- Environmental Factors
- Mineral Nutrients
- Weed & Pest Management
- Cleaning & Storage
Vegetable Seed Production: Onion
The onion (Allium cepa L.), is the most common vegetable of the genus Allium, which includes several important vegetables: Allium cepa (shallot, top set onion, multiplier onion), A. sativum (garlic), A. ampeloprasum (elephant garlic or great head leek), A. schoenoprasum (chive), A. fistulosum (Welsh onion, Japanese bunching onion), A. chinense (rakkyo), A. tuberosum (Chinese chive), and A. cepa x A. fistulosum (Beltsville bunching onion). These monocots were once classified in the family Alliaceae but have been reclassified into the subfamily Allioideae of the family Amaryllidaceae. Onion, is a biennial or a perennial by means of its bulbs in areas mild enough to over winter. Alliums are cool season crops, tolerant of frost but not prolonged freezing temperatures below the range from -9.5 to -6.5° C. Optimum temperatures for growth and development are in the range from 13 to 25° C, somewhat narrow compared to other vegetables. Flowering occurs after vernalization (Brewster, 2008, Rubatzky and Yamaguchi, 1997).
Alliums have a unique plant architecture. The plant consists of a short subconical stem from which the linear leaves arise in 1/2 phyllotaxy (all leaves lie in a single plane). The leaves arise from the short crown stem in a compact series, and the sheaths of the outermost leaves enclose the younger ones. The sheathing base of each leaf completely encircles the stem, and it is the development of the fleshy stem bases that, together with the lack of internodal elongation, results in the development of the bulb. Alliums, except leeks and chives, are composed of fleshy, enlarged leaf bases or scales. Chives do not produce swollen bulbs. Onion has long hollow leafless stems that arise from the terminal bud and bear the inflorescence. Flower stalks may also arise from lateral buds. The terminal inflorescence develops from the ring-like apical meristem. Scapes, one or more, elongate from 30 to more than 100 cm above the leaves. The scape is a long, leafless flower stalk extending between the spathe and the last foliage leaf. At first, the scape is solid but, by differential growth, becomes thin walled and hollow. The onion scape has a characteristic spherical bulge along its length. The total number of developing scapes depends on the number of sprouted lateral buds (Brewster, 2008, Rubatzky and Yamaguchi, 1997).
The garlic umbel inflorescence develops at the top of a straight and solid scape, is subspherical, and usually contains only bulbils or a combination of bulbils and flowers, which rarely if ever set seed in commercial clones. The infrequently formed flowers are lavender and usually wither and abort. Recently, researchers in Germany, Japan and USA have produced viable seeds from certain garlic clones particularly those belonging to subsp. ophioscorodon of A. sativum (Rabinowitch and Currah 2002, Volk et al., 2004, Pooler and Simon, 1994).
Alliums can be grown in a wide range of soil types but prefer fertile, well-drained, non-crusting mineral soils that are high in organic matter and have good moisture retention. Heavy clay soils and light sandy soils should be avoided unless frequent irrigation is available (George, 2009). Onions grow best in the pH range of 5.3 to 6.5. Above pH 6.5, deficiencies of copper, manganese, and zinc may occur (Smith et al., 2011). Onions should be grown in the same field once every four to five years to help prevent the buildup of soil-borne disease, nematodes, and weeds. The soil must be finely tilled to depth of at least 18 cm using rotovators or disks and free of clods, stones, and other impediments, especially for direct seeding, to ensure uniform emergence and good stand establishment. Pelleted seeds improve singulation and spacing with precision belt or air seeders. Planting Allium seed crops on raised beds improves drainage and encourages salt accumulation away from the root zone.
Onions are a shallow-rooted, cool-season crop that responds well to fertilization. Generally, Alliums have a high requirement for micro-nutrients and moderate need for N-P-K. Alliums are moderately sensitive to sensitivity to salinity especially at the time of germination and in the small seedling stage but become more tolerant as they mature. Significant yield reductions occur when salinity is in the range of 4 to 5 mmhos/cm (Voss et al., 2013). Alliums have a shallow fibrous root system restricted to the top 30 cm of the soil, so precision fertilizer placement is essential for efficient use. Fertilization needs vary depending on many factors such as temperature, cultivar, spacing, soil type, available water etc. Fertilization should be in accordance with soil test results so that costly mineral inputs are not wasted and become pollutants. Tissue and soil analysis combined cropping history are important for determining N fertilization needs (George, 2009). Quick tests of leaf tissue provide rapid assessment of N availability during the growing season.
