Onion

Onion, Allium cepa, is an herbaceous biennial in the family Liliaceae grown for its edible bulb. The stem of the plant is a flattened disc at the base and the tubular leaves form a pseudostem where their sheaths overlap. The leaves are either erect or oblique and there are 3–8 per plant. The onion plant produces pink or white flowers clustered on stalks. The bulbs are formed just above the flattened stem of the plant by overlapping leaves. The bulb is made up of several layers, each corresponding to a leaf. They are are generally oval but shape can be variable and occur in clusters of 3–18 to a plant. The bulb is protected by a membrane which turns to a papery coat. Onion plants can reach a height of 50 cm (20 in) and are grown as annuals, harvested after one growing season. Onion may also be referred to by cultivar and these include red or purple onion, shallots and spring onions or scallions. The origin of the onion has not been conclusively determined although it is likely to be somewhere in South East Asia where the gene pool is most diverse.


The bulb is an edible vegetable and is the most commonly used part of the onion, usually consumed after cooking although it can be eaten fresh. The stems and leaves are also edible.


Requirements Onions are hardy, cool season vegetables that grow best at temperatures of 12 to 24 °C (55–75 °F), growing particularly well in areas with cool spring weather and drier, hotter summer weather. They require a fertile, well-draining soil such as clay or silt loams with a pH of 5.5–6.5. The plants do not do well in acidic soils. Onions should be set out in full sun for optimum bulb development. Onions are biennial vegetables and if they are left in the ground for a second year, they will produce flowers and set seed. Seeds and transplants In milder climates, onion seeds can be direct seeded as soon as the soil is workable in the Spring, 4–6 weeks before the last frost date, or even earlier if starting seeds indoors to produce transplants. The planting site should be cultivated deeply and be free of stones. Work some compost into the soil and ensure that soil has a soft crumbly texture before planting. Seeds should be sown 2.5 cm (1 in) deep allowing 10–13 cm between plants and 30–45 cm (12–18 in) between rows. Onion transplants which have been started indoors can be transplanted to the garden at a similar time to planting seeds. Transplants are best produced in cell trays by planting 2 seeds per cell. Once the seeds have germinated, the seedlings should be thinned to one plant per cell. Transplants are ready to be planted when the root system has developed sufficiently to bind the soil in the cell together. Transplants should be spaced 10–13 cm allowing 30–45 cm (12–18 in) between rows. Sets Onion sets are small, immature onion bulbs which can be purchased from seed companies and garden centers for planting in place of seeds. Select the smaller sets for planting as they are less prone to bolting. Sets should be planted 2 cm (0.75 in) deep allowing 5–7.5 cm between sets and 25 cm (10 in) between rows. General care Onions should be watered thoroughly after planting and once every week thereafter, applying approximately 1 inch of water each time. Be careful not to allow the soil to become dry and cracked as onions have a shallow root system and this indicates a lack of water. Water adequately but do not overwater. Remove any weeds around young plants by hoeing shallowly so as not to damage the roots of the onions. Plants should be fertilized avery few weeks with a high nitrogen fertilizer. Harvesting When the soil at the base of the plants begins to crack, this indicates that the bulbing process has begun. Fertilizer applications should be stopped at this point. Avoid hilling the soil over the bulbs as onions mature best if they are on top of the soil. When the onion bulbs are mature, the leaves will begin to yellow and fall over. The tops can be bent right back to speed maturation. After a few days, the bulbs can be pulled and left on the soil surface to begin curing. Any damaged onions should be cooked and consumed right away as they will not store well. Onions should be cured for several weeks before storing. This can be done outdoors by placing them on a plastic sheet off of the ground, under cover if the weather is wet. Growing green onions Green onions, also known as scallions or Spring onions, are immature onions which are harvested before the bulbs mature. Seed can be purchased to grow onion varieties which are specifically bred to be harvested as green onions. Green onions, like regular bulbing onions can be started from seed or sets and can be grown as transplants indoors (see above). Green onions do not require the same amount of space that bulbing onions do and seed or transplants can be set out 2.5–3.8 cm (1.0–1.5 in) apart in the row. Allow 5 cm (2 in) between sets. The general care of green onions is the same as that for regular onions. They can be harvested when they reach 15 cm (6 in) in height or more.


Boyhan, G. E. & Kelley, W. T. (Eds.) (2008). Onion production guide. University of Georgia Cooperative Extension. Available at: http://www.caes.uga.edu/extension/thomas/anr/documents/onionproductionguide_b1198_2.pdf. [Accessed 01 March 15]. Free to access. CABI Crop Protection Compendium. (2013). Allium cepa (onion) datasheet. Available at: http://www.cabi.org/cpc/datasheet/4239. [Accessed 01 March 15]. Paid subscription required. Drost, D. (2004). Onions in the garden. Utah State University Cooperative Extension. Available at: http://extension.usu.edu/files/factsheets/onionsf.pdf. [Accessed 01 March 15]. Free to access. Schwartz, H. F. & Mohan, S. K. (Eds.) (2008) Compendium of Onion and Garlic Diseases and Pests.Second Edition. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopapspress/Pages/43573.aspx. Available for purchase from APS Press.