Last revised December 12, 2012
Dry bulb onion in the Pacific Northwest may be classified into two distinct categories--those grown on muck soils, and those grown on mineral soil. Mineral soil onions are produced in two different regions, west of the Cascade Mountains, and east, notably in the Treasure Valley and the Columbia Basin. By far, most are produced on mineral soils. On both sides of the Cascades, mineral soil onions may be spring-planted or fall-planted for overwinter production.
Overwinter production west of the Cascade Mountains is relatively new and generally restricted to the Willamette Valley. Overwinter onion production is riskier, but allows harvest of onions in June and July when onion prices are historically at their highest. Overwinter onions differ in shape, pungency, scale color and storage characteristics from spring planted varieties, so be sure of your market before planting these. Only certain varieties are suited for overwinter production and cultural practices also differ as noted in special notations concerning "overwinter" production in various sections of this guide.
Bulb onion varieties are classified according to day length. Except for the overwinter varieties, all bulb onions grown in the Pacific Northwest are classified as long-day varieties requiring 14 or more hours of day length before bulbing occurs. 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 northwest, except as pearl or boiler varieties, or for overwinter production.
Temperatures and light intensity and quality (red:far red) can modify onion bulbing response. High temperatures and bright days can "compensate" to some extent for some day length, causing onions to bulb sooner than they would otherwise. Overcast skies and 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 tops-down) is very important in bulb size uniformity and storage quality. Tops-down in hybrid varieties tends to occur rapidly, requiring only a day or two to complete top-fall. Non-uniform varieties may have tops falling 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 at harvest, while those whose tops resist falling do not cure properly, contributing to decay in storage.
Cool temperatures, poor stands and late plantings predispose varieties to bull necks, a trait that contributes to poor storability. This is especially true of late, varieties that may have poor bulbing uniformity qualities (sometimes more prevalent in non-hybrids).
Selecting the correct variety for your production area and understanding the climatic and other environmental factors that affect its performance is extremely important.
See the Vegetable Variety Selection Resources page to find varieties that have been shown to perform well in the Pacific NW.
The optimum temperature range for germination is 48 to 90 F. March and April plantings are most common in western and central Oregon.
In western Oregon, onions are grown on peat soils or silt loams and sandy loams. Overwintered crops must have soils with good internal drainage.
In western Oregon planting is from approximately March 20 through the first week of May.
Onion seed numbers approximately 9,500 per ounce. Most onions are direct seeded. Some later varieties of sweet Spanish onion are transplanted in the Willamette Valley.
About 120,000 to 140,000 transplants are needed to plant one acre (when planted 3-4 inches apart in rows 15 - 24 inches apart). Only garden onions are grown from sets. When sets are used, about 800 lb 15/16-inch or smaller diameter sets are needed per acre. The depth of transplant or set placement has an effect on onion shape (see section on seeding below).
This method of placing individual seeds at a predetermined spacing within a row produces a crop of more uniform size, less culls and higher yield of the desired size grade. Present recommendations are to use coated seed and to seed two lines 3 inches apart per row with 6-12 seeds per foot of row (depending on variety), and rows 10-12 inches apart. Stanhay, Beck, or Graymore planters are used with coated seed. New vertical plate vacuum planters, such as the Gaspardo or Stanhay, also may be used with uncoated seed.
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 growth occurs. Seeding should be completed by May 5 in Western Oregon.
Approximately 2-3 lb of seed are required per acre when using a Planet Junior planter with 2 to 4-inch scatter shoe. Planters should be set to drop 5-10 seeds per foot of row (depending on the variety and size of onion to be grown), 3/4 to 1 inch deep.
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.
Spacing between rows is usually 10-12 inches apart in Western Oregon, where sets of 4 or 5 rows are used between wheel spacings.
Overwintered onions are seeded in August to mid-September. The earlier seeding dates are preferred with the more bolting resistant varieties. Plant spacings and configurations are the same as described above, except that final spacings should be 3-4 plants per foot of row to obtain the jumbo sizes desired in these types of onions. Overwintered onions are grown for their mildness and succulent texture which will not stand crowding. Over crowding will cause these onions to become badly misshapen and flat-sided.
It has been observed that temperatures under 20 F during the winter months may lead to thickened and elongated necks at harvest that cure poorly. Some varieties (Walla Walla Sweet) are more susceptible to this problem than other varieties recommended.