Nitrogen requirements vary depending on nitrogen-supplying capacity of the soil, irrigation efficiency, and the amount of N loss due to rainfall and other environmental factors. Alliums are generally sensitive to ammonia and salts but tolerant of low pH compared to other vegetables. Composted manure (11 to 22 t/ha) can supply nutrients and fulfill early season N requirements. With efficient irrigation, a total of 260 kg/ha of N should maximize yield on most mineral soils, with less needed on soil types with high residual N. However, needs may vary from 110 to 450 kg/ha depending on soil, cropping history and water received. Higher amounts may be justified in fields receiving significant rainfall or lower mineral holding capacity. Excessive applications of N may cause excessive vegetative growth and delay flowering. Excess nitrogen also promotes unwanted secondary growth (Rosen et al., 2008).
Soils with bicarbonate extractable P greater than 30 ppm require a preplant application of no more than 56 kg/ha of P2O5, while soils testing at 10 ppm P or less may require as much as 224 kg/ha of P2O5. With adequate preplant application, in-season P additions are usually not required (Smith et al., 2011).
Soils exceeding 150 ppm ammonia-acetate-exchangeable K are unlikely to respond to additional fertilization. However, if soils test at less than 100 ppm, potassium additions of up to 170 kg/ha of K may be needed to ensure adequate fertility for seed crops. (Smith et al., 2011).
Seed to Seed
For the seed-to-seed method plants are direct seeded, vernalized in the field, bolt, and produce seed. Seed-to-seed production is less expensive because labor is reduced since bulbs are not lifted or replanted (Voss et al., 2013). Seed-to-seed production fields are seeded, usually in late summer, allowing plants to develop beyond the juvenile phase so that vernalization can occur during the fall or winter. Onion seeds germinate slowly at a base temperature of 4° C and optimal germination occurs at approximately 24° C. Before planting it is important to ensure the minimum isolation distances are met from other Allium cepa seed fields. Seed-to-seed onion crops are often planted on raised beds 100 to 105 cm wide although other widths are possible. For example, single rows may be planted on smaller 75 cm wide raised beds. Plate, belt, or vacuum precision seeders can plant to a final stand at optimal spacing so thinning is not required. The soil should be finely tilled with small ped size and kept moist until emergence. Target seeding depths are 10 mm to 2.5 cm deep with deeper planting on lighter drier soils. For 75 cm beds, the target plant population is 45 to 60 plants per m of row. For double rows, planted on wider 100 cm beds, the target plant populations are 50 to 70 plants per m of row or about 30 seed stalks per m2. Both single and double row seeding densities require 3.5 to 5.5 kg/ha of seed (George, 2009, Voss et al., 2013).
Bulb to Seed
Even though onion flowers are perfect their male and female flower parts develop at different times so they are cross pollinated by insects. The flowers are visited by a range of insects, including bees, flies, and other insects, that collect pollen and nectar (Parker, 1982). The duration of anthesis of an individual umbel is roughly 4 weeks. Honey bees reluctantly visit onion flowers to collect both nectar and pollen, but only nectar foragers will visit both male-sterile and male-fertile lines in hybrid onion production. Onion flowers, and specifically their nectar, are one of least favorite of honey bees who prefer flying longer distances to visit other flowers types instead. The sugar concentration of the nectar may be increased by potassium fertilizer. However elevated potassium in nectar may be another reason why bees find onion nectar unattractive. Therefore, seed producers should not plant crops more attractive to bees near onion seed fields. If honeybees are used, supplemental hives with colony stocking rates as high as 30 hives per hectare or more have been suggested (Goodwin, 2012).
