Yard and Garden: Planting and Growing Onions

Onions perform best in well-drained, slightly acidic, fertile soils in full sun

March 22, 2024, 12:05 pm | Aaron J. Steil

AMES, Iowa -- With spring upon us, it’s time to think about planting in home gardens. In Iowa, onions are a popular addition to the vegetable garden as they are easy to grow and take up little space. In this article, horticulturists with Iowa State University Extension and Outreach answer questions about growing onions.

What is a suitable planting site for onions?

Onions perform best in well-drained, slightly acidic, fertile soils in full sun. The planting site should receive at least six hours of direct sun daily. Heavy soils can be improved by incorporating organic matter, such as compost, into the soil.

Planting onions.While some sources claim onions are “light feeders,” onions require higher fertility levels than most other vegetables. Apply about two pounds of all-purpose garden fertilizer, such as 10-10-10, per 100 square feet and till into the soil before planting. Four to five weeks after planting, side-dress with additional fertilizer. Sprinkle one pound of an all-purpose garden fertilizer per 100 feet of row. Place the fertilizer in a narrow band about 2 to 3 inches from the base of the onion plants. 

When planting, space rows 12 to 15 inches apart with plants 4 to 6 inches apart when grown for mature storage onions, and closer (1 inch apart) when grown for green onions. 

Since onions do not create a canopy of foliage, they do not compete well with weeds. Gardeners can also plant three to four onions in a cluster, spacing the clusters 6 to 12 inches apart, making it easier to weed with a hoe between plants rather than hand pulling. 

Which onion cultivar should I plant?

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).Harvested young onions.
  • ‘Patterson’ (yellow-brown skin, globe-shaped, excellent storage) .
  • ‘Redwing’ (deep red skin, globe-shaped, excellent storage).
  • ‘Red Zeppelin’ (deep red, globe-shaped, excellent storage).
  • ‘Sierra Blanca’ (white, globe-shaped, short-term 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).

Onions rely on photoperiod or day length to determine when bulb development begins. Short-day cultivars meet their photoperiod requirement and begin to form bulbs earlier in the growing season than long-day cultivars. Intermediate (or day-neutral) cultivars meet their requirement between short and long-day cultivars.

Long-day and intermediate-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. More foliage means more food available to produce bigger bulbs. Short-day cultivars generally produce small bulbs in northern areas because of the small amount of foliage present when the bulb forms. Long-day and intermediate-day cultivars can produce more foliage before bulb initiation starts, so they produce larger bulbs. Intermediate-day cultivars may produce slightly smaller bulbs than long-day cultivars since they will begin bulb initiation slightly earlier in the season.

Which planting method is best when growing onions?

Onions can be grown from transplants, sets, or seeds. Gardeners typically select their planting method based on cost, use, availability and ease of planting.

Growing onions from plants is the preferred planting method for many home gardeners. Onions sold as plants are typically sold in bundles of small bulbs with green leafy growth on top. This method is easy, but the selection of specific cultivars available at garden centers can be limited.

Growing onions from sets (small bulbs) is easy. However, onion sets are typically sold as red, white or yellow onions, so specific onion cultivars are usually not available. Since the cultivar is unknown, the flavor, use and keeping quality of onions grown from sets vary considerably.

Growing onions from seeds is the most difficult planting method for most home gardeners, though it is the least expensive. Challenges include poor germination rates and long lead times, as transplants must be started early to be ready to plant in the garden in early to mid April. However, specific onion cultivars are readily available by seed, providing more variety. Onion seed typically loses vigor quickly, and new seeds should be purchased each year.

What is the proper way to plant onion plants?

Plant onion plants or transplants as soon as the ground can be worked in spring, typically from early April to early May. Select healthy green transplants and plant them 1 to 1½ inches deep. 

What is the proper way to plant onion sets?

Before planting sets, separate the bulbs into two size groups – those smaller than a penny in diameter and those larger than a penny. Though it might be counterintuitive, planting larger sets can result in a greater number of onion plants flowering, limiting the bulb's size. Sets smaller than ½ to ¾ inches in diameter will have the best chance of developing into mature onions and yielding well. Use the larger sets for green onions.   

Plant sets as soon as the ground can be worked in spring, from early April to early May. Sets should be planted at a depth of 1 to 1½ inches.

What is the proper way to plant onion seeds?

For the best success, start onion seeds indoors in a well-drained growing medium approximately eight to 10 weeks before you intend to plant them outdoors. Provide bright light and moderate to cool temperatures (55-65 degrees Fahrenheit) for best growth.

When the seedlings become 5 inches tall, clip them back to 4 inches with scissors. Trimming the tops is sometimes done two times or more before planting. Plants given this “haircut” often become more stocky and vigorous at planting. Harden or acclimate the onion seedlings outdoors for several days before planting them in the garden. Early April to early May is the best time to plant onion seedlings in the garden. When planting, place the roots and the lower white portions of the plants below ground level.

Onion seed can also be sown directly in the garden as soon as the ground can be worked in spring (early April to early May). Plant seeds in rows and cover the seeds with one-half to three-quarters inch of soil. When the seedlings are 2 to 4 inches tall, thin the planting. A full season of growth is needed for mature onions.

Shareable photo: Harvested young onions.

Adobestock Photo Credits: 1. Planting onion sets by 63ru78/stock.adobe.com. 2. Planting onion plants by Nadia/stock.adobe.com.

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