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.
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.
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.
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.
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.
Post-harvest black discoloration at neck; lesions on outer scales; black streaks under outer dry scales; entire surface of bulb turning black and shriveling
Wash hands thoroughly after coming into contact with fungus
Treat seeds with appropriate fungicide prior to planting to reduce rot in mature bulbs; avoid bruising bulbs during and after harvest; storing at temeperatures below 15°C (59°F) prevents mold from spreading but it will resume once temperature increases
Small white lesions with light green halos which may expand slightly as they age; in prolonged periods of moisture fungus may develop rapidly and cause leaf blighting
Disease emergence favors high humidity and warm temperatures; fungus survives on piles of crop debris or in soil; older leaves more susceptible to blighting than younger leaves
Plant onions in single rows allowing at least 30 cm between plants to promote good air circulation and quick drying of foliage after rain; time irrigation to allow plants time to dry out sufficiently; apply appropriate fungicide sprays when plants have at least five true leaves and early symptoms of disease
Stunted plant growth; reduced stand; bulbs rotting in ground or in storage; pest is a cream-white, bulbous mite
Damage to plants by bulb mites allows secondary invasion by other pathogens and can cause bulb rots
Do not plant successive crops of onion or garlic in same location; allow field to fallow to ensure that any residual organic matter decomposes completely - crop residues can harbor mite populations; treating garlic seed cloves with hot water prior to planting may help reduce mite populations
Pale spots or elongated patches on leaves; gray-purple fuzzy growth on leaf surface; leaves turning pale then yellow; leaf tips collapsing
Disease emergence favored by cool temperatures and leaf wetness
Avoid planting infected sets; rotate crops to non-allium species for 3-4 years; plant in well-draining areas and do not overcrowd plants; destroy all infected crop debris; apply appropriate foliar fungicides taking care to apply thoroughly to waxy leaves
Curving, yellow or necrotic leaves; necrosis begins at leaf tips and moves downward; wilting plants; infected bulbs may be brown and watery with rot spreading from stem plate to basal leaves; stem plates may have brown discoloration
Disease emergence favors moderate to high temperatures
Rotate with non-susceptible crops for at least 4 years, plant resistant onion varieties
Rotting seeds that are covered in mold; discolored root tips which may be pink, tan, yellow, red or black; slowly growing seedlings which wilt and die
Fungus survives in soil and disease emergence is favored by moist to wet soil
Plant only disease-free seed; treat seed with fungicide; rotate crops with cereals or grasses to reduce levels of pathogen in soil; steam treatment or fumigation of soil can help reduce levels of Fusarium in the soil
Thin, white, winding trails on leaves; heavy mining can result in white blotches on leaves and leaves dropping from the plant prematurely; early infestation can cause yield to be reduced; adult leafminer is a small black and yellow fly which lays its eggs in the leaf; larave hatch and feed on leaf interior
Mature larvae drop from leaves into soil to pupate; entire lifecycle can take as little as 2 weeks in warm weather; insect may go through 7 to 10 generations per year
Check transplants for signs of leafminer damage prior to planting; remove plants from soil immediately after harvest; only use insecticides when leafminer damage has been identified as unnecessary spraying will also reduce populations of their natural enemies
Water-soaked, dark green oval lesions or streaks on leaves; tipburn of leaves; dark spots on wrapper scales of bulbs; reddish-brown discoloration of inner scales; rot developing in ring-like pattern
Little is known about the pathogen; greatest damage occurs during winter; rapid spread of disease on infected plants is promoted by rainfall
Avoid fertilizing plants during winter' apply appropriate bacteriacidial sprays
Stunted or wilting seedlings; plant will commonly break at soil line if an attempt is made to pull it up; if infestation occurs when plants are bulbing, bulbs will be deformed and susceptible to storage rots after harvest; adult insect is a greyish fly which lays white, elongate eggs around the base of the plant; the larvae that emerge from the eggs are tiny and white and bore into the onion plant; mature larvae are about 1 cm (0.4 in) long with feeding hooks
Females can lay several hundred eggs during their 2-4 week lifespan; insect overwinters as pupae in the soil
Management of onion maggots is heavily reliant on good sanitation; all onion bulbs should be removed at the end of the season as maggots will die without a food source; commercial onion growers must often rely on the application of appropriate granular insecticides and, in some cases, insecticide sprays are also required; home gardeners should try to remove any volunteer wild onion and chive plants as these can act as an infection source; floating row covers may provide protection by preventing females from laying eggs around the plants
