How to Grow Big Onion Bulbs


Onions are a member of the allium family, as are garlic, leeks, shallots and chives, and are popular in all sorts of cooking. Many gardeners are frustrated when their onions are not as big as they would like them to be, but anyone can grow big onions with the proper conditions and a little know-how. You can grow onions from seeds, from seedlings that were started late in the preceding year (called "sets") or from transplanted onion plants. Growing onions from seeds produces very small onions; growing from sets produces larger onions. Of the three methods, growing onions from transplants produces the largest bulbs, You can purchase onion plants, or you can save money by starting your own seedlings indoors for transplant.

Things You'll Need

  • Onion seeds or plants
  • Garden claw or aerator
  • Garden rake
  • Sterile seed-starting medium (if starting from seeds)
  • Container or cardboard egg carton, four inches to six inches deep (if starting from seeds)
  • Fertilizer
  • Start seeds indoors in January or February, eight to 12 weeks before you want to plant them outside. Sow the seeds in dense rows or, if you're using egg cartons, plant three to a cell. Plant ¼-inch to ½-inch deep in sterile planting medium. Keep the lights on no more than 12 hours a day, or the bulbs may develop too early. Keep the tops trimmed to about ¾ inch, and water regularly.

  • "Harden off" the seedlings before you plant them outside. Put them outside for short periods of time, every day for about two weeks, so they will experience less shock when you transplant them.

  • Loosen the soil eight to 10 inches deep, breaking any clods and removing trash and rocks. Rake the soil smooth and work in fertilizer.

  • Plant the seedlings just deep enough to stay in the soil, about 4 inches apart.

  • Water frequently; onions need about an inch of water a week.

  • Fertilize again when the leaves are 6 inches tall, and again when the bulbs start to swell (they will push themselves up and seem to sit on top of the soil).

  • Harvest the onions when a quarter to half of the tops have fallen over and the onions have a papery skin.

Tips & Warnings

  • Loosen the soil when it is relatively dry, so it won't stick to your tools.
  • Plant seedlings as early in the spring as possible, once the threat of frost is over.
  • Since onions grow in layers, more leaves on a plant mean more layers in the bulb, and that means a bigger bulb.
  • Onions need at least six hours of direct sunlight a day. More sunlight is better.
  • You can eat the onion bulbs at any stage of the growing process.
  • Select onions to plant on the basis of the length of summer days where you live. For example, the southeastern U.S. averages 12 hours of daylight, so people in the Southeast should buy "short-day" onions.
  • Lack of adequate water when the bulbs are swelling will produce an onion with a very strong flavor.
  • Keep the area free of weeds---weeds will steal nutrients from the onions and keep them small.

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