7242

FRESH-MARKET BULB ONION

PRODUCTION IN CALIFORNIA

RICHARD SMITH, UC Cooperative Extension Vegetable Crop

Advisor, Monterey County; ANDRE BISCARO, UC Cooperative

Extension Agriculture and Environmental Issues Advisor, Los Angeles

County; MICHAEL CAHN, UC Cooperative Extension Irrigation and

Water Resources Advisor, Monterey County; OLEG DAUGOVISH, UC

Cooperative Extension Vegetable Crop Advisor, Ventura County; ERIC

NATWICK, UC Cooperative Extension Entomology Advisor, Imperial

County; JOE NUNEZ, UC Cooperative Extension Vegetable Crops Advisor,

Kern County: ETA TAKELE, UC Cooperative Extension Farm Management

Advisor and County Director, Riverside County; AND TOM TURINI, UC

Cooperative Extension Vegetable Crops Advisor, Fresno County

PRODUCTION AREAS AND SEASONS

Fresh-market bulb onions (Allium cepa L.) are

produced throughout California. The main

production areas are the low desert (Imperial and

Riverside Counties), the San Joaquin Valley (Fresno,

Kern, and San Joaquin Counties), the southern and

central coast (Monterey, San Benito, Santa Clara,

and Ventura Counties), and the high desert (eastern

Los Angeles County). Bulb onions are planted from

September through May. Harvest begins in April

or May and is usually completed in September.

Fresh bulbs and lightly processed bulbs (fresh onion

ring, whole-peeled, etc.) make up approximately 45

percent of the total bulb onion acreage in California,

which ranks among the top fresh-bulb-producing

states in the United States. Approximately 20 percent

of California’s fresh-market onions are stored for 3 to

6 months.

CLIMATIC REQUIREMENTS

Onions are cool-season biennial plants (requiring two

seasons to complete the cycle from seed to seed) that

are commercially grown as an annual crop. Bulbing

is highly dependent on day length. Varieties adapted

to California will initiate bulbing at day lengths of

approximately 12 to 15 hours. Dozens of varieties

are grown in California due to the great difference in

latitude between the Mexican border (32ºN) and the

Oregon border (42ºN), and because several market

classes are needed.

Short-day onions are planted in the fall and bulb

with 12 to 14 hours of daylight. They are adapted

to the desert regions of Southern California. Using

transplants, these varieties can be grown in the lower

San Joaquin Valley and south coast, with a maximum

latitude of approximately 36ºN. Intermediate-day

varieties are also planted in the fall. They are grown

throughout the San Joaquin and Central Coast valleys

and can be grown in the lower Sacramento Valley

and central Sierra foothills. Day length requirements

are approximately 13 to 15 hours, with optimal

adaptation in latitudes of 35º to 38ºN. As with the

short-day types, varieties with a continuum of

varying day lengths have been developed.

Onion growth is very dependent on temperature.

The minimum temperature for emergence is higher

than for most other cool-season vegetables at 55ºF

(12.8ºC) for 70 percent emergence in up to 2 weeks.

In addition, early growth rate is slow compared with

other cool-season crops. Optimal leaf growth rate

occurs at 68º to 77ºF (20º to 25ºC). However, total

plant growth rate depends on the amount of light

intercepted. Maximum light interception occurs with

a leaf area index of approximately 8 or higher.

Bolting is primarily driven by temperature when

plants with a leaf base greater than approximately 3⁄8 inch

(9.5 mm) in diameter are subjected to temperatures of 45º

to 50ºF (7.2º to 10ºC). A combination of factors, including

planting date, variety, plant size, and temperature, as

well as timing and duration of temperature, determine

whether and when bolting occurs.

Vegetable

Production Series

vric.ucdavis.edu

UC Vegetable Research

& Information Center

University of California

Agriculture and Natural Resources

http://anrcatalog.ucdavis.edu • Publication 7242

FRESH-MARKET BULB ACREAGE

AND VALUE

Year Acreage

Average yield

(tons/acre)

Gross

value/acre

2009 22,766 27.22 $6,821

2008 22,512 24.05 $5,298

2007 25,318 25.39 $6,289

2006 25,981 23.05 $7,114

Source: Compiled from agricultural commissioner reports

for Fresno, Imperial, Kern, Monterey, San Benito, San

Joaquin and Santa Clara Counties.

http://vric.ucdavis.edu
http://anrcatalog.ucdavis.edu

VARIETIES

Few public onion breeding programs exist in the

United States (none in California), but a large number

of private seed companies are involved in onion

variety development. Because of this, a large number

of varieties, most of which are hybrids, are available

for any given area.