Yellow streaks on bases of of first leaves; all leaves which emerge after infection have yellow streak or are completely yellow; leaves may be flattened or crinkled; bulbs are undersized; flower stalks yellow and twisted; flower clusters small and seed is of poor quality
Transmitted by several species of aphid, including the peach aphid; virus is not spread via seed or pollen
Controlling aphids by applying insecticides is not effective due to the short amount of time aphid needs to transmit virus; other control methods include: planting sets or transplants which were produced in areas free of virus; growing plants from seed; removing any infected plants and planting more tolerant varieties
Light pink roots which darken and turn purple; roots become transparent and water soaked; plant may look like it has a nutrient deficiency; infected seedling may die; stunted plants with undersized, shriveled bulbs
Fungus colonizes plant through root tips; fungus can survive in soil down to a depth of 45 cm (17.7 in)
Disease is most severe when onions are planted continuously or in a 1-year rotation, a rotation of 3-6 years is preferred; plant more resistant varieties; solarization and/or fumigation can help reduce the levels of pathogen in the soil
Small water-soaked lesions lesions on leaves or stalk with white centers; which enlarge to become zonate and brown to purple in color with red or purple margin surrounded by yellow zone; large lesions may coalesce and girdle leaf, killing any tissue between the lesions and the leaf tip; severely infected foliage may die
Disease emergence favored by wet foliage, with sporulation occuring during the night during periods of high humidity
Cultural controls include long rotations with non-hosts and the reduction of leaf wetness by planting in well-draining soil and timing irrigation to allow plants to dry adequately during the day; some fungicides are effective at controlling the disease but should be rotated for optimal control
Seeds water-soaked, mushy and decomposing; infected roots are gray and water-soaked; seedlings that have already emerged prior to infection collapse and die; older plants that become infected become severely stunted
Disease emergence favors high soil moisture and cool temperatures
Control of disease is dependent on minimizing soil moisture: break up compacted soil; plant in well-draining areas or raised beds; treat seeds with appropriate fungicides prior to planting
Small white flecks on leaves and stems which develop into circular or elongated orange pustules; severe infestations can cause leaves to yellow and die
Favors high humidity but low rainfall; spores can be transported over long distances by wind
No resistance known; use only disease-free seed and plant in well-draining soil; control weeds around crop; apply appropriate protective fungicide
Dark, thickened lareas on cotyledons (seed leaves) which may become large and cause leaves to bend downwards; raised blisters may be present on the base of scales in older plants; lesion mature and become covered in black powdery fungal masses; plant growth stunted; death of plant occurs within 3-4 weeks
Smut can persist in soil for many years and is mainly introduced through infected sets and transplants; spores can be spread by wind, on equipment or in irrigation water
No resistance to disease known in onion; plant onions during periods which promote rapid growth; plant only health sets and transplants - if smut is present in the soil they will not become infected
Discolored, distorted tissue; scarring of leaves; severly infected plants may have a silvery appearance
Thrips are most damaging when they feed on onions at the early bulbing stage of development; both onion thrips and western flower thrips have an extensive host range and can be introduced to onion from other plants
Natural enemies include some species of predatory mite, pirate bugs and lacewings; avoid planting onion in close proximity to grain fields as thrips populations build up on these plant in the spring; overhead irrigation of plants may help reduce thrips numbers; apply appropriate insecticides at first sign of thrips damage
Older leaves yellowing; stunted growth; death of all leaves; fluffy white growth on base of bulb which spreads up bulb to storage leaves
Fungus can survive in soil for 20 years and is one of the most damaging diseases of Allium crops worldwide, causing major crop losses
Fungicide treatment may not be effective at controlling white rot under conditions which are favorable to the fungi's development and control may have to rely on cultural methods: avoid transferring soil or plant material between sites; treat seeds with hot water prior to planting; use a long term rotation with non-allium crops; apply appropriate fungicides if available
Boyhan, G. E. & Kelley, W. T. (Eds.) (2008). Onion production guide. University of Georgia Cooperative Extension. Available at: http://www.caes.uga.edu/extension/tho.... [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/factsh.... [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/shopap.... Available for purchase from APS Press.
Keabetswe Which fungal and bacterial diseases affect onion seedlings?