Onions are an important crop for gardening year round. You can plant onion transplants (bunches), sow directly, transplant seedlings, or plant small starter bulbs called sets.
For optimum success, plant onions in full sun and well drained soil that is high in organic material. Add high nitrogen fertilizer close to, but not in, planting area at time of planting for starts, and in March for direct sown seedlings.
Direct Sow: In February, directly sow onion seeds 1” apart, in rows 6-8” apart. Sprinkle compost over the seeds if warm, dry weather is expected, otherwise, do not cover them. Keep the soil moist. When plants have 4 leaves, thin to 4” apart. Sow overwintering onion seeds in July and August.
Seedlings: Transplanted seedlings produce more uniform plants than direct sowing. Starts are available for much of the year, but to get a good size bulb, planting is in late winter or mid- summer. To grow your own seedlings, sow densely in January or early July and do not thin. Provide 12 hour “days” using supplemental light in January, and trim seedlings back to 3” tall whenever they reach 4” tall, this will prevent flopping over. Plant out when 1/16” in diameter, 2-3 weeks old.
Sets: Onion sets sprout & establish quickly for use as scallions and they produce bulbs about 3 weeks earlier than from seed. However, sets are notorious for bolting before growing larger as a result of erratic spring weather. Planting sets in the fall is best, otherwise as early as possible in late winter to give them plenty of time to bulb.
Bunches: Bunches are the easiest way to grow onions west of the Cascades. They are overwintered in the ground, and then dug up in preparation for sale. This gives them a big head start over seedlings. They are available only in February and March.
Bulbing: The size of the bulb depends on the amount of top growth before bulbing starts. Some onions are photoperiodic - bulbing occurs only when the length of day is just right. The day length needed for bulbing differs by variety but is generally near the summer solstice in Portland. For big bulbs, the best thing you can do is to promote rapid top growth in spring by providing consistent conditions and plenty of nitrogen so that you have the largest possible plant when the day length causes the plant to switch from growing to bulbing. Onions planted later than March but before June will never make large bulbs.
Harvest: As the tops begin to dry out and turn yellow, withhold water so the bulbs mature in dry soil. It’s time to harvest when just a couple green leaves remain at the top. Hang the newly harvested bulbs where they will get good airflow but no dew or rain for a week or two to cure them. Proper curing is essential to promote long storage.
Storage: Keep onions in mesh sacks so they get good ventilation, and hang sacks where air is dry and cool. Check occasionally and remove any sprouting or rotting onions. Do not store onions near potatoes, both will go bad faster.