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Yellow storage onions (vegetable crop management)

Vincent A. Fritz, Cindy B. Tong, Carl J. Rosen and Jerry A. Wright


Bulbing onion varieties are classified according to day length. All bulb onions grown in the Minnesota are classified as long-day varieties requiring 14 or more hours of day length before bulbing is initiated. Technically, all onions are "long-day" plants, in that bulbing begins as day length increases. Short-day varieties are those requiring only 10 to 12 hours of day length for bulbing to occur. These are grown in southern states generally below the 35th parallel, and are not suitable for bulb production in the midwest, except as pearl or boiler varieties, or for over-winter production.

Temperature and light intensity and quality can modify the onion bulbing response. High temperatures and bright days can "compensate" for some day length, causing onions to bulb sooner than they would otherwise. Overcast, cool temperatures delay bulbing. Time of bulbing is an important factor in determining onion bulb size. Early bulbing contributes to small bulb size, with delayed bulbing resulting in larger size.

Uniformity of maturity (rate of foliage collapse) is very important in bulb size uniformity and storage quality. Foliage collapse in hybrid varieties tends to occur over a period of three to four days, with close to 100% of the tops falling, however in non-uniform (often open-pollinated) varieties, this could occur over a period of several weeks with a percentage of the tops not falling at harvest. In such varieties, bulbs with early tops-down contribute to incidence of bald onions (skinless) at harvest, while those whose tops resist falling do not cure properly, contributing to decay in storage.

Cool temperatures and late plantings predispose varieties to bull (thick) necks, a trait that contributes to poor storability due to post-harvest disease infection. This is especially true of late varieties that may have poor bulbing uniformity qualities (sometimes more prevalent in open- pollinated varieties).

Selecting the correct variety for your production area and understanding the climatic and other environmental factors that affect its performance is extremely important.


Yields of the same variety will vary from year to year depending on weather conditions, planting schedules and soil types. As new varieties continue to be introduced into the market, the best recommendation for variety selection is to consult your seed company representatives and variety trial results conducted at universities for the variety that will best fit your needs. One source of current recommended varieties for the Midwest can be located at: Midwest Vegetable Production Guide for Commercial Growers. Methods, treatments, soil types, irrigation, etc. should be considered in evaluating field trials. Seed treatments, viability, freshness, and resistance to disease and pest problems are also important considerations. Seed treatments, germination rates, and time of packaging should be clearly stated on the label.

Soils and soil temperature

The optimum temperature range for germination is 48 to 90°F. Planting normally begins as early as April 10 and continues until month's end. In Minnesota, onions are grown primarily on peat soils, although silt loam, or sandy loam soils are also suitable.


Most onions are direct seeded. There are approximately 9,000 onion seeds per ounce. The depth of seed placement has an effect on onion shape (see following section).When sets are used, about 800 lbs of 15/16 inch or smaller diameter sets are needed per acre. Sets are more expensive and can limit variety selection due to availability.

Spring seeding

Onions may be seeded as soon as the land can be made ready, but mid- April is considered ideal. If seeded too late, bulbs will be small since bulbing will begin before adequate vegetative growth occurs. Seeding should be completed by May l.

It is recommended that onions be planted on raised beds (70" on center) in two sets of double rows (precision or broadcasted, depending on the planter) spaced approximately 4" apart with each set of double rows spaced approximately 18" apart. One of the most outstanding benefits from this type of culture is improved drainage. Onions do not grow well during extended wet periods which can contribute to root death, subsequent tip burn, and reduced plant vigor.

With the described planting configuration, a seeding rate of 11 - 13 seeds/ft. 1/4" deep is recommended for maximum yield potential. With precision seeders (Stanhay® and Nibex®) 1.25 to 1.5 lbs. of raw seed are used per acre. These planters are also designed to accommodate pelleted seed. Higher seeding rates will generally result in significant increase in boiler size onions (< dia.) and decrease in No. 1 size production.

If onions are to be planted without raised beds, it is recommended that the rows be spaced 12 - 14" with a seeding rate of 11 - 13 seeds/ft.

