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.
VARIETIES
Willamette Valley (spring-seeded):
Muck soils, yellow: Australian Brown, Capable, Columbia, Cuprum, Mercury, Sabroso, Sentinel, Simcoe,
Spartan Banner 80, Spartan Sleeper, Taurus, Topaz, Valiant.
Mineral soils; yellow:
- Early sales: Aires, Early Pak, Progress, Pronto, Rocket, Simcoe, Taurus, Yula.
- Storage: Aires, Australian Brown, Autumn Splendor, Canada Maple, Candy, Capable, Improved Autumn Spice, Rocket, Sentinel, Simcoe,
Spartan Sleeper, Super Sleeper, Taurus. For trial: First Edition (long storage), Mercury, Pinnacle, Sabroso, Valiant.
Mineral Soil Red: Tango. For trial: Benny's Red, Carmen, Fuego.
Red "Torpedo" types: Red Torpedo.
Mineral Soil White (white types are only marginally adapted to western Oregon):
Blanco Duro, White Delight.
Medford area (spring-seeded):
Yellow: Brahma, Bravado, Magnum, Sweet Amber, Valdez, Vega.
White: Blanco Duro. For trial: Avalanche, Glacier.
Red: Tango. For trial: Benny's Red, Carmen, Torpedo (long, spindle-shaped,
sweet).
Fall seeded overwintering onions (mineral soils only). These onions
are planted in mid-August to mid-September and harvested in June and July:
Yellow: Bison, Buffalo, Hi Ball, Hi Keeper, Imai, Keep Well, Senshyu Yellow Globe, Top Keeper,
Walla Walla (a number of strains exist: Early and
late French, Arbini, Locati and others; none are hybrids).
Red: Red Cross (a large, "Bermuda" or flat onion), Kurenai.
SOIL TEMPERATURE
The optimum temperature range for germination is 48 to 90 F. March and
April plantings are most common in western and central Oregon.
SOILS
In western Oregon, onions are grown on peat soils or silt loams and
sandy loams. Overwintered crops must have soils with good internal drainage.
SEEDING AND TRANSPLANTING
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).
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 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.
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 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.
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 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.
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 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.
FERTILIZER
The following guides for western Oregon onion fertility management have
been quoted from OSU
Fertilizer Guide 67 (for western Oregon muck soils) and are based on muck soil
and mineral soil research conducted by faculty of the Crop and Soil Science and Horticulture
Departments of Oregon State University.
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. Fertilizer application for onions should insure
adequate levels of all nutrients. Optimum fertilization is essential for top quality and yields.
Fertilizer suggestions are based on a 12 to 14-inch row spacing.
Recommended soil sampling procedures should be followed in order to estimate fertilizer needs.
Nitrogen (N):
Spring-seeded Onions on Mineral Soils:
N rate of 20-40 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. Note: 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).
An additional 100-120 lb/A should be applied during the growing season
when onions are 6-8 inches tall.
Fall-seeded Overwintering Onions (mineral soil only):
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.
Nitrogen solutions can also kill weeds in dry bulb onions and in
other Alliums. See the file Nitrogen Fertilizer Solutions Providing
Ancillary Weed Control in Alliums.
Mineral Soil Onions, both spring and fall-planted:
Phosphorus (P):
Phosphorus is essential for vigorous early growth of seedlings. All
phosphorus should be applied
at planting, regardless of whether spring-seeding or fall-seeding. It should
be banded at planting
2 inches to the side and 2 inches below the seed.
If the soil test* Apply this amount of
for P reads (ppm): phosphate (P2O5) (lbs/A):
0- 40 170-200
40-100 140-170
over 100 100-140
*Assumes extraction procedures similar to those used by the OSU Central
Analytical Laboratory. Specific information on soil test procedures is
available from the Dept. of Crop and Soil Science.
Potassium (K):
All K should be applied and worked into the seedbed before planting.
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) (lbs/A)
0-100 150-200
100-200 100-150
200-350 50-100
Over 350 0- 50
*Assumes extraction procedures similar to those used by the OSU Central
Analytical Laboratory. Specific information on soil test procedures is
available from the Dept. of Crop and Soil Science.
Magnesium (Mg):
When the soil test value is below 1.0 meq Mg/100g, broadcast 20 to 25 lb
Mg/A and work into the seedbed before planting. If deficiency symptoms appear, spray with 10 lb
Epsom salts in 100 gal water/A.
Magnesium can also be supplied in dolomite, which is a liming material and
reduces soil acidity to about the same degree as ground limestone. Dolomite should be mixed into the seed
bed in the fall.
Boron (B):
No consistent responses of onions to the application of B have been
observed in western Oregon. Where the soil test for B is below 0.25 ppm growers may wish to make a trial
application of 2 lb B/A. Too much B fertilizer can be highly toxic and suggested rates of B
application should not abe 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 2 ppm broadcast 10 lb Zn/A before planting or
include 3-4 lb Zn/A in the fertilizer band. A broadcast application of 10 lb Zn/A should supply Zn needs
for 2 or 3 years.
Other Nutrients:
Responses of onions to nutrients other than those discussed in this
guide have not been observed in western Oregon.
Lime:
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.
For acid soils low in magnesium (Mg) (less than 1.0 meq Mg/100 g of
soil) 1 ton/A of dolomite lime can be used as a Mg source. Dolomite and ground limestone have about the
same ability to neutralize soil acidity.
FERTILIZER ON MUCK SOILS:
Nitrogen (N):
An N rate of 20-40 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 section on "sulfur" below).
An additional 20-60 lb N/acre should be applied during the
growing season when onions are 6-8 inches tall.
Phosphorus (P):
Phosphorus is essential for vigorous early growth of seedlings. Phosphorus should be
banded at planting 2 inches to the side and 2 inches below the seed.
If the soil test* Apply this amount of
for P reads (ppm): phosphate (P2O5) (lbs/A):
0-40 120-150
40-100 80-120
over 100 60- 80
*Assumes extraction procedures similar to those used by the OSU Central
Analytical Laboratory. Specific information on soil test procedures is
available from the Dept. of Crop and Soil Science.
Potassium (K), Magnesium (Mg), Boron (B), Zinc (Zn):
As above for mineral soils.
Copper (Cu):
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.
Other Nutrients:
Responses of onions to nutrients other than those discussed in this
guide have not been observed in western Oregon.
Lime:
Experimental work on the muck soils of western Oregon has shown that
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.
The liming rate is based on 100-score lime. A lime application is
effective over several years.
If the soil test* Apply this amount
for pH reads: of lime (Tons/A):
below 5.0 4-5
5.0-5.5 2-4
over 5.5 0
*Assumes extraction procedures similar to those used by the OSU Central
Analytical Laboratory. Specific information on soil test procedures is
available from the Dept. of Crop and Soil Science.
Lime should be mixed into the soil in the fall.
For acid soils low in magnesium (less than 1.0 meq Mg/100 g of
soil) 1 Ton/A of dolomite
lime can be used as a Mg source. Dolomite and ground limestone have about the
same ability to neutralize soil acidity.
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.
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 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.
HARVESTING, HANDLING, CURING, AND STORAGE
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.
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 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.
TOPPING
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 (Quoted or modified from USDA Ag. Handbook 66 and other sources)
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.
PACKAGING
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.
SHIPPING
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.
SPROUTING INHIBITION
maleic hydrazide 2 lb ai/A
Royal MH-30
(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
onions.
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