Last revised February 12, 2010
Note: This file contains only information specific to production of pearl, set, and boiler onions. For more information on onion culture, refer to the files Dry Bulb Onions -- Eastern Oregon and Dry Bulb Onions -- Western Oregon.
Onion bulb size is influenced by a number of factors. Variety, plant density, photoperiod, and temperature are several important ones.
Small bulb onions are usually produced in the Northwest by using techniques that capitalize on the onion's response to day length, plant density, and temperature. Pearl onions are short-day varieties grown under the Northwest's long-day summer conditions and planted at very high plant populations to produce the necessary yields. Onions for "sets" or "boiler" production may be short, intermediate, or long-day types also grown at high populations and using planting dates that capitalize on optimum warm summer temperatures at bulbing.
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 light quality (red:far red) can modify onion bulbing response. High temperatures and bright days can "compensate" 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.
Selecting the correct variety for your production area and understanding the climatic and other environmental factors that affect its performance is extremely important.
Most pearl onions are produced from the species Allium cepa. In addition, there is another species of onion, Allium ampeloprasum L. var. sectevum that is often called pearl onion. The plants form a clump of small daughter bulblets that may be used as pearl onions. These are grown mainly in gardens in the eastern section of Germany and, until recently, were grown commercially in the Netherlands. The small bulbs can be grown from seed, but more typically are produced from bulblets. At maturity each plant forms a cluster of short-stalked bulblets with silvery white skins. These are traditionally pickled.
Pearl or cocktail onions: Crystal Wax (also known as White Bermuda), Eclipse (L303). For trial: Barletta (or White Pearl), White Mexican (or El Toro). All these are short-day varieties that bulb shortly after emergence to produce the desired bulb size at the latitudes of Pacific Northwest production areas.
Set onions: Generally long day varieties such as Stuttgart, Yellow Ebenezer, White Ebenezer, and Red Weathersfield are used.
Boiler onions: Small bulbs (1-1 3/4 inches) of any of the varieties listed above may be used. When planting specifically for boilers, Southport White Globe or the yellow types such as Stuttgart, Australian Brown, or Yellow Ebenezer are commonly used.
The optimum temperature range for germination is 48 to 90 F. March and April plantings in western and central Oregon and late February to April in eastern Oregon and the Columbia Basin.
In western Oregon, onions are grown on peat soils or silt loams and sandy loams. For pearl or cocktail onions, soils must be friable to allow rapid mechanical separation of onions from the soil at harvest. Loamy sands or similar sandy soils are best.
Onion seed numbers approximately 9,500 per ounce. These onions are all direct seeded at high seeding rates using specially designed planters that allow for uniform scattering of the seed and close spacings between rows. This is usually done using gangs of planters mounted on multiple tool bars.
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.
Plant density also has an effect on onion shape, with onions tending to be taller at high density. When round onions are desired at the high densities used for pearl onions, use varieties that tend to be flat-round in the southern U.S. production areas where they are normally grown as fresh market bulb onions.
All these onions are spring seeded. Onions may be seeded as soon as the land can be made ready, but April is considered ideal. Seeding should be completed by May l in Eastern Oregon and May 5 in Western Oregon.
Pearl onions are seeded at 80 to 110 lb seed/acre depending on the size distribution of the onions desired. The lower densities provide a higher percentage of "boiler" size onions. A Planet Jr. seeder plate number 23 to 26 will provide approximately 80-110 lb seed/acre on a total broadcast basis, at a planting speed of about 3 MPH. To accomplish total broadcast plantings, gangs of planters are mounted on several tool bars in a staggered arrangement and 4-inch scatter shoes are used to cover the desired bed width.
Onions for sets are seeded at about 55-75 lb/acre. Distribute seed in a wide band so that plants are approximately 1/2 inch apart. Uniform distribution of seed is important in obtaining good yields and size distribution. When planting in rows, growers in western Oregon use custom built planting units that meter six rows at a time. These are spaced to plant 18 rows across a 40-inch bed. These planters use metering units such as those used in dispensing granular materials. These are fitted with flexible tubes that direct the seed to scatter shoes mounted on Planter Jr. or similar seeder frames.