Depth of seeding has an effect on bulb shape since the onion stem plate (the base of the onion bulb) forms at the point where the seed germinates. Shallow planting results in flatter bulbs, while deeper seed placement results in taller, and sometimes top-shaped bulbs.


A soil test is the most accurate guide to fertilizer requirements. The following are general guidelines:

Good management practices are essential if optimum fertilizer responses are to be realized. These practices include use of recommended varieties, selection of adapted soils, weed control, disease and insect control, good seed bed preparation, proper seeding methods, and timely harvest.

Because of the influence of soil type, climatic conditions, and other cultural practices, crop responses from fertilizer may not always by accurately predicted. Soil test results, field experience, and knowledge of specific crop requirements help determine the nutrients needed and the rate of application. Fertilizer application for onions should insure adequate levels of all nutrients. Optimum fertilization is essential for top quality and yields.

Recommended soil sampling procedures should be followed in order to estimate fertilizer needs. The University of Minnesota Soil Testing Laboratory ( can provide you with soil sampling instructions and soil sample bags and information sheets.

Primary nutrients

Nitrogen (N)

Onions require a good supply of available N; however, excess N applications can result in late maturity, large necks that are difficult to cure, soft bulbs, and poor storage quality.

A higher percentage of fertilizer N is absorbed by the plant if the fertilizer is applied when the onion root system is well developed. Split applications of nitrogen are used more effectively by the plant than a single pre-plant broadcast application. Fertilizer N applied before planting should be incorporated into the plow layer or banded well away (6") from the seed on the furrow side of rows in two-row onion beds.

Sidedressed nitrogen applications or nitrogen applied in irrigation water can be an effective means of providing supplemental nitrogen to the crop during the season. A nitrogen source such as 28% N (Urea ammonium-nitrate) is a recommended form to use for fertigation.

Excessive rainfall and irrigation that cause nitrogen movement beyond the root system can reduce the amount of nitrogen available to the plants. Additional nitrogen may be required during the season under these conditions. Cool weather during the growing season can reduce nitrogen release from organic matter. Under these conditions, the nitrogen rates should be increased particularly on organic soils.

Excessive nitrogen available to onions, particularly later in the season (mid-July), can delay onion maturity and make field curing of onions more difficult. Inadequate field curing before storage can result in greater losses due to rot.

Onion varieties that differ markedly in yield potential can be expected to differ somewhat in their requirements for nitrogen. Higher yielding varieties may require slightly more nitrogen. Plant populations of lower than 4 plants per foot of row will require less nitrogen.

Because of the mobility of nitrate in soils and the complex transformations from organic matter, soil tests for nitrogen are not reliable for predicting nitrogen fertilizer needs in the eastern half of Minnesota (Area 1), particularly on sandy irrigated soils. Therefore, Nitrogen recommendations are based on yield goal, previous crop, and soil organic matter content.

In western Minnesota (Area 2), the amount of residual N in the soil varies considerably because of lower amounts of rainfall. A soil test for nitrate-N (NO 3 -N) helps in evaluating the N carry-over from the previous year, and it should be used to adjust N applications. Soil samples for NO 3 -N should be taken from the 0-24 inch soil depth in the spring prior to the application of N fertilizer.

Refer to Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota (AG-BU-5886-F) for more information and reference map.

Table 1. Nitrogen recommendations for dry bulb onions

Approximate Yield Goal2

Soil Organic Matter Level (O.M.)1

Organic Soil





-----------N to Apply (lb/A)-------------







Suggested Method of Application 2,3,4,5
1/4 banded, 3/4 sidedress 4 - 5 weeks after emergence
1 Low O. M. = less than 3.1%, Medium O. M. = 3.1 - 4.5%, High O. M. = 4.6 - 19%, Organic soil = greater than 19% O. M.
2 Recommendations are based on attaining approximate yield goals listed.
3 Suggested methods of application are a general guide and can be modified when appropriate.
4 On sandy soils, sidedress applications may be split 1 - 2 more times (not to exceed total recommended unless a need is indicated).
5 Up to 30 lb N/A of the broadcast application can be banded if equipment is available.

