Onion Agronomy Guide
A cool-season crop, onions are part of the Amaryllidaceae family and are related to crops such as garlic, leek, chives and shallots. They prefer cool weather with temperatures between 55°F and 75°F. Periods when temperatures are warmer than 90°F tend to compromise onion growth.
Depending on when onions are harvested, plants may produce dry bulb onions or green bunching onions. To harvest dry bulb onions, wait until the bulbs reach full maturity and the leaves die. The onions serve as a good source of vitamin C and fiber. If the leaves are green at harvest and bulbs lack development, then the onions are known as green bunching onions.
Onion planting sites should have well-drained soil with 3 percent to 5 percent organic matter. In the fall before planting onions, farms may raise a cover crop, such as rye or barley, to contribute organic matter to the soil. Good water infiltration and moisture-holding capacity are important site characteristics, and the soil pH level should range from 6.2 to 6.8. The selected site should also lack soil compaction, offer air drainage and provide full sun. Before planting, eliminate weeds from the selected site.
If an operation chooses to grow onions with a mild flavor, then the plants benefit from a growing location that has a low sulfur level; less than 20 ppm is an estimate. Sites with high sulfur levels generally don’t produce onions that are as mild as those produced at sites with lower sulfur levels.
Conduct a soil test annually, and use the results to adjust soil fertility levels. After identifying the appropriate fertilizer rate for a specific site, apply the fertilizer through a broadcast application before planting or a banded application at planting. In general, onions benefit from a low-nitrogen, high-phosphorus, high-potassium fertilizer, but depending on soil conditions, an additional sidedress nitrogen application may be needed early in the growing season.
Other nutrients important to onion production are copper, manganese, zinc and molybdenum. Sweet onion production may require an application of calcium or potassium nitrate. As a guide, assume that the crop will require a sidedress application that provides 100 pounds of the fertilizer per acre.
When choosing onion cultivars, three types are available: long-day, short-day and intermediate-day cultivars. These types vary according to how long the day must be for bulbs to begin forming. Missouri growers typically raise long-day onions. To develop bulbs, these long-day varieties require at least 15-hour day lengths.
Other characteristics to consider when choosing a cultivar are bulb color, shape and flavor. Bulb onions may have a red, yellow, white or brown skin color. Their shape can vary from round to flat to globe. From a flavor perspective, sweet and pungent cultivars are options.
To raise onions, producers may choose to plant seeds, sets or transplants. Commercial growers tend to use transplants or sets. A set refers to a small, dormant bulb. Post-planting, the small bulb will grow into a marketable onion. If possible, then growers who prefer to plant sets should choose small sets. Those that are large are more likely to flower during the growing season, and a flowering onion plant tends to yield smaller bulbs that have challenges during storage. In the event that onion plants do flower, remove the flower heads by hand. Transplants typically start from seed, and when they reach 12 weeks old, they can move to the field. Transplants are often used if raising large, sweet onions.
When soil is ready to be worked in the spring, operations may begin planting onions, often as early as late February or March. Plant field-raised onions in rows spaced from 12 inches to 24 inches apart. Within a row, allocate at least a 2-inch spacing. To grow large, sweet onions, however, increase the within-row spacing to 4 inches to 5 inches.
Constructing raised beds and covering them with plastic mulch is an option for operations that choose to plant transplants. The raised beds can promote drainage. Raised beds should have a 4- to 6-inch height.
Onions lack well-developed roots, so irrigation is important, particularly after transplanting and when bulbs are expanding. Depending on the growing season stage, weekly water needs range from 1 inch to 1.5 inches. When the tops fall, dry conditions are beneficial. In such dry conditions, onions reach maturity more quickly, and they develop into a higher quality product. Drip irrigation systems are an option to supply irrigation water. Overhead irrigation systems have the potential to increase disease risk, considering that these systems expose above-ground vegetation to moisture.
Weed management is a priority when growing onions. The plants lack well-developed root systems, so weeds can compete for nutrients and water. Plus, the plants themselves don’t produce enough above-ground vegetation to create shade that could reduce weed pressure.
To control weeds, possible practices include mulching, applying herbicides, using mechanical cultivation, removing weeds by hand and rotating crops appropriately. With cultivation, however, it must occur before bulbs form. Operations that grow onions in plastic-covered raised beds may find that the plastic mulch minimizes weed pressure. Applying an organic mulch may also control weeds.
