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About 14 to 16 weeks before your last frost date is the best time to start onion seeds indoors if you want to grow giant onions. Onion seedlings can be transplanted into the garden early, as soon as the soil can be worked.
Fill a deep flat or pots 4 to 6 inches deep with good seed starting mix to about 1 inch of the top. (Make sure your seed starting mix has been moistened). Then either broadcast the onion seeds over the surface or plant them in rows in the seed starting tray. Cover the onion seeds with about 1/4 inch of seed starting mix. Place the seed tray or pots in a large plastic bag or cover with clear domes until the seeds germinate, usually in 6 to 16 days. After they emerge, remove the plastic covering and place in a sunny window or other bright light and keep the soil moist. Feed the onion plants with a weak solution of a liquid natural fertilizer if the leaves become pale green. Bright light and cool temperatures are important for seedling growth. It will take about 8 to 10 weeks for the onion seedlings to reach the 6 to 8 inch stage. At that point, cut the tops with a scissors to 3 to 4 inches high.
Once the soil in the garden has thawed and is dried enough to work up, spread a well-balanced fertilizer over the soil surface and work it in. Plant the seedlings about 2 inches deep and space about 2 to 4 inches apart. Using Planting Paper will keep the soil warm, hold in moisture and keep weeds from growing around the onions until they are established. Fertilize a couple times during the summer and keep the rows weed-free as onions do not like competition. Harvest when the tops die back naturally in fall. If conditions are dry, lay the onions on top of the ground for a few days to cure. If rain threatens, move the onions to an airy shed or garage. After curing, cut the tops to about 1 inch from the bulb and store them in a mesh bag in a cool, dry, airy place.