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Onions: How to Grow It

Updated April 22, 2022
Professional headshot of Rhoda Burrows

Rhoda Burrows

Professor & SDSU Extension Horticulture Specialist


Variety of freshly harvested onions on a cutting board.
Variety of freshly harvested onions. Courtesy: Canva

General Description: Onions have been cultivated for over 5,000 years, and they have been used, not only for food, but for medicine for various illnesses and joint problems. Ancient Greek soldiers and athletes believed eating onions would give them strength from the gods. Ancient Egyptians considered onions symbols of eternity, and they used them as preservatives, as well as for food.

Onions contain cancer-fighting chemicals, as well as compounds that reduce blood cholesterol and promote insulin production and possibly may reduce the risk of Alzheimer’s disease. Of course, most people eat them for the flavoring they add to everything from soups and burgers to casseroles, stir fries, dips and even pickles!


Left: Harvested yellow, Valencia onions on a table. Right: Row of red onions ready for harvest in a garden.
Figure 1. A) Valencia onions, a type of yellow onion. B) Red onions in a garden. Courtesy: All-American Selections; Ansel Oommen,
  • Common Onion: These are the bulb onions you see in the grocery store. There are a variety of common onion types, including:

    • White onions are mild, sweet and do not store as long.
    • Yellow onions are more-pungent, harder and longer-keeping (Figure 1-A).
    • Red onions tend to be spicier than yellow onions and not keep as well (Figure 1-B). For South Dakota, avoid short-day varieties, as they will not form bulbs under the long days of our summer.

  • Egyptian Walking Onions: These hardy perennials are planted in the fall and produce bulblets on the end of seed stalks, as well as small, strongly-flavored bulbs at their base. Young plants arising from bulblets can be used as green onions.

Left: Basket of harvested red, ‘Echalion’ shallots. Right: Bunches of ‘Warrior’ green onions on a countertop.
Figure 2. A) 'Echalion' shallots. B) 'Warrior" bunching onion. Courtesy: All-American Selections
  • Shallots: Clusters of small, elongated bulbs. Despite their small size, they require a full growing season (Figure 2-A).

  • Green or “Scallions/Bunching” Onions: These onions are harvested as immature bulbs (Figure 2-B). They may be bulbing varieties simply harvested early, or non-bulbing varieties developed specifically for scallions.

  • Multiplier or “Potato” Onions: These perennials produce a cluster of 3 to 20 small-tomedium-sized bulbs, which can last up to a year or more in storage. Green immature offsets may be used fresh, pulled as soon as the bulbs are large enough for eating.


Timeline for starting transplants, planting and harvesting onions. For an in-depth description of this timeline, please call SDSU Extension at 605-688-4792.
Figure 3. Timeline for starting transplants, planting and harvesting onions.

Timeline: Onion transplants can be started mid-February and grown through the end of March. Planting can take place from mid-April through mid-May. Green onions can be harvested June through September, while bulbs can be harvested August through October (Figure 3).

Two gardeners transplanting rows of onion seedlings.
Figure 4. Transplanting seedling onions. Courtesy: Alton N. Sparks Jr., University of Georgia,

Site Selection and Preparation:

  • Onions should be planted in a sunny spot (at least 8 hours of sun daily) with rich, well-drained soil.
  • Onions are not tolerant of high-salt soils.
  • Well-aged compost can be mixed in prior to planting to increase organic matter and nutrient availability.
  • Onions do not compete well with weeds, so avoid weedy areas for planting.
  • Raised beds can give excellent results. If the onions are direct-seeded, the bed needs to be especially well-prepared, level and clod-free.

Sowing and Transplanting: Onions are usually grown in gardens from seedling transplants or from sets, or small bulbs (Figure 5).

Onion sets with two mature bulbs for size comparison.
Figure 5. Onion sets with two mature bulbs for size comparison. Courtesy: Marco Verch
  • Transplants can be purchased (usually as bundles of bare-root seedlings) or grown indoors from seed. Seed should be started about two months before transplanting. They are ready to transplant when they have at least two or three leaves. The seedlings can be trimmed back to about three-inches tall if the leaves are drooping over. Plant 1 to 1¼ inches deep; and 2 to 4 inches apart into moist soil. Alternatively, plant clumps of three seedlings six inches apart.
  • Sets. Although they are easier to handle, sets have more of a tendency than transplants to “go to seed” (i.e. produce a flower stalk rather than a bulb). To avoid this problem, choose sets that are smaller than one-inch in diameter. Plant them about 1 inch deep and 2 to 4 inches apart several weeks before the average last frost date. For square-foot gardens, plant 9 sets per square foot. Onions grown from sets will usually be somewhat smaller than onions grown from seedling transplants.
  • Green onions can be grown by seeding directly into the ground, but the ground MUST be kept moist for the 2 weeks it can take them to germinate.

