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Onions require well-drained soil with a pH of 6.0-7.0 and full sun. Start onions indoors and transplant outdoors when soil temperature has risen to 68-75F. Bunching onions: 24 seeds/ft. rows 12-18" apart. Thin to one per inch. Bulbing onions: 12 seeds/ft. rows 12-18" apart. Thin to one every 4 inches.
Water at least one inch per week, particularly during bulbing.
Fertilize once prior to transplanting or planting, and throughout the growing season. Onions are heavy feeders.
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Onions can be ready to harvest as young green onions or left to bulb. When necks become soft and tops are falling over, pull and sun-cure at least 2-7 days, depending on weather. Move to a protected location to finish drying. When dry, clip off tops and roots and store in onion bags or shallow boxes at near freezing and 65-70% humidity. Note: Walla Walla onions are not long storage onions.
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Bulbing in onions is triggered by daylength. Short day onions are best for southern states, whereas long day onions are better for northern states.