Table 2. Nitrogen credits for previous crops

Alfalfa (good stand)

70 lb N/A

Alfalfa (poor stand)
Alsike clover
Birdsfoot trefoil
Grass-legume hay
Grass-legume pasture
Red clover

40 lb N/A

Snap beans

20 lb N/A

All other crops

0 lb N/A

Phosphorus (P)

Phosphorus is essential for vigorous early growth of seedlings. All phosphorus should be applied at planting, if necessary. P can be banded at planting 2" to the side and below the seed. On low soil testing soils a combination of broadcast and row applied should be used.

Table 3. Phosphorus recommendations for dry bulb onions

Approximate Yield Goal1

Soil Test P Level (ppm)

















--------P2O5 to Apply (lb/A)2------









1 Recommendations are based on attaining approximate yield goals listed.
2 Recommended rates are for total amount to apply: broadcast + starter. Up to 70 lb P2O5 can be banded at planting.

Potassium (K)

All K should be applied and worked into the seed bed before planting. Onions require medium levels of available soil potassium. The K soil test onions (dry) is based on K present in the surface soil (0-12").

Table 4. Potassium recommendations for dry bulb onions

Approximate Yield Goal1

Soil Test K Level (ppm)








----------K2O to Apply (lb/A)2--------








1 Recommendations are based on attaining approximate yield goals listed.
2 Recommended rates are for total amount to apply: broadcast + starter. Up to 30 lb K2O

Secondary macronutrients

The secondary macronutrients, calcium, magnesium, and sulfur, are generally not limiting to crop production in most Minnesota soils except under certain conditions.

Magnesium (Mg)

Magnesium deficiency may occur in acid sandy soils. Soil tests less than 100 ppm Mg are considered low. If Mg deficiency is known or suspected, the use of dolomitic limestone is the best long range approach. Apply low rates (approximately) 1,000 lb/A if maintenance of soil acidity is desired. Otherwise lime with dolomitic limestone according to soil test recommendations. Other more immediate available sources of Mg include potassium-magnesium sulfate (11% magnesium) or Epsom salts (10% magnesium). Recommended rates of magnesium application based on a soil test are presented in the following table:

Table 5. Magnesium recommendations for dry bulb onions

Magnesium Soil Test

Relative level

Magnesium to apply



-------- ppm ---------


---------- lb/A ---------

0 - 49




50 - 99




100 +




Sulfur (S)

Onions require relatively high levels of available S. Sulfur fertilizer requirements will vary with soil texture, leaching losses, and S content of the soil. S is frequently contained in fertilizers used to supply other nutrients such as N, P, and K and may be present in irrigation water, which can be tested for S content.

Plants absorb S in the form of sulfate. Fertilizer materials supply S in the form of sulfate and elemental S. Elemental S must convert to sulfate in the soil before the S becomes available to plants. The conversion of elemental S to sulfate is usually rapid for fine ground (less than 40 mesh) material in warm, moist soil.

Elemental S (a strong acidifier) should be applied the year preceding the crop, using finely ground material. A soil test for sulfate-S is not recommended if elemental S was applied the preceding year. S in the sulfate form can be applied at planting time.

Sulfate is susceptible to leaching on sandy soils and deficiency is most common on sandy low organic matter soils. Soil tests for sulfur are only accurate for low or medium organic matter soils. If deficiency is known or suspected, refer to the following table for sulfur soil test recommendations.

Table 6. Sulfur recommendations for dry bulb onions
Sulfur Soil Test

Relative level

Sulfur to apply

------- ppm -------





---------- lb/A --------

0 - 6


20 - 30

10 - 15

7 - 12


trial only

12.1 +





Manganese (Mn)

Most manganese in soils is precipitated as manganese oxide or hydroxide. The form available to plants is the Mn2+ ion. Manganese deficiency may occur on organic soils with a pH greater than 5.8. On low pH soils (pH less than 4.8), manganese can be toxic to plants. For organic soils, Mn recommendations are based on soil pH. Soil or foliar applications can be made according to the tables below.