Onion varieties are generally classified according

to day length (short, intermediate, and long),

market use (green, fresh bulb, dehydrator bulb), and

bulb color (within the fresh market class: yellow,

brown, red, and white). A continuum of day length

varieties has been developed, sometimes making

the distinction between the different day length

classes difficult. Adjustments can also be achieved by

varying planting dates and using transplants instead

of direct seeding.

Short-day fresh-market bulbs are usually Granex,

Grano, or combination Granex-Grano types. Granex

varieties are flat to thick-flat in shape; Grano types

are large globe or top-shaped. Some varieties that are

grown include Texas Early Grano 502, Don Victor,

Matahari, Red Rock, Red Grano, Kristal, Solano and

many others. Most fresh-market short-day varieties

are relatively low in soluble solids (5 to 7%) and

pyruvic acid (2 to 6 moles/kg), with a relatively high

sugar to pyruvate ratio. This makes them sweeter

than other varieties. Other factors, such as sulfur

content in the soil and temperature, influence the

level of sweetness as well.

Short-day onions are usually preferred by the

onion ring industry because varieties have been

developed with a single center (growing point)

and numerous, thick fleshy rings; however, more

intermediate and long-day varieties are also being

developed with a high percentage of single centers.

Short-day varieties are considered to be a nonstorage

or short-storage crop because they are soft, easily

bruised or cut, and have thick necks and thin outer

scales. They are harvested slightly immature to meet

market demands. When allowed to mature fully, and

if harvested carefully, short-day varieties maintain

good quality for several weeks, and even months.

Intermediate-day varieties are frequently selections

or derivatives from Sweet Spanish types (which may

also be long-day). These are usually globe-shaped but

may be flattened and must have some resistance to

bolting since they grow through winter. They include

Stockton Red Globe, Early Red Burger, Cimarron,

Caballero, Red River, Rumba and many others.

Intermediate-day varieties are also intermediate in

solids, pyruvic acid (pungency), and storability.

Few long-day varieties are produced for the fresh

market in California. This is because onion growers in

the northern region of the United States can produce

and store fresh bulbs at a much lower cost. However,

the acreage of long-day storage varieties grown in

the San Joaquin Valley has increased in recent years,

and the acreage in the Antelope Valley (in the higher

elevations of Los Angeles County) has remained

steady. The potential for production and storage in

the northern mountain valleys of California has not

been fully developed. Numerous Sweet Spanish

and Fiesta types (Sweet Spanish by Yellow Globe

cross) have been adapted for California production

between 37ºN and 42ºN latitude. Color types of red,

white, and yellow are grown. Some long-day bulb

varieties with intermediate to good storage periods

are Frontier, Tamara, Sedona, Norstar, Vaquero,

Granero, Red Wing, Mercury and many others. Long-

day varieties are harvested in late summer and early

fall. Because half of all onions grown in the United

States are harvested during this same period, long-

day varieties must withstand some storage in order

to be marketable. In general, successful varieties have

very firm bulbs, more and tighter outer scales, thinner

necks, solids content of 8 to 12 percent, and pyruvic

acid levels of 10 to 20 moles/kg.

PLANTING

Most commercial acreage is direct seeded, but

transplants are used in some fall-planted fields for an

earlier harvest of short-day (particularly in regions

north of the southern deserts) and intermediate-day

(in the San Joaquin Valley) varieties, and to achieve

uniform, jumbo-sized bulbs.

Onions are most commonly grown in multiple

rows on raised beds 40 to 42 inches (102 to 107 cm)

wide, but some production systems use 36-inch (91-

cm) beds or wide beds of 60 to 80 inches (152 to 203

cm). Distribution of rows across the bed depends

on the irrigation method and planter. With drip or

sprinkler irrigation, rows are spaced equidistantly

across the bed at approximately 4-inch (10-cm)

intervals. When furrow irrigation is used, the center

of the bed is left empty for salt accumulation, with 2

or 3 rows planted on either side of the center of the

bed. Most seeding is done with precision air planters,

although some mechanical plate planters are used,

especially in the southern desert.

Onions are planted to a stand at 3 to 4 inch (7.6

to 10.2 cm) spacing between seeds using 180,000 to

209,000 seeds/acre (445,000 to 516,000 seeds/ha). Good

singulation is critical to avoid doubles. Both pelleted

and raw seed are used; primed seed is used to facilitate

rapid emergence in cool soils. The shallow planting

of onion seed—at approximately 1⁄2 inch (12.5 mm)—

requires a soil surface that is well prepared and must be

kept moist until germination. More shallow plantings

may increase the tendency for flatter bulbs, and deeper

planting may result in lower germination and rates of

emergence, as well as deeper-shaped bulbs.

