Potato Onions Growing Guide: from seed to kitchen

What are Potato Onions?

Potato onions are a self-sustaining, adaptable, and endlessly generous onion — once you grow potato onions, you may never go back!

Potato onions are a true perennial bulbing onion and one of the most versatile and rewarding crops a gardener can cultivate.
Generally, potato onions are larger and more pungent than shallots (which a considered more mild and complex in falvour), while regular onions have the strongest, most pungent flavours.

Unlike conventional onions, potato onions can be grown from seed or from replanted bulbs, left in the ground to multiply on their own, or managed as a long-term perennial planting. That flexibility is one of their greatest strengths. You can eat your best onions and replant them — because they divide, you never have to choose. If you would like more on conventional onions see their growing notes.

First year onions grown from seed can reach the size of a large baseball or bigger under ideal conditions. From year two onward, replanted bulbs divide into clusters called nests of 2–8 or more bulbs, typically ranging from golf ball to baseball size. Fertility, water, and spacing all influence final bulb size.

Potato onions are remarkably patient plants. They will persist through less-than-ideal soil, low fertility, and even competition from grass, gradually sizing up over two, three, or even four seasons. Onions grown in challenging conditions also tend to delay flowering, which is simply not possible with a typical biennial onion.

The potato onion we offer is a long day onions and best suited growers in southern Australia. Long day onions require extended day length to produce larger bulbs.

For more on growing/cultivating onions see our growing guide.

Seeds per Gram: ~250 – 350 seeds

Germination Temperature: 18 – 23 °C (germination in 7 – 10 days; slows <10 °C, patchy >28 °C)

Feed Requirements:

Moderate.
Work 5 cm compost plus a balanced organic fertiliser (about 60 g m⁻²) into the bed.
Side-dress with nitrogen mix (eg. blood & bone) at 4-leaf stage and again when bulbs begin to swell.

Seed Starting

Sow seeds in late winter or early spring.
Direct sowing is possible but winter weeds can soon overtake the slower growing onions.
We prefer sowing in trays or nursery pots in the greenhouse and transplanting in spring.
Cover seeds with 3–6 mm of potting mix and keep moist until germination. Onions are cold germinators.
Transplanting when seedlings are tall and robust — pencil-thick is ideal — reduces the need for early weeding and coddling.

Cultivation

These are day-length sensitive onions, ready for harvest around or shortly after the summer solstice.

First year seedlings: Space a minimum of 10–15 cm apart in rows 30-40 cm apart.
Second year and beyond: Space 20–30 cm apart, as established bulbs will divide and send up multiple leaf and flower stalks.

From year two onward, most onions will flower after vernalisation (overwintering).

To maximise bulb size, pinch flower stalks as soon as they appear and repeat as new stalks emerge. Alternatively, allow some plants to flower for seed saving while pinching others for larger bulbs.

Harvest & Storage

First year: Harvest when plants begin to flop over. Seed will mostly produce only 1-2 bulbs in the first year.
Second year and beyond: Harvest ‘nests’ of bulbs once the leaves start to flop for best storage length. If plant plants flower put these bulbs aside to eat first as they usually do not store as long.

After the summer solstice, bulbs may continue to grow and divide, but prompt harvesting after the plant flops gives the best storage results. Ideally stop watering a couple of weeks before harvest to allow for better curing and less risk of fungal problems.

Cure harvested onions in a dry, well-ventilated spot out of direct light until the leaf stalk has dried completely through, hanging is ideal for air flow.

Once cured store onions in a cool, dry location. We keep ours in potato bags.
Do not store in the fridge as that may encourage sprouting.

Eat any sprouting bulbs first, as some will store right through winter without sprouting (these will be the best ones for replanting and multiplying in the second year and beyond).