Table 7. Manganese recommendations for dry bulb onions (organic soils only)

Soil pH
Amount of Mn to apply





5.7 or less



5.8 - 6.3



6.4 or more



A few suspected Mn deficiencies have been reported in onions grown on alkaline mineral soils with a pH greater than 7.3.

Table 8. Manganese recommendations for dry bulb onions (mineral soils only):

Soil pH

Amount of Mn to apply





7.3 or less



7.4 or more



Boron (B)

No consistent responses of onions to the application of B have been observed in Minnesota. Where the soil test for B is below 1.0 ppm growers may wish to make a trial application of 1-2 lb B/A. Too much B fertilizer can be highly toxic and suggested rates of B application should not be exceeded. B should be evenly distributed over the field and not banded.

Zinc (Zn)

The application of Zn has increased the yield of onions where soils have been deficient. When the soil test for Zn is below 0.5 ppm broadcast 10 lb Zn/A before planting or include 2 lb Zn/A in the fertilizer band. A broadcast application of 10 lb Zn/A should supply Zn needs for 2 or 3 years.

When soil test for Zn is between 0.5 to 1 ppm, broadcast 5 lb Zn/A or apply 1 lb Zn/A in the fertilizer band.

For crops showing Zn deficiency during the growing season, foliar applications of zinc chelate (2 oz/A actual zinc) are suggested.

Copper (Cu)

Muck soils tend to be deficient in Cu and "fix" Cu. If Cu levels are less than 2.5 ppm, apply 10 lb Cu/A to the soil before planting, OR two foliar applications of 0.15 lb Cu in 100 gal water /A may be applied to onion leaves. Soil Cu applications need not be repeated every year. The best indicator for copper is leaf analysis.

Consider copper applications when leaf copper levels are below 5 ppm. When Cu levels are between 2.6 to 5 ppm, broadcast 6 lb Cu/A or apply 0.1 lb as a foliar spray.

Other nutrients

Responses of onions to nutrients other than those discussed in this guide have not been observed in Minnesota.


Onions are classified as being "very sensitive to soil acidity" when grown on mineral soils. Lime applications should be considered when mineral soil pH is 6.0 or below. Lime should be mixed into the soil in the fall for best results. Keep mineral soil pH between 6.0 and 6.8. Use the dolomitic form if soil Mg is low.

Onions will produce good yields over a fairly wide range of muck soil acidity. Lime applications should be considered when the soil pH is 5.5 or below. Keep muck soil pH between 5.6 and 6.0. Do not apply lime when the muck soil pH is above 6.0.

The need for lime can be determined from a routine soil test. For further information on lime, lime sources, and reference map refer to Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota (AG-BU-5886-F), pgs. 2-4, which is distributed through University of Minnesota Extension. Area I (acid subsoils) refers to Eastern Minnesota and Area II refers to Western Minnesota (alkaline subsoils). Lime should be applied before planting and incorporated to a depth of 6 inches.

Tissue analysis

Plant tissue analysis can be used during the growing season to monitor the nutrient status of the plant and help diagnose nutritional problems. Although not a substitute for soil testing, it can help to provide additional information related to crop nutrition and the effectiveness of a particular fertilizer program. Sufficiency levels can be determined from young mature leaf (top, no white portions) taken at time of mid-growth. Depending on the field size, at least 25 leaves should be taken for an adequate analysis. Plant analysis can be done at the University of Minnesota Soil Testing Lab or other commercial laboratories in MN on a fee basis.

Refer to, Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota (AG-BU-5886-F) University of Minnesota Extension, for nutrient concentration sufficiency ranges and more information on plant analysis and tissue testing.


Onions are shallow-rooted, and unless moisture supply is constant, they bulb early and the resulting sizes may be small. Light, frequent irrigations should be used when onions are small to minimize leaching of nitrogen from the root zone. Increase water applications as plants and roots increase in size. Maintaining moisture near the surface, at the onion stemplate is important in root generation. Onion roots generate at the stemplate only when moisture is present. Proper moisture management is important in alleviating pink root problems, general root health, and therefore bulb growth vigor. Also, maintaining an even soil moisture is important in reducing incidence of double-center bulbs.