2 • Fresh-Market Bulb Onion Production in California

Onion seed is susceptible to loss of vigor from

high temperature and humidity; germination tests

are critical. The storage life of opened seed containers

can be shortened very rapidly if not immediately

placed in an environment low in temperature and

humidity. Seeding rates should be adjusted for lower

germination. Fresh-market onion seed is planted 2 to

3 pounds per acre (2.4 to 3.6 kg/ha).

SOILS

Onions will grow in a wide range of soil types:

sand, loam, clay, and organic-peat. Onions are

shallow rooted and need a friable soil that retains

moisture well. Excessively dense clay soils interfere

with root growth and frequently result in a serious

clod problem at harvest. Sandy soils require more

frequent irrigation. Seed germination and seedling

establishment require a seedbed that is uniform, clod-

free, firm, and several inches deep. Compared with

planting on flat or small ridges, raised beds provide

better drainage and an area for salt accumulation

away from the root zone.

Onions are sensitive to salinity, primarily at the

stages of germination and emergence. Once plants

are established, higher levels of salinity are tolerated.

Yield reductions may occur in soils with an electrical

conductivity greater than 1.2 dS/m (mmho/cm) or

irrigated with water with an electrical conductivity

greater than 0.8 dS/m. A 50 percent yield reduction

may occur at soil electrical conductivity levels

of 4 to 5 dS/m. Onions are sensitive to boron in

concentrations greater than 0.5 mg/L in irrigation

water.

IRRIGATION

Overhead sprinklers are typically used for

germination and stand establishment. Onion seed

must not dry out during germination, and the

soil surface should not be allowed to crust during

seedling emergence, which may last 10 to 20 days

following the initial irrigation. After emergence, the

crop is usually irrigated by furrow or drip. Drip

tape is typically placed on the bed surface when the

plants are 6 to 8 inches in height (2 to 2.5 months

after planting). Some growers bury the tape an inch

below the surface before planting so the crop can be

irrigated earlier with the drip system than when the

tape is placed on the bed surface.

Onions require frequent irrigation throughout the

season for several reasons. Ninety percent of the

roots are in the upper 12 inches (30 cm) of the soil,

so very little water is extracted from a soil depth of

more than 24 inches (61 cm). Moisture is required

near the surface of the soil to stimulate new root

growth because onion roots are mostly nonbranching,

and all roots originate at the stem, or basal plate,

of the plant. Rates of transpiration, photosynthesis,

and growth are lowered by even mild water stress.

Periods of water stress may reduce bulb size and

may also cause doubles and splitting; water-stressed

plants are usually higher in pungency.

The amount and frequency of irrigations depend

on the method, soil type, weather conditions, and

development stage of the crop. After emergence,

when plants are small, water needs are low, so the

irrigation interval with overhead sprinklers may

range between 7 to 10 days. As plants begin to bulb

water use increases and irrigations become more

frequent to maintain uniform soil moisture in the

beds. Under drip irrigation, onions may be irrigated

every 2 to 4 days, and under furrow irrigation,

about every 5 to 6 days. Water is cut off when the

bulbs have reached maturity. Usually a minimum

of 1 month without water is needed for the tops to

dry down before harvest. An onion crop typically

uses 20 to 30 inches (50 to 76 cm) of water to meet

evapotranspiration requirements. Most growers

apply 30 to 48 inches (76 to 122 cm) to achieve

maximum yields.

The combination of soil moisture monitoring and

weather-based irrigation scheduling can be used

to determine water needs of onions. For optimal

plant growth on most soil types, irrigate to maintain

the soil water tension less than 30 centibars (kPa)

in the upper 12 inches (30 cm) of the soil. Keeping

soil moisture too high (
to reduce the storage life of bulbs. Water use is

highest when the crop reaches full canopy cover.

Water extraction of onions can be estimated using

reference evapotranspiration data adjusted with a

crop coefficient, which is closely related to the

percentage of ground covered by the leaf canopy.

At a maximum canopy cover of 85 percent, the crop

coefficient is nearly 1.0. Because evaporation represents

a majority of the water loss during the early stages of

growth, a crop coefficient between 0.3 and 0.5 should

be used for overhead sprinklers, depending on the

irrigation frequency, until the leaves shade more than

30 percent of the ground. Maximum onion yields are

reached with applied water from 100 to 150 percent

of crop ET. The California Irrigation Management

Information System (CIMIS) coordinated by the

California Department of Water Resources provides

daily estimates of reference evapotranspiration

for most production regions of California (see
http://wwwcimis.water.ca.gov).

