Onion sets for bulb onions are small, dormant bulbs that skip the seed stage and jump straight to bulb formation, giving you a head start on the growing season. From zones 1 through 13, these frost-hardy Allium cepa bulbs mature in 85 to 90 days and thrive in well-drained soil with a pH between 6.0 and 7.0. Using sets instead of seeds means faster harvest and more reliable results, especially for gardeners in cooler climates or those wanting to extend their growing window.
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Onion sets eliminate the guesswork of seed starting and deliver mature bulbs in under three months, making them one of the fastest routes to a full harvest. Their frost-hardy nature means you can plant them as soon as soil is workable in spring, even in zones as cold as 1. Space them just 2 inches apart in rows 12 inches apart, keep the soil consistently moist, and you'll have bulbs ready to cure and store by mid-to-late summer.
Onion sets grow into full-sized bulbs for storage, cooking, and fresh use. The mature bulbs work in any savory dish where onions are called for: caramelized in soups and stews, roasted alongside root vegetables, sliced fresh into salads, or minced into sauces and salsas. Because sets mature quickly and store well when properly cured, they are especially useful for gardeners looking to preserve a harvest for winter cooking.
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If starting from seed indoors, sow 10 to 12 seeds in a 4 to 6 inch pot, then thin seedlings carefully. If seedling tops grow over 5 inches before transplant, cut them back to 3 inches to reduce transplant shock. Separate seedlings gently and plant in a shallow trench when ready to transplant outdoors, filling the trench around them and watering in well. Otherwise, onion sets are planted directly into the garden in spring.
Direct plant onion sets in spring as soil becomes workable. Space sets 2 inches apart in rows 12 inches wide. Press sets firmly into prepared soil at their original depth. Germination temperature range is 50 to 70°F, so soil should be warming toward the upper end of that range for quickest growth.
Harvest when bulbs have reached full size and about half the tops have naturally fallen over, typically 85 to 90 days after planting sets. Push over any remaining standing tops and wait about 1 week before pulling bulbs. Harvest only in dry weather, as onions harvested in wet conditions do not cure properly and are prone to rot. Loosen soil around bulbs gently before pulling to reduce damage, then brush off soil and lay bulbs in a warm, well-ventilated area to cure before storage.
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