Culture: Set seedlings out 1–2" deep and 6–8" apart in shallow trenches, 1–2' between rows. Onions survive light frosts. After half the onion tops fall, push over the remainder and harvest within a week. Field-cure in the sun about 10 days until dry, covering with a tarp in wet weather. In the event of extreme heat or prolonged damp conditions, we recommend sheltered curing in a well-ventilated barn or greenhouse. Curing is essential for long storage. Store cured onions in mesh sacks in a cool dry well-ventilated place, periodically removing sprouting or rotting bulbs. In spring, put your remaining onions in the fridge to extend storage until your new crop is ready.
Onions are triggered to form bulbs in response to day length. Day length differs depending on latitude, so different onion varieties were developed to have different day-length needs. In the north, the earlier onions are set out, the more chance they have to make top growth while the days are lengthening. High fertility and steady water is crucial for large onions. Side dressing is recommended. After summer solstice they begin bulbing.
All the varieties we list are suitable for northern growers. If you live farther south, note our latitude specifications at the end of each description.
Long-day: Must be north of 36° latitude, though some long-day types perform best north of 40°. These onions need 14-16 hours of sun a day to trigger bulb formation. May not perform well in continually hot soil temps.
Intermediate-day: Also called day-neutral onions, generally need 12-15 hours of daylight to bulb. Some can do well in parts of the upper southern U.S. all the way up through Maine. Others are best for mid-latitudes only (35-40°). All intermediate-day onions in our catalog have performed well repeatedly in our Maine trials.
(Short-day: Suited for the South, below latitude 36°, bulbing when the day length measures between 10–12 hours. We don’t offer seed for short-day varieties.)
Onion seed is short-lived. Retest 1-year-old seed before using. Discard anything older.
Click for Onion sets and plants.
Culture: Start allium seeds indoors in February or March. Minimum germination soil temperature 45°; optimal range 60-70 °. We discourage using bottom heat because alliums germinate poorly in soil temps above 70°. Transplant in spring soon after the ground can be worked.
Alliums are heavy feeders and want generous amounts of organic matter, fertilizer and water. Late transplanting and poor fertility can result in small onions or failure to form bulbs. Alliums are notoriously intolerant of weeds. Slugs love to munch them, and in areas above 40° latitude, root maggots may be a problem.
About allium seed: Allium seed is short-lived. We do not hold over hybrid onion seed because of precipitous decreases in germination. Test 1-year-old seed before using. Discard anything older.
DM Downy Mildew
PR Pink Root
ALERT: Leek Moth is emerging as a serious pest potentially affecting all Alliums in the Northeast. Consult your local Cooperative Extension for more info.
For the latest results of our germination tests, please see the germination page.