F-1 hybrid cultivars are more uniform and higher yielding than open pollinated onions and this has led to their adaptation. Hybrid onion seed production requires special considerations. The time from seeding to bulb planting to flowering differs among inbred lines crossed to make F-1 hybrids. To achieve simultaneous flowering or ‘nicking’ of male and female parental lines, planting in most cases will need to occur at different times. An alternative approach is to direct seed one parental line and plant bulbs of the other. Seed producers avoid planting bulbs of both parental lines. The ideal situation is to develop inbred lines for seed production that have similar maturities but unfortunately this doesn’t always occur. The ideal situation is to have the male pollen parent flower shortly before, during, and after the female parent flowers. To achieve pollen production over an extended period, split plantings are sometimes employed. For success, seed companies must share information about flowering characteristics of hybrid parental lines with seed producers (Voss et al., 2013).
Many popular modern onion cultivars are F-1 hybrids produced using a male sterile inbred line as the female parent to ensure crossing by bees without costly and time-consuming emasculation and hand pollination. Bees tend to find male-fertile lines more attractive than male-sterile lines and have a tendency to move up and down rows instead of crossing between male-fertile and male-sterile lines, which may reduce pollination. Because only bees foraging for nectar will visit both male-fertile and male-sterile lines, colonies introduced to onion fields should have large numbers of adult bees and should not be fitted with pollen traps or be fed with sugar syrup, as both methods promote pollen collection instead of nectar foraging (Goodwin, 2012).
The minimum recommended isolation distance between different cultivars is 2000 m. Some authorities stipulate shorter distances than this for cultivars with the same bulb color. In some countries, there are declared zones in which only cultivars of a specific bulb color can be grown for seed (George, 2009).
After seeding, the soil should remain moist so germination and early seedling development are uniform. Moist soil also prevents crusting which may delay or inhibit seedling emergence, reduce uniformity, and potentially delaying maturity. Since seeding occurs in the summer, often at high temperatures, sprinkler irrigation is used to cool the soil and prevent crusting by evenly hydrating the seed bed. Since onions have a shallow root system that is composed of straight non-branched roots that extend from the basal stem plate, plants must receive a consistent supply of water throughout the growing season. Onions cannot access water at soil depths below 60 cm, and most available water is extracted from the top 30 cm of soil (Voss et al., 2013). The general recommendation for at least one inch of water per week, especially during bulbing, applies to onions. Photosynthesis and expansive growth are reduced by even mild water stress, because unlike many crops, onions cannot reduce their leaf water potential by osmotic adjustment to compensate for dry soils. Stressed onion plants exhibit poor flower and pollen development, reduced seed yields, lower seed weights, reduced nectar production and lower seed vigor when germinated (Voss et al., 2013). The amount and frequency of irrigation required depends on soil type, stage of crop development, environmental conditions (humidity, wind, irrigation methods, rainfall, evapotranspiration etc.). Evapotranspiration from a field can be monitored by pan evaporation with tensiometer measurements of soil to determine when water application is needed. Crop coefficients published for various locations determine how much of the evaporative losses must be reapplied when the soil moisture tension threshold is reached. Irrigation is usually needed when 25% of available water in the top 60 cm has been depleted. In general, onion seed crops use 65 to 90 cm of water per growing season. With 70 to 80% efficiency water application of 90 to 115 cm may be required to produce a seed crop (Voss et al., 2013).
Growth and Development
Onion food crops are grown as an annual for fresh use when immature (green onions) or mature (dried bulbs) (Welbaum, 2015). However, for seed production, onions require two seasons to complete their reproductive growth cycle. The initial growth rate is slower than many other cool-season crops because of slow leaf area development and poor light interception that limits photosynthesis. As seedlings become established and grow, new foliage and roots continue to be produced, along with a slight elongation and widening of the compressed stem (Brewster, 2008, Rabinowitch and Currah, 2002). Onion seed production requires low-humidity conditions during spring and summer as seeds are maturing. Disease management, pollination, and seed maturation are enhanced by warm temperatures and low relative humidity. Foliage diseases are more common under humid conditions and bee pollination is reduced during rainy weather. Pre- or post-harvest drying is enhanced in climates with low humidity. Climates that are cool in the winter with warm to hot spring and summer seasons with low humidity and limited rainfall are near ideal for onion seed production. Although onion seed production requires two growing seasons, both can occur within a single 10- to 12-month window if the crop is seeded in late summer. The first season the plants grow vegetatively from seed and produce a bulb based in response to day length. The optimum range of temperatures for best growth and development are 20 to 25 °C.
