All About Onions

In Iowa, onions are a popular addition to the vegetable garden as they are easy to grow and take up little space. Onions (Allium cepa) are in the Amaryllidaceae family and are closely related to other popular vegetables, including garlic, chives, shallots, and leeks. These plants have a similar growth habit with dark green, flat, strap-shaped leaves and bulbs underground. Onions vary in color, shape, and taste. Bulbs can be white, yellow, or red and round, flattened, or torpedo-shaped. Some bulbs are sweet, while others are pungent. They are a staple in many dishes across the globe.

onions growing in a garden — Onions are a popular vegetable to grow at home. ⁽¹⁾

Types of Onions

In the home garden, onions are often grown as green onions or dry (storage) bulbs. Dry and green onions can be grown from any variety of onion. They differ in how they are planted and when they are harvested. Specific bunching onions or scallions (Allium fistulosum) can also be grown. This species does not form bulbs and has to be harvested and used as green onions or scallions.

Green Onions

Green onions (also called scallions) are harvested young before the bulb starts to form. They are long and slender with white bottoms and dark green tops. The entire plant is used, often raw, in salads, eggs, dips, and other dishes. They are milder in flavor than dry onions. A row of onions can be planted with plants set close together. Green onions can be harvested, leaving behind some at a wider spacing to grow into dry onions. The closely related spring onion is similar to green onions but is harvested a little later, just as the bulb starts to form. They are used in a similar way to green onions.

Dry Onions

Dry onions (also called storage onions) are harvested after the bulb fully develops and plants begin to dry. They are larger, often about the size of a baseball. When harvested and dried properly, they can be stored for many months. They are eaten raw or cooked and have a much stronger flavor than green onions.

Photoperiod

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.

Growing Conditions

Onions perform best in well-drained, slightly acidic (soil pH 6-7), 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

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.

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

Transplants

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 selecting specific cultivars available at garden centers can be limited.

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.

Sets

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.

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 more 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.

planting onion sets — Onion sets are small bulbs planted 1 to 1.5 inches deep ⁽⁷⁾

onions growing from seed — Onions can be started indoors from seed and transplanted out in the garden ⁽⁶⁾

Seeds

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.

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.

Care

Mulched onions in the garden — A layer of straw mulch helps control weeds ⁽⁸⁾

Fertilization

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.

Watering

Water once a week during dry weather.

Weeding

Control weeds by shallow hoeing or hand-pulling. Onions are shallow-rooted and require careful cultivation around plants to avoid injury to the bulbs. One to two inches of mulch, such as straw, applied between plant rows help control weeds and conserve soil moisture.

Bolting

Onions occasionally bolt (produce a flower or seed stalk) in response to stress or unfavorable growing conditions, such as cold temperatures. Once the bolting process begins, the quality of the bulb deteriorates rapidly. These onions should be harvested at the first sign of bolting and promptly used as they do not store well.

Harvest

Green Onions

Green onions can be harvested when they reach a usable size. This is typically 4 to 6 weeks after planting. Usually, a small trowel or garden fork will be needed to loosen the soil to avoid breaking the plant off at the soil line.

Dry Onions

Dry or storage onions should be harvested when the tops have fallen over and start to turn brown. Most onion cultivars mature 90-120 days after planting. Carefully pull or dig the bulbs with the tops attached. A garden fork or spade can be used to loosen the soil. Avoid hitting the onions with the fork, as these bulbs will not store well. If you accidentally slice an onion bulb, plan to use it right away.

Storage

Green Onions

Green onions are stored in a plastic bag for up to a week in the refrigerator. A moist paper towel can be added to increase the humidity.

Dry Onions

braided onions — The tops can be braided and hung to dry onions ⁽¹³⁾

Before storage, curing onions by spreading them out in a single layer to dry ⁽¹²⁾

Dry onions need to be cured before storing. After harvesting, dry or cure the onions in a warm, dry, well-ventilated location, such as a shed or garage. Spread the onions in a single layer on a clean, dry surface. Cure the onions for two to three weeks until the tops and necks are thoroughly dry and the outer bulb scales begin to rustle. After the onions are properly cured, cut off the tops about 1 inch above the bulbs. As the onions are topped, discard any that show signs of decay. Use the thick-necked bulbs as soon as possible, as they don’t store well. An alternate preparation method is to leave the onion tops untrimmed and braid the dry foliage together.

Place the cured onions in a mesh bag, wire basket, or crate. The storage container must allow air to circulate through the onions. Store the onions in a cool, moderately dry location. Storage temperatures should be 32-40°F. The relative humidity should be 65-70%. Possible storage locations include a basement, cellar, or garage. Hang the braided onions from a rafter or the ceiling. If storing the onions in an unheated garage, move the onions to an alternate storage site before temperatures drop below 32°F.

