Dry bulb onion in the Pacific Northwest may be classified into two distinct categories: those that are grown on muck soils, and those grown on mineral soil. By far, most are produced on mineral soils. East of the Cascade Mountains, mineral soil onions are produced in two different regions, notably in the Treasure Valley and the Columbia Basin. Mineral soil onions may be spring-planted or fall-planted for overwinter production. Overwinter production has been well-established in the Milton-Freewater area of Oregon and the Walla Walla area in Washington. Overwinter onion production is riskier, but allows harvest of onions in June when onion prices are historically the highest of the year. 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.

Sweet onions have long been produced in the Walla Walla area of Washington and the adjacent portion of Umatilla County in Oregon. Sweetness in onions is attributable to several factors including the accumulation of simple sugars such as fructose, glucose, and sucrose. Fructose contributes the most to sweetness relative to its concentration in the bulb. Onion flavor is influenced by three other factors: tear producing compounds, compounds such as pyruvic acid that cause pungency, and other volatile flavor components. Pungency is measured by determining the pyruvic acid content of the bulb. Onions may be classified as to pungency according to the following scheme:

• very mild sweet onion..... 1- 4 mmoles pyruvic acid/kg weight of bulbs
• mild sweet onion............. 5- 7 mmoles
• intermediate pungency..... 8-10 mmoles
• pungent ........................11-15 mmoles
• very pungent .................... >15 mmoles

Spring-seeded Spanish types:

Yellow, early: Brahma, Cima, Early Shipper, Maritime, Pinnacle, Yula, Zenith. For trial: Bullring, Condor, Envoy, Fabius, Festival, Frontier, Golden Eagle, Regiment, Tenshin, Viceroy, Viper.

Yellow, main season: Bravo, Copra, Dai Maru, Vaquero. For trial: Big Mac, Bravado, Golden Treasure, Quest, Ringmaker, Seville, Summit, Sweet Amber, Sweet Perfection, Takii 6404, Tesoro, Valdez, Vega, Winner.

The above information is based on multi-year studies by Oregon and Washington State Universities that provide additional information on bulb size distribution, storability, diseases, and bolting. Oregon State University reports are available from: Dr. Clinton Shock, Malheur Experiment Station, 595 Onion Ave., Ontario, OR 97914. Washington State University onion variety reports are available from: Mr. Gary Pelter, WSU Cooperative Extension, Courthouse, POB 37, Ephrata, WA 98823.

Red: Mambo, Tango. For trial: Carmen, Bennies Red, Fuego, Red Sweet Spanish.

White: Blanco Duro, White Delight. For trial: Avalanche, Blizzard, Glacier, Sterling, White Keeper, White Sweet Spanish.

Fall-seeded overwinter: Planted in mid-August to mid-September for harvest in late June through July.

Yellow: Walla Walla (a number of strains exist: Arbini, Locati, Early and late French and others; none are hybrids; adapted especially to northern Umatilla County), Top Keeper, Hi Keeper, Buffalo, Keep Well, Hi Ball, Bison, Senshyu Yellow, Imai Yellow.

Red: Red Cross (large, flat), Kurenai.

SOIL TEMPERATURE

The optimum temperature range for germination is 48 to 90 F. March and April plantings in central Oregon and late February to April in the Treasure Valley of eastern Oregon and in the Columbia Basin.

SOILS

In Eastern Oregon, onions are grown on a range of mineral soils. Sandy-loam or silt-loam soils are preferred.

SEEDING AND TRANSPLANTING

Spring seeding: In eastern Oregon's Treasure Valley and lower Columbia Basin onions are planted from late February or the first of March to mid-April. Seeding early increases the risk of bolting with susceptible varieties. Seeding late results in smaller bulbs at harvest. Seeding should be completed by early May.

Onion seed numbers approximately 9,500 per ounce. Most onions are direct seeded. A few acres of overwintering Walla Walla Sweet onions are transplanted in northern Umatilla County.

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 of 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 below).

Precision Seeding: 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 where jumbo onions or soft bulb varieties are used, and to seed two lines 3 inches apart per row with 4-6 seeds per foot of row (depending on variety), and rows 12-14 inches apart. Stanhay, Beck, or Graymore planters are often used with coated seed. New vertical plate vacuum planters such as the Gaspardo or Stanhay also may be used with uncoated seed.

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.