Sow 2-3 lb of seed to obtain enough plants for each acre to be transplanted. One acre of seed-bed can produce enough transplants for 12 acres of onions. Seed is drilled to a depth of l inch or slightly less.
Seed may be drilled with a wide shoe attachment which scatters the seed over a 4-inch band in rows with 16-inch centers. Light sprinkler irrigation following seeding will often be necessary to obtain a satisfactory stand.
Loosen plants before pulling, and tie in bundles of 100 to 200. If being moved any distance, or if transplanting will be delayed, store at 32-36 F with relative humidity of 90%. Immediately before planting, trim the plants to leave l/2 inch of the original root and 4 inches of the green top. Set in rows 12-16 inches apart, with 3-4 inches between plants.
See the guide, Nutrient Management for Onions in the Pacific NW for fertilizer recommendations.
Fertilizer recommendations for muck (high organic matter) soils:
Muck soils tend to be deficient in Cu and "fix" Cu. If Cu has not been applied for 3 years , apply 15-25 lb Cu/A to the soil before planting, OR two foliar applications of 1-2 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 4 ppm.
Do not apply lime when the muck soil pH is above 6.0. Decreased yields have resulted in Willamette Valley experiments when liming raised the soil pH to 6.5.
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-inch depth. In western Oregon, 20-22 inches of water may be required. Watering should be terminated after the bulbs have reached full size, and tops have begun to fall.
In overwintering onion production, use fall season water sparingly. Apply only enough water to establish a good stand, and enough growth so that onions will over-winter successfully. Onion plants should be about 1/4 inch in diameter as they go into the winter and go dormant. In spring, irrigate as necessary to maintain vigorous growth. Irrigation is generally terminated two weeks before harvest.
Soil type does not affect the amount of total water needed, but does dictate frequency of water application. Lighter soils need more frequent water applications, but less water applied per application.
See also the OSU Irrigation Guide for this crop.
For spring-seed onions, harvest may range from August 20 to October 20 with the prime harvest season being from the first to the end of September. The average yield of storage onions in Western Oregon is approximately 20 tons/acre with good yields of about 25 tons/acre.
Studies on the effect of harvest date on yield indicate that yield of bulb onions increases dramatically in the 4 weeks before 100% tops-down. In one study, yields increased approximately 10,500 lb/acre from the start of top-fall (at 12% tops-down) to 100% tops-down, while mean bulb weight increased 2.25 oz. Furthermore, after the start of 100% tops down, yield increased by 2900 lb/acre for the next 5 days (Davis, G.N. and H. Jones).
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 lb 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.
Research indicates that, from the standpoint of maximum storage life (before bulb sprouting) optimum harvest would be when 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 due to bald onions in storage.
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 common. After an appropriate interval, the undercut onions are lifted and windrowed. This may be done with tops on or tops may be removed in the windrowing operation. Onions are also commonly windrowed with tops on to protect them from sunscald. Windrows may also be mechanically "fluffed" to facilitate curing.
Onions may be topped or harvested and stored with tops on. Topping may be done by hand or by machines such as the Vegi-Vac, Top-Air. These machines perform the lifting, topping and windrowing operations at the same time. When onions are allowed to cure without windrowing, machines can also harvest, top and load all in one operation.
With good air movement and proper placement of onions in storage, onions have been found to store best with their tops on, however, keeping the tops on may complicate removal of onions from bulk storages and necessitates extra handling at packing time.
If onions are to be bulk-stored it is best to store them without their tops. This facilitates handling, loading and unloading the storage. Onions that are to be stored in bins are usually stored with tops on.
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. 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.
Early market and "overwinter" 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 sunlight and temperature (above 90 F).
Cull onion disposal: Oregon Department of Agriculture regulations specify rules for cull onion disposal in order to minimize the spread of diseases. Consult these for rules appropriate to your production area. In western Oregon, many of the cull onions are disposed of as sheep feed. Due to certain chemical constituents of onion, their use in animal feed is restricted by animal species and feed concentration.
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 storages 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.
Onions are held in either common or cold storage. The storage quality of onions is influenced by cultivar and by the conditions under which they are grown and stored.
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. and 60-75% relative humidity. 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 onions at 2 cf./m. air flow per cubic foot of onions. Electric or gas-fired infrared radiation has been shown to be an effective heating system for rapid curing.
Onions are considered cured when the neck is tight and the outer scales are dry and will rustle. 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 most common form of decay is gray mold rot, which occurs at the top of the bulb - whence its name "neck rot."
