Yard and Garden: Start Onions the Right Way

Learn how to start spring onions successfully in your home garden

February 28, 2020, 11:43 am | Richard Jauron

AMES, Iowa – Onions are an indispensable vegetable in the kitchen. Onions are easy to grow and take up little space in the garden. Follow the recommendations of Iowa State University horticulture specialists for successful onion growing. Contact the Hortline at hortline@iastate.edu, or 515-294-3108 with any additional questions.

Which onion cultivar should I plant?

onions growing in soil.When considering onions for a home garden, the suggested onion cultivars in Iowa include:

  • ‘Blush’ (brownish pink skin, globe-shaped, excellent storage).
  • ‘Candy’ (yellow-brown skin, globe-shaped, short term storage).
  • ‘Copra’ (yellow-brown skin, excellent storage).
  • ‘Patterson’ (yellow-brown skin, globe-shaped, excellent storage).
  • ‘Red Burgermaster’ (bright red, globe-shaped, good storage).
  • ‘Redwing’ (deep red skin, globe-shaped, excellent storage).
  • ‘Red Zeppelin’ (deep red, globe-shaped, excellent storage).
  • ‘Stuttgarter’ (light brown skin, flattened globe, excellent storage, from sets).
  • ‘Walla Walla’ (yellow-brown skin, flattened globe, short-term storage).
  • ‘Yellow Sweet Spanish’ (yellow-brown skin, globe-shaped, short-term storage).

An important aspect of onion bulb development is the photoperiod, or day length. Day length (the number of daylight hours) determines when bulb development begins. Short-day cultivars meet their photoperiod requirement and begin to form bulbs earlier than long-day cultivars.  

Long-day onion cultivars are the best choice for gardeners in Iowa and other northern areas. The amount of onion foliage present at bulb initiation is important, as the onion’s foliage manufactures food for bulb development. Short-day cultivars generally produce small bulbs when grown in northern areas because of the small amount of foliage present at bulb initiation. Long-day cultivars produce larger bulbs, as they can produce more foliage before bulb initiation occurs.  

Onions can be grown from seeds, sets or transplants. Gardeners typically select their planting method based on cost, use, availability and ease of planting. Growing onions from seeds is the least expensive planting method. However, it is also the most difficult as seed germination is sometimes sporadic, resulting in poor stands.

How do I sow onion seeds indoors?

Using a well-drained growing medium, sow onion seeds in flats or plug trays approximately eight to 10 weeks before you intend to plant them outdoors. As soon as the seeds germinate, place the onion seedlings under fluorescent lights. The light fixture should be placed no more than 4-6 inches above the seedlings. Growing temperatures should be 55-65 degrees Fahrenheit. Insufficient light and temperatures above 65 F promote spindly growth.

Thin the onion seedlings (within two to three weeks of germination) if the plants are crowded. When the seedlings become 5 inches tall, clip them back to 4 inches with a pair of scissors. Harden or acclimate the onion seedlings outdoors for several days before planting into the garden. Initially, place the plants in a shady, protected location. Then gradually expose the plants to longer periods of sunlight. Bring the seedlings back indoors if nighttime temperatures are forecast to drop to 32 F or below. Early April to early May is the best time to plant onion seedlings in the garden.  

Plant onion transplants from early April to early May. When planting, place the roots and the lower white portions of the plants below ground level. Space transplants 1 inch apart when grown for green onions and 2-3 inches apart when grown for mature storage onions.  

How do I sow seeds in the garden?

Onion seeds should be planted as soon as the ground can be worked in spring (late March or early April in southern Iowa, early to mid-April in central Iowa, and mid to late April in northern portions of the state). Cover the seeds with one-half to three-quarters of an inch of soil. Thin the planting when the seedlings are 2-4 inches tall. For large storage onions, plants should be spaced 2-3 inches apart after thinning.  

Before planting sets, separate the bulbs into two size groups — those smaller than a nickel in diameter and those larger than a nickel. The larger sets often bolt (produce a flower stalk) and don’t produce good-sized bulbs. Use the larger sets for green onions. The smaller sets can be allowed to develop into mature onions.  

Plant sets from early April to early May. Sets should be planted at a depth of 1 to 1.5 inches.  Space sets 1 inch apart when grown for green onions and 2-3 inches apart when grown for mature storage onions.

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