Bunching Onions (Allium fistulosum)
How to Grow Bunching Onions from Seed
Choose a site with full sun to partial shade. Onions are a biennial plant that prefers loose, well-drained soil. Amend with compost, but avoid applying excess nitrogen which promotes top growth at the expense of bulb size.
Onions can be grown from starts (young plants) or seed. Regardless of method, choose the right type of onion for your particular area. The proper day length is critical to promote bulbing. Long day onions do best in the northern U.S., while short day onions are good for southern states. Intermediate day onions do well in areas in between.
Plant onion starts outdoors four weeks before the last frost or when soil temperatures reach 50˚F. Space starts 6" apart in rows 12" apart. Place onion starts in a hole to a depth of ½" and lightly firm soil around the plant. Keep moist until well established and mulch to conserve moisture and minimize weeds.
Sow onion seeds ½" deep and 1" apart. Seeds germinate best when soil temperatures are between 50-90˚ F. Seeds will sprout in 6-16 days. Thin seedlings so that mature plants are 3-6" apart. For northern growers: Sow onion seeds in early spring into flats of rich soil. Transplant seedlings in late spring and harvest in late summer when long days have initiated bulb formation. For southern growers: Sow seeds outdoors in late summer for overwintering. Cover with light frost protection as needed. Onions can be harvested in the spring once 10 hr days promote bulbing.
Young onion leaves can be harvested for scallions. Mature onions are ready to harvest when the tops begin to turn yellow. Use the back of a rake and push plant tops over horizontally to interrupt the flow of water and nutrients to the bulb. Once tops turn fully brown, usually within a couple of days, choose a sunny day to remove onions from the ground. Dry the onions outdoors or if rainy, choose a warm dry place indoors. Lay the tops of one row of onions over the bulbs of the other to prevent sunscald.
How to Save Bunching Onion Seeds
Onions are biennial, meaning they flower, set seed and die during their second year of growth. Harvest seed when flowers dry and turn brown. Store seeds in a cool, dry place. Seeds remain viable for 1-2 years.