When planting primarily for boiler onions, a seeding rate of about 20 to 30 lb/acre is used. Seeds are 3/4 to 1 inch apart. They may be planted as described for pearl or set onions above.
For the most current advice, see Nutrient Management for Sustainable Vegetable Cropping Systems in Western Oregon, available as a free download from the OSU Extension Catalog
A soil test is the most accurate guide to fertilizer requirements.
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. The fertilizer application for onions should insure adequate levels of all nutrients. Optimum fertilization is essential for top quality and yields.
N rates of 80-100 lb/A should be broadcast and worked into the seedbed before planting, using fertilizers such as ammonium sulfate, 16-20-0 or a mixed fertilizer containing sulfate sulfur. Caution, when mild-flavored onions are desired, reduce sulfur applications to the minimum recommended for proper plant growth since sulfur applications have been shown to increase onion pungency (see sections on "sulfur" below).
If necessary, additional N may be applied during the growing season. Small onions do not require high rates of N.
Sidedressed nitrogen applications or N applied in irrigation water can be an effective means of providing supplemental nitrogen to the crop during the season. DO NOT use aqua or anhydrous ammonia in sprinkler irrigation.
Shallow incorporation of high rates of topdressed or broadcast fertilizer N can seriously reduce onion stands.
For Nitrogen liquid fertilizer formulations having weed control effects, refer to the file Nitrogen Fertilizer Solutions Providing Ancillary Weed Control in Alliums. Refer to the files Dry Bulb Onions -- Eastern Oregon and Dry Bulb Onions--Western Oregon for more information on requirements for P, K, Ca, Mg, lime, and minor elements.
Onions are shallow-rooted, and require a constant supply of moisture. 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.
Excessive irrigation causes N movement beyond the root system and reduces N available to the plants. 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.
Although small in size, yields of pearl onions can be about 20 tons/acre when the correct plant population and other cultural practices are realized.
Harvesting begins after tops have fallen and dried thoroughly. Beds of pearl onions intended for processing are topped using flail choppers or mowers just before (about 24 hours) digging the onions. This allows the tips to dry yet does not allow the onions to green or scald. In western Oregon, "set" onions may be harvested and temporarily stored without removing the tops. Tops are removed before packaging. When tops are not to be removed, "set" onions are undercut and allowed to cure in the field. Undercutting is accomplished in such a way as to lay the tops over the windrow to protect the onions from sunscald. "Set" onions must not have been treated with sprout inhibitors commonly used in bulb onion production.
When pearl onions are harvested for processing, they are undercut to remove as much of the root system as possible and harvested using specialized equipment designed specifically for pearl onion production. Harvesters lift the entire bed and convey the onions over closely spaced rod conveyers to bins or bulk trucks for transport to the processing plant.
Onions intended for storage 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 temperatures are 75 to 90 F.
Pearl, set and boiler onions may be held in either common or cold storage. Storage quality is influenced by cultivar and by the conditions under which they are grown.
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.
Store at 32 F and 65 to 70 % relative humidity. The temperature and humidity requirements for onion sets, used primarily as planting stock for early green onions, as well as pearl and boiler onions, are essentially the same as those for large dry onions. Because of their small size, these onions tend to pack closely, so they should not be placed into deep piles. They are usually held in ventilated storage in shallow, slatted trays rather than in bags or crates.
Set onions are handled in mesh or Kraft paper bags for marketing. Low relative humidity and low temperature are important to keep the sets sound and dormant and free from sprouting and rooting. At humidity much above 70 % and at warmer temperatures (40 to 50 F), more of the sets will sprout, develop roots and decay. Onion sets should be stacked to allow good air circulation. A storage life of 6 to 8 months is possible for good quality sets.
Set onions are handled in mesh or Kraft paper bags for marketing. Low relative humidity and low temperature are important to keep the sets sound and dormant and free from sprouting and rooting. At humidities much above 70 % and at warmer temperatures (40 to 50 F), more of the sets will sprout, develop roots and decay. Onion sets should be stacked to allow good air circulation. A storage life of 6 to 8 months is possible for good quality sets.
Onions should not be stored with other products that tend to absorb odors. They may be stored with garlic.