The storage life of onions is determined by the variety and storage conditions. When properly stored, good keepers, such as ‘Copra’ and ‘Stuttgarter,’ can be successfully stored for several months. Poor keepers, such as ‘Walla Walla’ and ‘Sweet Spanish,’ can only be stored for a few weeks. If the storage temperatures are too warm, the onions may sprout. Rotting may be a problem in damp locations. Inspect the stored onions regularly in fall and winter. Discard any that are starting to rot.

onions in storage — Store dry onions in a well-ventilated container in a cool, moderately dry location. ⁽¹⁴⁾

Suggested Cultivars

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).
‘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).

Potential Problems

Fusarium Basal Rot

Onions with Fusarium basal rot will have bulbs with brown and watery rot spreading from the base up to the leaves. The stem plate at the bottom may also be brown. Leaves can wilt or curl, turn yellow, or die from the tips downward.

Utilizing cultural control methods is the best way to manage this disease. Plant in well-drained soils. Practice good crop rotation and do not plant onions or onion relatives (like garlic) in the same spot for 3 to 4 years. Finally, select varieties that are resistant to Fusarium basal rot.

Learn more in this publication from the University of Wisconsin: Onion Disorder: Fusarium Basal Rot.

Purple Blotch

Caused by the fungus, Alternaria porri, purple blotch causes small water-soaked lesions on the leaves or stalk with white centers. As the disease progresses, the spots enlarge and form concentric rings that are brown to purple in color with a red or purple margin surrounded by yellow. Large lesions may coalesce and girdle the leaf, killing the leaf from the lesion out to the leaf tip.

Utilizing cultural control methods is the best way to manage this disease. Practice good crop rotation and do not plant onions or onion relatives (like garlic) in the same spot for 3 to 4 years. Avoid overhead irrigation and promptly remove and destroy any infected materials. After harvest, remove and destroy all foliage. Yellow varieties of onions are generally more resistant to purple blotch.

Learn more in this publication from the University of Wisconsin: Onion Disorder: Purple Blotch.

Black Mold

Black mold of onion is caused by the fungus Aspergillus niger. This disease can occur in the field or during storage. As the common name suggests, the primary symptom is a black discoloration of tissue. Infected bulbs may show blackening at the neck, streaks or spots of black on or beneath the outer scales, and black discoloration in bruised areas. In advanced stages, the entire bulb appears black and shriveled.

The fungus that causes black mold is common in the soil and on dead plant material. Infection of bulbs usually occurs through wounds (especially through the neck as onions mature and tops break over or are cut). The fungus may also enter through wounds on the roots or bruised outer scale tissue.

Control of any foliar diseases will help reduce black mold during storage. It is also important to avoid wounding when bulbs are harvested, transported, or stored. Finally, proper drying and storage will help prevent black mold. Warmth and moisture favor disease development. For successful storage of onions, thorough ventilation, a low temperature, a dry atmosphere, and protection against freezing are essential.

Learn more in this article from Cornell University: Black Mold of Onion.

Onion Maggot

Adult flies lay eggs at the base of the plant. Larvae hatch, bore into the plant stalk, and feed on the bulb, roots, and stalk. There is more than one generation each year. Damage from feeding is worse in cool, wet springs and includes stunted or wilted plants, yellowing foliage, leaves that break at the soil line when pulled up, and damaged and deformed bulbs. Infected bulbs are more prone to storage rot diseases.

Using row covers in spring to exclude the female flies from accessing the plants for egg-laying can be effective. Remove weeds from the garden and surrounding area that could serve as a host, such as wild garlic or wild onion. Practice good crop rotation and remove all plant debris after harvest. The most damaging spring generation can be avoided by planting later (after June 1st). This practice can work well for fast-maturing varieties and green onions.

Learn more in this article from the University of Minnesota: Cabbage and Onion Maggots.

Onion Thrips

Onion thrips feed on a wide range of plants. Populations can grow rapidly, and up to five overlapping generations can occur in one growing season. Onion thrips feed on leaves in between the leaf sheaths. While they prefer younger foliage, damage on older foliage is not unusual. The damage from feeding creates small brown or whitish blotches that create a speckled or sandblasted appearance. Damage can become severe and is particularly detrimental to young seedlings.

Insecticides are available for control but can be difficult to apply since onion thrips are primarily tucked into the leaf sheaths where it is difficult to get pesticide. Apply them at the first sign of damage. Onion thrips have a wide range of host plants, including many grain crops. Isolation from nearby fields can potentially help. Overhead watering can have some success at dislodging the insects. Encourage or release natural predators, including lacewings, predatory mites, and pirate bugs.

Learn more in this article from Penn State University: Onion Thrips.

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Authors:

Aaron Steil

Last reviewed:

July 2024