I plant all varieties of potato onions during October, the same time I plant my garlic. If you have good results with fall-planted garlic, the same should hold true for potato onions. In the past I've planted the onion bulb neck below soil.
Mulch. I rake the leaves from my yard and coat the gardens with them so they are 6-12 inches deep (15-30 cm).
In spring, remove the mulch at the first signs of new growth, side-dress with fertilizer.
Winter - mild climates
Back in 2020 I watched a video in which the guy planted his potato onions on Christmas day. He did not mention his location, but he was wearing jeans and a long sleeve shirt. There was no snow on the ground and I could not see his breath.
Growth Period
When spring-planting bulbs, time to maturity is 70-100 days.
Potato onions have two distinct growing phases: vegetative and bulbing. During the vegetative phase they produce the top leafy greens. When the leaves stop growing, triggered by day-length, they enter the bulbing phase. It is during this phase that the plant uses those greens to help enlarge the base of the onion - the onion bulb.
In the past I did not pay attention to when potato onion bulbs started to form, so I made a video of it.
Considering my onions started to bulb around June 5th when there is 15-1/2 hours of daylight, my bulbs are considered a long-day variety.
My seed bulbs were purchased from Southern Exposure Seed Exchange, which is based in Virginia (an intermediate-day location). According to the Maine Potato Lady, SESE purchased their seed bulbs from her (a long-day bulb location). She got her seed bulbs for a gentleman who has since died, therefore, she didn't know the original source of his bulbs.
So my bulbs thrive in both intermediate- and long-day environments.
Watching when potato onions bulb
How Many Bulbs Should I Plant?
Decide how many onions you want to eat the following season. Divide that number by four. That is how many bulbs you should plant. Example:
I use 1720 onions during any given year (keep in mind that potato onions are smaller than commercial onions).
1720 divided by 4 = 430 bulbs need to be planted.
Why am I dividing by four? I plant a mixture of small, medium and large bulbs and I average five bulbs per nest, therefore I will plant one bulb from each nest and eat four. So, if I plant 430 bulbs and average five daughters per mother, I will harvest 2150 bulbs minus 430 bulbs that I will keep and replant for the following season, leaving me 1720 bulbs to eat, or just shy of five potato onions per day. I hope that makes sense. You may have to make adjustments depending on the average number of bulbs your onions produce, and the size of the bulbs you plant.
430 bulbs uses 240 square feet (22 square meters) of my garden.
Keep in mind that I started by planting 145 potato onions in 2020 and I was not completely sustainable until 2023. I had to build up to the number of planting bulbs I needed.
Best Bulbs
Since there isn't much information available about potato onions, I thought "best bulbs" meant those that were oddly shaped and obviously bulging, ready to burst with two or three daughters. Evidently, that is not so. It is suggested that the best bulbs are those that are round, firm and resemble the perfect onion, no matter the size.
Watering
Water them consistently, keeping the soil evenly moist but not wet. Generally, I water once a week with a soaker hose for 15-30 minutes. During extremely hot seasons, twice a week. Ideally, you want to provide a thorough soaking to a depth of six inches once a week, rather than lightly sprinkling them every day.
Weeding
Keep your onions well weeded, as they do not compete well with weeds. You may think I'm crazy; I weed my gardens by hand. Being organic, it's the only way to truly keep the weeds under control.
Can I Plant Bulbs in Containers?
Yes, absolutely, although personally I have no experience with this method. I would think one large bulb in an 8" (20 cm) container would do well. As long as potato onions have soil, sunlight, water, fertilizer, and enough space for the nest to grow, they should do well! Some vendors even sell single bulbs pre-planted in pots.
First-timers
If you're new to potato onions, do what you think is correct (or do what I suggest above). Aside from weeding, potato onions and garlic are the easiest and least time-consuming plants I grow. Learn by trial and error for your climate. I knew nothing about potato onions when I started. I simply put them in the ground one fall and harvested them the next summer.