Irrigation should thoroughly wet the soil to the 24 in. depth. Watering should be terminated after the bulbs have reached full size, and tops have begun to fall.

Soil type does not measurably affect total crop water requirements but does determine application frequency. Sandy soils hold very little water and therefore must be irrigated lightly but often for best results. Some sandy soils hold less than an inch of available water per foot, while silt loams may retain two to three times that amount. Irrigations exceeding soil water-holding capacity not only waste water but also leach fertilizer, especially nitrogen, from the root zone.

Harvesting, handling, curing and storage

As onions mature, tops begin to fall and dry. Sprout inhibitors are applied when onions are intended for long term storage. They are applied when tops are about 50% down, and there are 5 to 8 green leaves per bulb to absorb and translocate the sprout inhibitor. Do not apply sprout inhibitors when temperatures exceed 80-85°F to avoid crystallization on leaf surfaces. Use of a spray adjuvant is suggested . Avoid early sprays before maturity to reduce spongy onions. Maleic hydrazide (Royal MH-30) at 2 lbs ai/A is most commonly used. Apply in sufficient water to insure adequate coverage. Consult label for latest information on application rates, timing and precautions.

Optimum harvest from the standpoint of maximum storage life (before bulb sprouting), occurs while the onion foliage is still partially (30-40%) erect, and long before maximum yield is attained (when tops are completely down and dry). Since yields may increase 30-40% between the stage when tops begin to go down, and the leaves are fully down and dry, it is tempting to leave onions to cure in the field as long as possible. The optimum time for harvest therefore, must be a balance between highest yields and reduced bulb storage quality. Furthermore, excessively field-drying onions increases the risk of bald onions in storage.

Digging and windrowing

To facilitate curing onions for harvest and storage, onion rows are undercut, lifted and windrowed for field curing. Rod-weeder diggers and knife undercutters are most often used. After an appropriate interval, the undercut onions are lifted and windrowed. This may be done with tops on or off, but most commonly with tops on to protect the onions form sun scald damage. Windrows are often mechanically "fluffed" to facilitate curing and later combined to facilitate loading.


New harvesting machinery can load and top onions in one operation with minimal damage. Other toppers top onions while they are still in the ground or top them as they are dug and windrowed for further curing.

If onions are to be bulk-stored it is best to store them without their tops. This facilitates handling, loading and unloading the storage. If onions are to be topped and stored, tops must be totally dry, or only the dry portion cut and removed. Cutting through any portion of the top while it is still green or moist may result in excessive Botrytis neck rot in storage. In very wet years, do not top onions until after they have been cured. When all or a portion of the onion top is left on, the remaining tops are removed during grading and packing using roller toppers at the storage or packing facility.

In the field, mechanically undercut and windrow "storage types" and pearl onions when 65-100% of the tops are down and cure by windrowing in the field. Machine "fluffing" of the windrow a few days after digging will shorten the drying period. This should also be done after each rainfall. After field drying has occurred, the onions may be topped and placed in storage buildings. Specialized harvesters are available for the various types of onions.

Spanish onions for immediate sale or short-term storage are mechanically undercut and may be green-topped by hand or machine and partly cured in sacks or boxes in the field prior to packing. Since these onions are not to be stored, complete curing of necks and scales is not as important.

When these onions are intended for storage, complete curing is mandatory. Care must be exercised in handling these onions to guard against sun-scald and damage since these onions are much more succulent and have very few protective scales. When mechanically undercut and windrowed for curing, be sure onion tops provide adequate protection from sun-scald during periods of high temperature (above 90°F) and sunlight.

Drying (curing)

Onions should be adequately cured in the field, in open sheds, or by artificial means before or in storage. Adequate curing in the field or in open sheds may require 2 to 4 weeks, depending on the weather. The best skin color develops at 75 to 90°F. The most common method of curing in northern areas is by forced ventilation in the storage by blowing heated air at 75 to 85°F, through the bottom of the onion pile to the top, at two to three cubic feet of air per minute for each cubic foot of onions (or 30-50cfm/ton). Use the higher air-flow rate initially to remove surface moisture and seal necks. If the weather is cool and wet, forced air at 75 to 85°F and 70% relative humidity is recommended. If the onions are also wet, forced air at 85°F and a relative humidity of 25-35% relative humidity should be used as soon as storage loading is completed. This should be continued until the outer skins and neck are dry.