Raised beds are commonly used and two rows are planted per raised bed. These are often split into 2 lines/row with rows 12-18 inches apart on top of beds that are on 30 to 44-inch centers. Highest yields of jumbo onions resulted from 8-12 onions per bed foot, and highest gross returns per acre were at 10-16 onions per bed foot depending on variety. This is equivalent to about 150,000 plants per acre. With precision seeders, 1.25 to 1.5 lb raw seed are used per acre.

In a 1996 plant population study at the OSU Malheur Experiment Station, in which nine rows were planted on wide beds (64-inch beds on 88-inch centers) using three drip irrigation lines per bed, highest gross returns were obtained with plant populations of 125,000/acre. These returns were a function of onion size-grades and recovery after storage. The configuration described requires a number of changes in present production practices that must be carefully examined, cost-verified, and tested by growers before adoption.

Fall bedding: Fall bedding is practiced by some growers in the Treasure Valley area. This is done with the application of certain herbicides (see weed control section). In some cases fertilizer is incorporated in the fall-listed beds with as much as 100 lb N/A added at this time. The addition of N to fall-listed beds is highly discouraged. Even though fall and winter rains in the Treasure Valley area are limited, and leaching over winter may be minimal, leaching of this nitrogen would occur upon commencement of irrigation in the spring before seeded onions are large enough to utilize this amount of nitrogen.

Fall Seeding: 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. Overcrowding 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.

Transplant Production:

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 that 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.

FERTILIZER

The following guides for onion fertility management have been quoted from OSU Fertilizer Guide 65 (for eastern Oregon) and are based on research conducted by faculty of the Crop and Soil Science and Horticulture Departments and the Malheur Experiment Station of Oregon State University. It applies primarily to the Treasure Valley area.

Spring-Seeded Onions

Nitrogen:

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 onion 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 N are used more effectively by the plant than a single preplant application.

Sidedressed N 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. Do not apply N after the end of June.

Excessive rainfall and irrigation that causes N movement beyond the root system can reduce N available to the plants. Additional N may be required during the season under these conditions.

Shallow incorporation of high rates of topdressed or broadcast fertilizer N can seriously reduce onion stands. A high rate of 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.

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

Onion varieties which differ markedly in yield potential can be expected to differ somewhat in their requirements for N. Higher yielding varieties may require slightly more N.

Onion stands (plant populations) of lower than 4 plants per foot of row require less N. Onion stands reduced as a result of poor emergence or inclement weather will require less fertilizer N.

The amount of N fertilizer required depends on the N carry-over from the previous crop, the amount of N in the irrigation water and the amount and type of residue to be plowed under. The amount of residual N in the soil varies considerably. A soil test for nitrate-N (NO₃-N) helps in evaluating the N carry-over from the previous crop in the case of mineral soils with organic matter content below 2%. Soil samples for NO₃-N should be taken from the 0-12 inch soil depth in the spring before application of N fertilizer.

OSU soil test results for N are reported in ppm. One ppm N in a 12-inch depth of soil equals about 4 lb N/acre. The total NO₃-N soil test values are used to estimate the N fertilizer requirement as indicated in the following table:

    Nitrate-N Soil Test (ppm)               N application
       (in 12 inch depth)                       (lb/A)
                0                                 280
               10                                 240
               20                                 200
               30                                 160
          over 30                           up to 150

These application rates are suggested for silt loam, loam, and clay loam soils. For sandy soils the application rates should be increased by 40 lb/acre.

When straw is incorporated after Sept. 1, increase N fertilizer rate by 30-50 lb/acre.

For information on the use of Nitrogen liquid fertilizer formulations for weed control, see the file Nitrogen Fertilizer Solutions Providing Ancillary Weed Control in Alliums.

Phosphorus (P)

Onions require relatively high levels of available P. The P soil test is based on P present in the surface foot of soil.

P should be banded. The fertilizer band should be located 2" to the side and 2" below the seed.

      If the soil test*                Apply this amount of
     for P reads (ppm):               phosphate (P2O5) (lb/A):
          0- 5                                170-200
          5-10                                140-170
         10-15                                110-140
         15-20                                 80-110
       Over 20                                 50- 80

Potassium (K)

Onions require medium levels of available soil potassium. The K soil test is based on K present in the surface foot of soil.