Insects and Diseases
Insect pests known to target onions are onion maggots and thrips. Certain nematodes may also influence onion production. To control insect pests, operations may spray pesticides. Adopt a regular insect scouting plan to guide your spraying needs and timing. Note, however, that insecticide resistance can develop, so avoid unnecessary pesticide applications that can encourage resistance problems to develop, and rotate among insecticides with different modes of action (refer to the IRAC code on the insecticide label for details). Another strategy to minimize insect pressure involves adopting a crop rotation. Crops to add to a rotation include carrots, potatoes, corn, sorghum or small grains.
From a disease perspective, bulb rot, neck rot, leaf blast, purple leaf blotch, white rot, downy mildew, pink root and smut may present challenges when raising bulb onions. Foliar diseases may be more likely to develop during hot, humid weather. To reduce disease incidence, consider selecting resistant varieties, rotating crops and using fungicides. Sites with the right soil characteristics and those that offer good air drainage may have fewer disease outbreaks.
A type of physiological disorder that can affect onions, tipburn refers to yellowing and browning of the top 1 inch to 2 inches on mature onion leaves. Variables contributing to tipburn include dry weather, thrip damage, nutrient deficiencies and disease pressure.
Harvest and Storage
For onions that will be stored, onion plant tops having fallen can indicate that a field is ready for harvest. Additionally, the necks on the onions should be dry. Onions reach maturity after more than 95 days. If operations are concerned about onions sprouting post-harvest, then they may apply maleic hydrazide before harvesting the onion bulbs. For the application to be effective, treat onions after they have fully matured but still have green leaves. Half of the tops should have fallen, and the necks should still be soft.
To begin the harvest, use a disk to loosen the soil. Next, pull onion bulbs from the soil, and cut all but an inch of the tops from the bulb. Gently brushing the bulbs can remove soil. Some machinery is equipped to not only harvest the bulbs but also remove tops from the bulbs.
Curing is an important step of the post-harvest process as it enables an onion bulb’s outer scales and neck to dry. They also shrink during curing. Preferably, the curing site will provide temperatures between 75°F and 80°F. The curing site’s relative humidity level should range from 70 percent to 80 percent, and the curing location should offer good air circulation. Weather conditions will dictate how long onions must cure; the process may take two weeks to four weeks. Options for curing onions include leaving them in the field or moving them to a shaded location. Operations located in wet areas with high humidity tend to cure onions away from the field. Forced-air curing is another option. If using forced-air curing, then the curing process can take less time. For example, this approach could subject onions to temperatures that range from 86°F to 113°F for 12 hours.
To store onions post-curing, a cool, dry environment is beneficial. For best results, store onions bulbs at 32°F, and the relative humidity level should range from 65 percent to 70 percent. During storage, keep onions away from ethylene, which may cause onions to sprout or fungi to grow. Variety type tends to dictate potential storage life. Compared with sweet onions, onions that have a pungent flavor tend to store better. Onions with a mild flavor tend to store well for roughly a month or less, but those with a pungent flavor may store well for as long as six months to nine months.
Johnson, Debbie. 2013. Planting, growing and harvesting onions. University of Missouri Extension. Columbia, MO 65201.
Engel, Daniel S., Ricky Foster, Elizabeth Maynard, Mohammad Babadoost, Ajay Nair, Cary Rivard, Megan Kennelly, Mary Hausbeck, Zsofia Szendra, Bill Hutchison, Touria Eaton, Celeste Welty and Sally Miller (eds.). 2019. Midwest Vegetable Production Guide for Commercial Growers 2019.
Kaiser, Cheryl and Matt Ernst. 2017. Onions. University of Kentucky. Lexington, KY 40506.
Orzolek, Michael D., Lynn F. Kime, R. Matthew Harsh and Jayson K. Harper. 2013. Onion Production. Penn State Extension. Building University Park, PA 16802.
Rutgers. 2019. 2019 Mid-Atlantic Commercial Vegetable Production Recommendations. Rutgers New Jersey Agricultural Experiment Station. New Brunswick, NJ 08901.
Suslow, Trevor. 1996. Recommendations for Maintaining Postharvest Quality: Dry Onions. University of California. Davis, CA 95616.
Zandstra, B. 2016. Onions: Commercial Vegetable Recommendations. Michigan State University. East Lansing, MI 48824.