Planting, growing and harvesting onions

  • Published: Friday, March 15, 2013
Your Show-Me Garden: MU Extension brings you gardening tips from experts around the state.

COLUMBIA, Mo. – Although onions may make you weep, the world would be a sadder place without the flavor and aroma they bring to our meals.

They are also one of the most productive of garden vegetables, says David Trinklein, University of Missouri Extension horticulturist.

There are three basic groups of onions: The common, the aggregate (which includes shallots) and the proliferous, often called the Egyptian or walking onion. The common onion is the type we plant in our gardens in the spring, Trinklein said.

“It produces a single, large bulb that usually matures by midsummer in the Midwest climate,” he said. “Green onions are simply plants of this species that are pulled before the bulb is well-formed.”

The spring-planted common onion is grown from sets, transplants or seeds. Planting should be done as soon as the soil can be worked in the spring. Onion sets are the most common means of planting onions.

“Sets are small bulbs that develop quickly to produce green onions or allowed to mature to produce (dry) bulbs. To produce green onions, plant the sets in a well-drained soil about an inch apart,” Trinklein said.

“For larger dry bulbs, sets should be placed no closer than 2 inches apart,” he said. “Small sets are more desirable than larger sets, which tend to flower more easily. If flowering occurs, remove the flower head as soon as it is visible. Onions that flower form smaller bulbs that do not store as well as bulbs harvested from non-flowering plants.”

Onion transplants are seedlings started by a specialist propagator. Transplants are grown in the south, pulled at an early growth stage and shipped north for sales as propagules or “starts.”

“Large, sweet types such as sweet Spanish and Bermuda types frequently grow from transplants,” Trinklein said. “Space them 4 to 5 inches apart within rows spaced 12 to 18 inches apart. As a rule, ‘sweet’ onions do not store as well as the more pungent types.”

The onion is a cool-season crop that takes more than 95 days to mature. For this reason, seeds planted outdoors don’t do well in Missouri because of the hot summers. If grown from seed, Trinklein suggests starting onions indoors well in advance of outdoor planting.

The onion is a photoperiodic plant, meaning it responds to day length. Short-day onions will form bulbs only when the length of day is 12 hours or less. Long-day varieties need day lengths of at least 15 hours to form bulbs.

“Varieties grown in Missouri are typically long-day varieties. This is another reason why onions seeded directly into the garden do not perform well in our state,” Trinklein said.

Onions grow best under cool temperatures (55 to 75 degrees Fahrenheit) in a loose, friable soil. They are sensitive to soil pH, needing soil in the 6.2 to 6.8 range. Apply fertilizers according to soil test recommendations. When fertilizer is needed, choose one that’s low in nitrogen but high in phosphorus and potassium, such as 5-10-10, Trinklein said.

Onions don’t compete well with weeds. Adding mulch after onions are established will help control weeds and conserve moisture.

“Common problems with onions include insects such as thrips and onion maggots, and fungal diseases such as downy mildew, neck rot, pink root and smut,” Trinklein said.

You can use onions any time during their production; for storage, harvest bulbs when the “neck” dries and the tops have fallen over, Trinklein said. After digging, cure onions by placing them in a warm location with good air circulation and low humidity for several weeks. After curing, store onions in relatively cool, dry conditions.

Onion trivia

Source: National Onion Association

  • Each year, U.S. farmers plant approximately 125,000 acres to produce 6.2 billion pounds of onions.
  • The largest onion ever produced weighed 10 pounds, 14 ounces.
  • The average American consumes more than 20 pounds of onions each year.
  • Men eat 40 percent more onions than women.
  • During the Middle Ages onions were considered so valuable they were used to pay rent and as wedding gifts.
  • Most people tear up when cutting onions because of sulfur-containing compounds that are released; chilling an onion before cutting will help curtail the crying.
  • Libya has the highest per capita consumption of onions at 66.8 pounds per person each year.
  • A single serving of onions contain 45 calories and is high in vitamin C and fiber.
  • There are fewer than 1,000 onion farmers in the U.S.
  • An old English rhyme claims the thickness of an onionskin can help predict winter weather: “Onion skins very thin, mild winter coming in. Onion skins very tough, coming winter very rough.”
  • Eating parsley will eliminate “onion breath.”
  • During the Civil war, Ulysses S. Grant reportedly told the War Department in Washington, “I will not move my army without onions” because of their medicinal use during that era.

Writer: Debbie Johnson

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David Trinklein

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