      If the soil test*                     Apply this amount of
     for K reads (ppm):                      potash (K2O) (lb/A)
          0-100                                     150-200
        100-200                                     100-150
        200-350                                      50-100
       Over 350                                       0- 50

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. Sulfur 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 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.

If S levels in the soil are not known, apply 40-60 lb S/A. Caution, when mild-flavored onions are desired, reduce sulfur applications to the minimum rate recommended for proper plant growth since sulfur applications have been shown to increase onion pungency. Onion pungency is also strongly dependent on the variety being used.

Elemental S is a strong acidifier. S in the sulfate form can be applied at planting time.

Zinc (Zn):

An application of Zn is suggested when the Zn soil test value is below 0.8 ppm.

Where Zn is required, either 10 lb/A of Zn should be broadcast and worked into the soil prior to planting or 3-4 lb/A of Zn should be banded with the fertilizer at planting time. An application of 10 lb Zn/A should supply Zn needs for 2 or 3 years.

To correct Zn deficiency during the growing season, thoroughly wet plants with a solution containing 1 lb Zn in 50-100 gallons of water.

Banding an acidifying fertilizer such as ammonium sulfate can reduce Zn deficiency. Zn deficiency is most likely to occur on calcareous soils where land leveling has exposed a calcareous subsoil.

Other Nutrients:

Responses of onions to other nutrients and lime have not been observed on mineral soils in eastern Oregon.

Fall-seeded overwintering onions:

Use only 30-50 lb N/A at time of planting. Apply all additional N in the spring in March, April and May. Be careful to minimize growth in the fall (by limiting N and water applications) so that plants go into the winter about 1/4 inch in diameter. Otherwise excessive bolting will occur in the spring.

Sidedressed N applications or N applied in irrigation water can be an effective means of providing supplemental N to the crop during the season. DO NOT use aqua or anhydrous ammonia in sprinkler irrigation.

For phosphorus, potassium and other nutrients, follow guidelines above.

IRRIGATION

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 N 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 the Treasure Valley area, 30-35 inches may be needed depending on seasonal variation, variety, planting date and location. Approximate summer irrigation needs for the Hermiston area are: 3.5 inches in May, 5.0 in June, 7.5 in July, and 7.0 in August. Watering should be terminated after the bulbs have reached full size, and tops have begun to fall.

Onions are often grown with furrow irrigation in eastern Oregon. Water soluble polyacrylamide (PAM) is useful for flocculating soil particles in irrigation furrows and reducing erosion of soil from the furrow. In addition, research at the Oregon State University Malheur Experiment Station indicates that the application of 800-1000 lb/acre straw mulch to irrigation furrows reduces soil erosion and improves water penetration and irrigation efficiency.

Research at the Malheur Station also indicates that drip irrigation can be used to advantage when onions are grown on wide beds with multiple rows per drip line. Research completed in 1996 indicates that three onion rows may be used per drip line. Nine lines were planted on each wide bed (see Seeding and Transplanting section, above).

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 overwinter successfully. Onion plants should be about 1/4 inch in diameter as they go into the winter and go dormant. In spring and early summer, irrigate as necessary to maintain vigorous growth. Irrigation is generally terminated in June, 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.

HARVESTING, HANDLING, CURING, AND STORAGE

The onion harvest season ranges from mid August to the end of October in eastern Oregon's Treasure Valley and the lower Columbia Basin with the prime harvest being from about the first of September to the 10th of October.

In eastern Oregon's Treasure Valley, yields of sweet Spanish onions average approximately 27 tons/acre with good yields about 30 tons/acre. Excellent field yields are 40 to 45 tons/acre.

For more information on Harvesting, Topping, Storage, Packaging, Shipping, and the Translucent Scale disorder, see the file Dry Bulb Onions -- Western Oregon.

PRODUCTION COSTS

Eastern Oregon production costs for 1990 computed by L. Jensen and G. Schneider, Malheur County Extension are $2889 for total cost per acre. These are composed of $2547 cash costs and $342 non-cash costs and result in a $6.15 per cwt net packout. Costs are based on:

1. 470 cwt/acre net packout.

2. owner-operator labor @ $8.50/hr.

3. Regular hired labor @ $7.00/hr.

4. Temporary hired labor @ $5.75/hr.

5. Land charge based on $2500 an acre land value charged @10% interest (50% equity) plus $25 and acre property taxes.

6. Management costs estimated at 4% of all cash costs.

7. General overhead estimated at 3% of all cash costs.