In the northern onion-growing states, onions of globe types are generally held in common storage. Average winter temperatures in the principal northern onion-producing states are sufficiently low to permit common storage during the winter months. However, they should not be held after early January unless they have been treated with maleic hydrazide in the field to reduce sprout growth.
Refrigerated storage is often used for onions to be marketed late in the spring. Onions to be held in cold storage should be placed there immediately after curing. A temperature of 32 F will keep onions dormant and reasonably free from decay, provided the onions are sound and well cured when stored. Air circulation should be sufficient to prevent heating and to remove moisture from within bins or sacks.
Sprout growth indicates too high a storage temperature, poorly cured bulbs, or immature bulbs. Root growth indicates too high a relative humidity. A comparatively low relative humidity (65 to 70 %) is recommended for successful storage of onions. However, humidities as high as 85 % and forced-air circulation have given satisfactory results. Higher humidities, at which most other vegetables keep best in storage, dispose onions to root growth, rot, and surface mold. Excessive drying however may result in cracking of the outer bulb scales and bald onions.
Globe onions can be held for 6 to 8 months at 32 F. The mild types such as those produced in the Walla Walla area of Washington can be held in cold storage; but, because of their poorer keeping qualities, they usually are stored for much shorter periods (1-2 months at 32 F) than the standard globe cultivars.
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 cultivars is completed during storage at 32 F.
Onions are damaged by freezing, the damage appearing as water soaked scales when the thawed onions are cut. 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. Conditions that predispose onions to translucent scale are:
Poor top growth and poor stands that reduce foliar cover and shading of the bulbs during the final stages of growth, thus exposing bulbs to prolonged sun exposure and resulting in increased bulb temperature
An extended period of maximum temperatures above 90 F in the final two months before harvest, particularly if half the daily maxima are above 90 F or 30 percent above 95 F.
A delay between harvest and cold storage. A delay of as little as 15 days increased incidence of translucent scale.
Larger bulbs (over 3 inch) were more likely to develop translucent scale than were smaller bulbs.
Sunburn is not necessarily a cause, but incidence of translucent scale increases when sunburn is present.
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.
Onions can also be stored at high temperatures of 85 to 95 F for short periods before marketing or before processing. Dehydrated flakes produced from onions that had been stored at 85 F for 4 months discolored less in storage than flakes made from onions stored at cold temperatures.
Controlled atmosphere storage tests with onions have been only moderately successful. An atmosphere of 5% carbon dioxide with 3% oxygen reduced losses from sprouting and root growth.
In Oregon curing is generally accomplished by forcing air of a low relative humidity through the bottom of the onion pile to the top. One to two cubic feet of air per minute for each cubic foot of onions is recommended, with the higher air-flow rate used initially to remove surface moisture and seal necks. If the weather is cool and wet, forced air at 75 to 85 F and 60-70% 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. The crop 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.
After curing, the relative humidity in the storage should be maintained between 60 and 70%. If the storage is too dry, the outer bulb scales will crack excessively, resulting in bald onions during packing. The less fluctuation the better.
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.
Consult also other appropriate storage publications such as Oregon State University PNW 277 Onion Storage Guidelines for Commercial Growers.
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 2, 5 and 25-lb mesh sacks.
Dry bulb onions from the northwest are shipped across the U.S.A. at ambient temperatures but protected from freezing. When shipped internationally, several methods are used.
When non-refrigerated containers are used, one door should be removed to eliminated the build up of humidity from onion respiration. Shipping companies can arrange to do this.
When refrigerated containers are used, as is necessary with sweet onions that have limited storage characteristics, arrangements must be made to bring the temperature in the container to ambient 24-48 hours before arrival at destinations where local humidity may result in the onions getting wet from condensation. Arrange for the shipping company to raise the container temperature from 32 F to 50 F two days before arrival, then from 50 F to 70 F one day before arrival. This gradual increase in temperature will prevent condensation on onions upon their removal from the container and reduce risk of spoilage.
maleic hydrazide 2 lb ai/A
(dry bulb onions only)
Apply when bulbs are fully mature with soft necks and 5 to 8
green leaves, or when approximately 50% of the tops have fallen, but are
still green. Should be applied at temperatures below 80 to 85 F to
avoid crystalization on leaf surfaces. Use of a spray adjuvant is
suggested in arid regions west of the Rocky Mountains. Avoid early
sprays before maturity to reduce spongy onions. Do not treat seed