Bulbing is a change in leaf morphology initiated when sufficient exposure to a critical day length is exceeded, although temperature has an influence as well. Each cultivar has a critical day length for bulbing induction. The duration of light exposure is most important, and the exposure process is cumulative. A brief exposure to the appropriate day length stimulus is not sufficient to initiate the bulbing process. When cultivars reach their critical day length before adequate vegetative growth is achieved, resulting bulbs will be small. Cultivars that require long days to bulb will not bulb when grown during short days (Brewster, 2008, Rubatzky and Yamaguchi, 1997).
Bolting is the formation of a seed stalk and associated inflorescence. Cultivars differ greatly in their response to low temperature and the duration of exposure needed for vernalization, the induction of bolting. A period of exposure to 5°-10°C for 4 to 6 weeks is adequate for the vernalization of many cultivars (Rubatzky and Yamaguchi, 1997). Chilling degree days can be calculated from weather or storage room growth data using 10°C degrees as a base temperature for chilling hour accumulation. For some cultivars, temperatures between l0°C and 13°C are adequate to stimulate bolting. Insufficient vernalization results in low seed yields. It is possible to devernalize a plant by exposure to high temperatures subsequent to vernalization. The pattern of foliage growth and development is also altered after bolting (seed stalk development) is initiated because the developing seed stalk is a strong sink for photoassimilates. However. rapid vigorous bulbing can suppress seed stalk emergence even if it is already initiated as both processes compete for available photoassimilates. It is possible to have bulbs and seed stalks developing simultaneously.
During autumn of first season, remove weeds, plants with off-type foliage, off-type bulb or stem color and plants bolting prematurely in first year. During flowering the following spring, check inflorescences and flower characters for gross abnormalities remove any weeds that have emerged since the previous roguing, rogue plants with off-type foliage or off-type bulb color (George, 2009).
Check inflorescences and flower characters for gross abnormalities. Flowering heads of plants infected with Ditylenchus dipsaci tend to be bent over; infected plants should be removed and burned. Before bulb maturity the field should be rogued to remove, plants with off-type foliage, off-type bulb or stem color, plants bolting prematurely in first year and late maturing plants. Early bolters, bull necks, bottle-shaped bulbs, split bulbs, doubles, damaged and diseased bulbs should be discarded prior to storage (George, 2009).
Managing pests in an important part of onion seed production. Both chemical and nonchemical strategies can be effective employed. Crop rotations to non-Alliums for multiple years are a simple way to control many soil borne diseases, insect pests, certain weeds, and nematodes. Site history should be evaluated to make sure fields do not harbor noxious weed, insect or disease pests of onion. Areas near seed fields must also be managed to prevent alternate hosts from harboring pests which may migrate onto a seed crop. Weed control in areas adjacent production fields will reduce the introduction of weed seeds. Solarization and sustainable fumigation techniques should be considered if less invasive technique do not effectively control pernicious soil-borne pests CABI, 2017, Chaput, 2011).
Onions compete poorly with weeds because of the long period required for plants to achieve foliage cover. The long growing season required for onion seed crops allows for successive flushes of weed growth to occur. Since onion seed crops are planted in late summer and remain in the ground through the winter and spring in mild climates, they may encounter different types of weeds such as perennials, winter annuals, summer annuals, grasses and broadleaves. The limited availability of pre- and post-emergence herbicides makes site selection, preplant weed management, via cultivation an essential components of onion seed production. For annual broadleaf weed control, the stale bed technique may work effectively. For this technique, the bed is prepared as if for planting and irrigation is applied to encourage weed growth. After the first flush of weeds emerges, the weeds may be killed by shallow application of herbicides before the onion seed crop is planted. During winter months, cultivation may not be possible because of wet soils. Care must be taken when hand weeding because onion root systems may be damaged when weeds are removed close to plants. Two cycles of careful hand weeding along with careful site selection and management are necessary to keep weed populations low. Post emergence herbicides, where available, may be applied and the three to four leaf stage and later to provide effective control. Soil solarization prior to establishment is another weed control tool for areas with extended periods of intensity solar radiation. Morning glories and specifically field bindweed are particularly bothersome weeds because the seed is similar in size and shape and color to onion seed making it difficult to remove during post-harvest processing. In some countries, bindweed is a federally designated noxious weed meaning there is zero tolerance for contamination of onion seed lots (Voss et al., 2013).