Onions are considered cured when the neck is tight and the outer scales are dry and make a rustling sound when handled. This condition is reached when onions have lost 3 to 5 % of their weight. If not adequately cured, onions are likely to decay in storage. The common form of decay is gray mold rot (Botrytis), which occurs at the top of the bulb - hence its name "neck rot". High temperatures and high humidity (80%) during curing with good air circulation favor development of desirable skin color.


After curing, onions should then be allowed to cool gradually slowly reducing its temperature by drawing outside air, anytime the storage air has a humidity above 75% and the outside air is cooler than the onions, until the temperature of the onions reaches the desired holding temperature, or (32°F and above).

Storage may be either in bags, crates, in pallet boxes that hold about a half ton of loose onions, or in bulk bins. Bags of onions are frequently stored on pallets and should be stacked to allow proper air circulation. Modern air-cooled storage facilities have forced ventilation systems in which air, heated if necessary, is introduce through floor racks beneath the bins of onions. Onions in bins are stored about 10 to 15 feet deep, but soft onions at the bottom may be distorted in shape.

In Minnesota, storage onions are generally held in bulk bins in common storage. However, they should not be held after early January unless they have been treated with maleic hydrazide in the field to reduce sprout growth.

During dry, cold weather (above 32°F) the doors may be opened or the fans used to circulate cold air throughout the pile. Be sure that the dew point of the outside air that is used is always 2 or more degrees above the temperature of the onions in the pile, otherwise water may condense on the onions. During wet weather or very cold weather (below 32°F) the building should remain closed and the air within should be recirculated periodically or outside and inside air may be mixed as needed to keep onions cool and dry.

Onions of the Spanish type grown in the Minnesota are often stored, if well matured and in properly constructed forced air storage facilities, until January or February (4 to 5 months); or they can be held at 32°F until April or May. Mild onions are usually stored for 1 to 2 months at 32°F. Onions will sprout and decay rapidly when stored at temperatures between 40 and 50°F. However, they will not sprout or be as susceptible to decay if they are in the resting stage, which lasts 30 to 60 days. The resting of most varieties is completed during storage at 32°F. Air circulation should be sufficient to prevent heating and to remove moisture from within bins or sacks, at least 1 cfm/ton of bulbs.

Sprout growth indicates too high a storage temperature, poorly cured bulbs, or immature bulbs. Sprouted onions are subject to decay and should not be marketed. Root growth indicates too high a relative humidity. Relative humidity of 65% to 70% is recommended for successful storage of onions. Higher humidities, at which most other vegetables keep best in storage, may cause onions to grow roots, rot, and develop surface mold. Excessive drying, however, may result in cracking or loss (bald onions) of the outer bulb scales.

Onions are damaged by freezing, which can occur at about 31°F. Freezing injury appears as water soaked scales when the thawed onions are cut, and the onions can become soft and decay easily. Onions only slightly frozen may recover with little perceptible injury if allowed to thaw slowly and without handling. Translucent scale of onions (a clearing of the scales which somewhat resembles freezing injury) has been found particularly in large bulbs stored several months. Prompt cold storage after curing reduces its prevalence.

Onions should not be stored with other products that tend to absorb odors. They may be stored with garlic. When onions are removed from storage in warm weather, they are apt to sweat because of moisture condensation. This may favor decay. Warming onions gradually, for example, to 50°F over 24 to 36 hours with good air movement should avoid this difficulty.


Dry onions are sorted, cleaned sized and graded, just prior to packaging. They are commonly packaged in 50 lb. sacks. Some are now also packaged in consumer packs of 1-10 lbs. mesh sacks, then palletized in master containers or in 48-50 lb. cardboard boxes.

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Reviewed 2010

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