Plant Care

Watering: Onions have a very shallow root system, so keep a close eye on the soil. They will need about an inch of water per week, more during hot, dry windy periods and less during the cooler spring. Once bulbs begin to mature, they do not need further watering, and may have a tendency to rot if the soil is too wet or water accumulates at the base of the leaves.

Weeding: Onions do not compete well with weeds, as their leaves will not shade out competitors. Weeds can also increase moisture around the plant, increasing the risk of fungal or bacterial disease. Early summer weed control (hand weeding and/or hoeing) is essential!

Fertilizer: Onions require a steady supply of nutrients, in part because they do not have extensive root systems. If planting into a raised bed with imported soil, test the soil for nitrogen, phosphorus and potassium (N, P, K) prior to planting. Most (but not all) existing soils in South Dakota will have enough potassium, but they will need nitrogen and possibly phosphorous. A recommended fertilizer ratio is 1:2:2 N:P:K, for example, 5-10-10, but 10-10-10 will also work. Spread one to two pounds (two to four cups) of the dry fertilizer per 100 feet of row. An extra application of nitrogen just as the plants begin to expand their bulbs can help achieve greater size, for example one pound of fertilizer per 100 feet of row. A lawn fertilizer (without herbicide) can be used for this side-dressing: for a typical lawn fertilizer of 34-0-0, apply about 2/3 cup per 100 feet of row, or a tablespoon per 10 feet of row.

Pests and Diseases

Major Pests: Onions grown in South Dakota generally have few pest problems. Proper crop rotation (that is, not planting onions in the same place two years in a row) will minimize potential problems. If any disease problems appear, don’t plant onions or garlic in that spot for four years.

Minor Problems: Flower Stalks. Onions under drought or heat stress may form flower/seed stalks. If so, cut the stalks off to prevent the bulb from sending its energy into forming flowers and seed.


Rows of onion bulbs ready to harvest in a garden.
Figure 6. Onion bulbs ready to harvest.

Bulbs: Bulbing onions can be harvested at any point that the bulb is the desired size. However, for best keeping quality, they should not be harvested until the tops begin to fall over (Figure 6). Pull the bulbs gently, and place them in a warm, dry well-ventilated area out of the sun for a few days. This can be on the ground, as long as it isn’t wet (although a drying rack is ideal). Avoid direct sun, as some types of onions can get sunburned as they are curing. Don’t remove the foliage yet – the drying leaves will help draw moisture away from the neck of the bulb and decrease storage rots.

The bulbs can finish drying indoors in a warm, dry area for a week or two. When the leaves have dried, you can remove them. Once the outer bulb scales begin turning a brown, tan or a reddish color (red onions), the onion is cured. You can carefully remove a few of the outer scales if they still are coated in dirt, but do not remove all the outer scales, because these act as a wrapper for the bulb, protecting the fleshier inner-bulb scales and protecting the bulb from excessive drying.

Green Onions: About two weeks before harvesting green onions, pile about 2 inches of soil around the base of the stem. This will blanch the stem, resulting in long, white, tender stems. Green onions can be harvested in as little as a month from transplanting, or 2 months from direct-seeding.

Storage and Preparation

Storage: Once the onions are cured (see the Harvest section above), you can transfer them to mesh sacks, a plastic ventilated crate or a box with holes in it. Store the dried bulbs in a warm, dry location (they will actually keep longer at 80°F than at 60°F, as long as relative humidity is below 70%) until you are ready to use them. They may also be stored in a refrigerator. Yellow onions will keep in storage the longest; red onions the shortest.

Average Yield: Ten feet of single row onions should yield 7 to 14 pounds.

Nutrition Facts: One medium-sized onion has only 60 calories and supplies 20% of the recommended daily value for Vitamin C.

Cooking: For recipes and tips for preparing onions, view our "Pick it! Try it! Like it!" resource for Onions.

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