Disease problems encountered when producing onion seed are similar to those encountered for edible bulb production. Downy mildew is caused by the oomycete organism Peronospora destructor, which infects first the leaves and later bulbs of onions and other Alliums in mild, humid weather in spring and early summer as seed stalks develop. Downy mildew is often worse on direct-seeded crops. White rot (Stromatinia cepivora or syn. Sclerotium cepivorum) results from a soil-borne fungus, which can persist in the soil for many years and may be controlled by crop rotation. Purple blotch (Stemphylium vesicarium and Alternaria porri) are potentially serious fungal diseases that cause purple discoloration of foliage. Pink root (Phoma terrestris) and soil-borne fungus discolor roots and stunt onion plants. Neck rot (Botrytis aclada and B. allii) is primarily a postharvest storage disease that results in sunken and scales that turn gray to dark brown during storage for bulb-to-seed production. Proper curing can prevent this disease. Moist conditions favor botrytis leaf blight (B. squamosa). The disease causes leaf spots (lesions) and maceration of leaf tissue resulting in leaf dieback and blighting reducing onion growth and seed yield. Onion smut (Urocystis cepulae), a soil-borne seedling disease common in Northern areas, typified by distinctive narrow elongated dark streaks, usually on the cotyledon or first true leaf. Infected seedlings fail to emerge or usually die within a few weeks after emergence (CABI 2017, Chaput, 2011, Schwartz and Mohan, 2008).
Field scouting is important to identify troublesome insects as they appear. Both onion and flower thrips (Frankliniella occidentalis) feed on leaf sap, cause leaf distortion, reduce seed set, and by vectoring viruses such as IYSV. Thrips cause economic losses when they become too numerous on umbels. Onion maggots (Delia antiqua) feed on onion roots and overwinter in soil. Crop rotation can effectively control maggots. Corn seed maggot, wireworms, leaf miners, armyworms, mites, and cutworms may also attack onion seed fields. Broad spectrum chemical insecticides to control onion insect pests mustr be used with extreme caution, and not during bloom to protect insect pollinators at work in onion seed fields (Gillespie et al., 2013). Nematodes can also be a problem for onion seed production in some areas. The stem and bulb nematode (Ditylenchus dipsaci) and root knot nematodes (Meoidogye spp.) can stunt growth and reduce seed yields. Fields with a history of nematodes should be avoided.
Determining when to harvest is one of the most challenging aspects of onion seed production. This is largely because there are conflicting objectives: 1. To allow maximum seed maturation on the mother plant and 2. Minimize the loss of seeds from mature umbels due to shattering. The optimum time for seed harvest is based on cultivar characteristics and local weather. The seeds are black when ripe and can be clearly seen against the silvery colored capsules. Traditionally onion seed heads are harvested when about 10% of black seed are visibly exposed in the umbel. This visual stage of development corresponds to a seed or whole umbel moisture content of approximately 65%. Shattering increases sharply below an umbel seed moisture content of 50 to 55%. The seed heads shatter (seeds separate and fall from the head) if harvest is delayed so harvest should commence before 5% of seeds have been lost from the umbel. The optimum time for seed harvest is based on cultivar characteristics and local weather (Voss et al., 2013).