FERTILIZATION

Because onions are shallow rooted and usually

planted in cool soils, they are responsive to

fertilization. Soil analyses are the best indicators

3 • Fresh-Market Bulb Onion Production in California

http://wwwcimis.water.ca.gov

for phosphorus (P) and potassium (K). Soils with

bicarbonate extractable phosphorus greater than 30

ppm should require a preplant application of no

more than 50 pounds of P2O5 per acre, while soils at

less than 10 ppm phosphorus may require up to 200

pounds of P2O5 per acre. With adequate preplant

application, in-season phosphorus application

is seldom warranted. Soils in excess of 150 ppm

ammonium-acetate-exchangeable potassium are

unlikely to respond to potassium fertilization, while

up to 150 pounds of K2O per acre may be required

to ensure adequate potassium supply in soils at less

than 100 ppm potassium. Most California soils have

adequate availability of all micronutrients; where

micronutrient deficiency occurs, zinc is often the most

limiting nutrient.

Nitrogen (N) requirements vary depending on

many factors, including the nitrogen-supplying

capacity of the soil, irrigation efficiency, and the

amount of leaching from rainfall. If irrigation is

efficiently managed a seasonal total of 250 pounds

of nitrogen per acre should be adequate to maximize

yield under most field conditions, less if the field has

significant residual nitrate-nitrogen concentration.

Higher seasonal nitrogen rates may be justified in

fields receiving significant rainfall, or in which high

irrigation efficiency cannot be achieved. Nitrogen

fertilizer should be delivered in multiple applications

through the season, with no more than 20 to 30

percent of the seasonal total applied preplant.

INTEGRATED PEST MANAGEMENT

UC IPM Pest Management Guidelines for onions are

available for weed, insect, disease, and nematode

pests. Sanitation, crop rotation, resistant varieties, and

frequent monitoring are essential in the prevention

and control of onion pests. Visit the UC IPM Web site

at http://www.ipm.ucdavis.edu.

Weed management. Onions compete poorly with

weeds because they initially grow slowly and the

crop canopy does not provide complete ground

cover. In addition, the long growing season allows

for successive flushes of winter and summer weeds.

There are a number of herbicides registered for use

on onions; weed control in conventionally produced

onions consists of applying preemergence followed

by postemergence herbicides. Organic producers

rely on cultural practices such as rotating onion

plantings into fields with low weed pressure and the

use of preirrigation followed by shallow tillage to

kill an initial flush of weeds. Under both production

systems, hand-weeding is required to control weeds

later in the growth cycle. However, the cost of

hand-weeding can vary widely, depending on the

effectiveness of early season weed control efforts.

Insect identification and management. Thrips

(western flower and onion) and maggots (seed corn and

onion) are the most frequent serious insect problems for

onions, but bulb mites, wheat curl mite, leafminers, and

armyworms are occasionally serious pests.

Bulb mite (Rhizoglyphus spp.) is most damaging

when plant growth is slowed by cool, wet weather.

The mites cut off the radicle of germinating seed

before the plant becomes established or penetrate the

outer layer of tissue and allowing rotting organisms

to gain entry to bulbs. Decaying cole crops or

other crop residues cause a rapid buildup of mites.

Allowing complete decomposition of organic matter

causes the mite population to crash, which is the best

management option.

Maggots (Delia spp.), similar to bulb mites, are

favored by cool, moist soils with high levels of

nondecomposed organic matter. Seed corn maggot

larvae attack germinating seedlings, feeding on

the developing roots and epicotyl. Onion maggots

also damage seedlings but continue feeding on the

expanding bulb, causing bulbs to be lost at harvest or

rot during storage. Allow complete decomposition of

organic matter prior to planting or transplanting, or if

this is not possible, a soil applied insecticide may be

used.

Thrips (Thrips and Frankliniella spp.) can reduce

yield and storage quality of onions. Thrips are most

damaging when they feed during the early bulbing

stage of plant development. Sprinkler irrigation can

help reduce thrips numbers, but for high populations

foliar insecticide applications are required to prevent

injury. Onion thrips (Thrips tabaci) adults can transmit

Iris yellow spot virus (IYSV), but the virus is acquired

during the larval stage. Insecticide treatments do not

completely prevent IYSV transmission.

Disease identification and management. Downy

mildew (Peronospora destructor