Indirect harvesting occurs when crops are hand harvested and dried prior to thrashing. For indirect harvesting, seed heads with approximately 10-20 cm of scape (seed stalk) attached are removed by cutting with a sharp knife or hand clippers. When cutting, the umbel is supported in the palm of the hand and held between the fingers to avoid loss of seed. Depending on crop uniformity and location, a single once-over hand harvest may be possible, but several successive hand harvests may be necessary for maximum seed yield. For indirect harvest, the seed heads are further dried on tarpaulins or sheets either in the open or in structures. For drying, umbels are placed 15 to 25 cm deep on a large canvas or plastic tarp and tried under ambient conditions for 2 to 3 weeks in areas without rainfall and low humidity. The piles are turned by hand to facilitate drying. Canvas tarps breath and are preferred to encourage air movement. Onion seed generally has a relatively short storage life and viability decreases rapidly at high temperatures and or higher seed moisture contents. To retain the highest seed quality, the seeds should dry quickly after harvest without an excessive buildup of heat on tarps. Frequent turning and shading will help maintain seed quality. Air circulation is key and fans can be employed to speed drying by lowering humidity. An alternative system designed to reduce labor cuts the stems at approximately 15 cm above ground level with a cycle-bar mower. The cut umbels with attached seed stocks are collected mechanically using a conveyer and placed in mobile bins. The material is then further dried on sheets as above. The attached stems add biomass to the piles increasing heating and requiring more frequent turning to facilitate drying prior to thrashing. In less arid regions, indoor drying may be necessary. Some producers use tiered boxes or a crate system to encourage air circulation. In climates with high humidity, forced-air drying or drying beads may be necessary (Bashir et al., 2016, Voss et al., 2013). After drying, umbels or umbels with stalks are thrashed using conventional combines in the indirect system and partially cleaned (scalped) before transport to a milling facility for further processing. (George, 2009).
Dried umbels are ready for threshing when the seeds can be separated from their capsules by rubbing in the hand. In order to avoid damaging the brittle seeds, threshing should not be delayed beyond this stage. Several threshing methods have been adopted for onion depending on the scale of operation and include flailing, rolling, threshing machines and combines. Onion seeds are very easily damaged during processing and frequent checks should he made to ensure that the seed coats are not accidentally cracked during processing. Examination of samples with a hand lens can confirm damage. Effect processing should separate seeds from the dried flowers without including pieces of the dried flower since this debris is difficult to separate from seed during cleaning. The light debris ("tailings") must also be examined to ensure it is free of good seed (George, 2009).
Flower pedicels may be removed by either a magnetic separator (the iron powder is added which adheres to the pedicels and not the seeds) or by flotation. If the latter process is used, only the light fraction is placed in water to avoid seed damage. During this process, which should not exceed three minutes per seed-lot, the good seeds sink while the poor-quality light seeds and pedicels float off. After spin drying and further drying in racks, additional debris is removed by an air-screen cleaner. Seed cleaning removes some good seed as well as contamination, so only the minimum number of steps required to meet the desired purity standards are used. The final seed lot must dried to 8% moisture content or less depending on the method of storage and packaging. (Voss et al., 2013).
After cleaning samples are analyzed for moisture, germination, and purity. If results are substandard, further milling may be required to upgrade the lot. Once minimum germination is achieved (commonly 85% in many countries), moisture content is again checked and adjusted below 8% before the seed is stored for packaging. Seeds may be treated with various materials such as biologicals, minerals, coating materials etc. and packaged in metal cans or plastic buckets for larger quantities or foil or plastic packets for smaller quantities. Since onion has a relative short shelf life, seeds must be protected from heat and humidity during storage and shipment using temperature controlled storage and resealable packaging with a barrier to keep seed moisture low (Voss et al., 2013).
The best yield from open-pollinated crops produced under ideal conditions is 2000 kg/ha but 1000 kg/ha is more common. In some areas yields as low as 500 kg/ha may be deemed satisfactory. The yield from F-1 hybrids is normally lower than from open-pollinated crops and is often as low as 50-100 kg/ha. There are approximately 9000 onion and leek seeds in one ounce (George, 2009).
Bashir, A. A., Sinha, J. P., Jha, G. K., & Chopra, S. (2016). Drying kinetics for vegetable seeds with Zeolite beads. Indian Journal of Agricultural Sciences, 86, 1630-1634.