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GREEN-Seeds.com
vegetables & herbs
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FARMER'S BOOKSHELF An information system of crops in Hawaii
Department of Horticulture
University of Hawaii at Manoa
ONION 1
Climate
 Day Length Requirements
of Bulb Onions and Background Information on Green Onions
In Hawaii, special attention
needs to be placed on day-length when growing bulb onions. Bulb onions differ
as to the photoperiod (length of day) required to produce bulbing at favorable
temperatures (60 - 80 degrees F.). Our longest day never exceeds 13 .83
hours (13 hours, 50 minutes) and varieties which require photoperiods of
13.5 or more hours for bulbing should not be planted. Ripening of bulb onions
(drying of the neck and tops falling over) generally requires just as long
days as bulbing or longe r and higher temperatures (70 - 80 degrees F.)
together with dryness of both soil and atmosphere.
Bulb onions grows best
under cool conditions (50 - 70 degrees F.) with plenty of moisture during
the early stages. The best adapted varieties for Hawaii are those that bulb
at 12 - 13 hours daylength and ripen at 13 - 13.5 hours daylength.
The following paragraphs
are for background information and may provide some appreciation for the
different classes of bulbing type onions.
Short day onions - have 12 - 13 hours daylength threshold for bulbing.
Also termed European types are basically mild fleshed, soft and unsuitable
for storage. Generally grown south of the 35 degree latitude.
Intermediate day onions - have 13.5 to 14 hours threshold for bulbing.
Like the short day onions, it is relatively soft fleshed and used for
the fresh markets and grown between 32 and 38 degrees latitude.
Long day onions - have 14.5 plus hours daylength threshold for bulbing.
Also termed American types, are very pungent, hard and stores well.
Long day onions, if grown in the southern states would not be exposed
to the threshold daylength required for bulbing, and only green onions
would be produced. On the other hand, short day onions grown in the
northern states bulb very quickly and become little more than sets in
size (could be marketed as "pearl onions").
For more exact groupings
of bulb onion daylengths it may be convenient to adopt Magruder and Allard's
1937 groupings of varieties into six groups based on minimum daylength required
to produce 100% normal bulbs under optimum temperatures.
GROUPINGS
OF BULB ONIONS ACCORDING TO MINIMUM DAYLENGTH REQUIRED FOR
100% NORMAL BULB FORMATION AT 60 - 80 DEGREES F. 
| TABLE 1 |
Minimum
Group DaylengthVarieties Required |
Yellow Granex Hybrid, Yellow Granex 33, Grano
1015 Y, Early Texas Grano, Awaiha, Red Creole,
I 12 HoursWhite Creole, Yellow Bermuda, Calred |
Early Harvest, San Joaquin, Pronto S, Ebenezer,
II 13 HoursCalifornia Early Red, Yellow Danvers Flat |
Early Yellow Globe Mountain Danvers, Ohio Yellow
Globe, Australian Brown, White Portugal,
III 13.5 HoursSouthport Yellow Globe, Sweet Spanish No. 1 |
Red Wethersfield, Southport Red Globe, Italian
Red, Flat Madera, Round Madera, Bloodred Flat
IV 14 HoursDutch No. 2 |
| V 14.25 Hours Yellow Globe Danvers |
Yellow Flat Dutch No. 1 and No. 2, Sweet Spanish
No. 2, Yellow Flat Giant, Yellow Flat
VI 14.9+ HoursDutch No. 1, Zittau Yellow & Red, Wolska |
Table
2 below from Nakagawa, 1957, illustrates the daylength periods including
civil twilight for Hawaii, latitude 21 degrees N.
TABLE 2. DAYLENGTH PERIODS FOR HAWAII LATITUDE 21 DEGREES NORTH.
Shortest
Day
11 hours, 16 minutes
Longest Day
13 hours, 50 minutes
11.27 - 12 hour days
January 1 - February 24 (55 days)
12 - 13 hour days
February 24 - April 15 (50 days)
13 - 13.5 hour days
April 16 - May 13 (28 days)
13.5 - 13.83 - 13.5 hr days May 14 - August 1 (80 days)
13.5 - 13 hour days
August 2 - August 31 (30 days)
13 - 12 hour days
September 1 - October 18 (48 days)
12 - 11.27 hour days
October 19 - December 31 (75 days)
Early maturity seems
to depend upon the ability of the plant to start bulb formation at short
daylength and to proceed very rapidly with the process after the minimum
period for bulbing is reached. To assure a good crop, seeds need to be planted
at least 50 days before the minimum daylength for bulbing. The corresponding
period for transplants is 30 days.
True pearl onions are
Allium ampeloprasum because they form just one storage leaf. However,
in practice, short-day onions grown in northern latitudes will develop pearl-size
bulbs and be marketed as such.
Green onions;
shallots, scallions, multiplier and non-multiplier types of bunching onions
are used in the immature stage. Non-multiplier green onions generally are
bulbing type, white cultivars harvested at the miniature bulb stage. Scallions
are white cultivars of Allium cepa that do not form bulbs. Some multiplier
green onions are cultivars of Allium cepa of Group Aggregatum with
white flesh and yellow or brown scales. These are distinguishable from the
shallot by its red scales and supposedly delicate flavor. The shallot can
be used both in the immature stage and as a dry bulb. A cross between the
shallot and a multiplier type green onion of Allium fistulosum gave
rise to the "Beltsville Bunching" green onion.
Localities
for Bulb Onion Production
Group I bulb onions
can be grown almost anywhere in the state where rainfall is very low during
the maturing, ripening and harvesting phase of the production cycle. There
is no one place with the ideal natural climatic requirements in the state.
Ideally , bulb onions prefer moist conditions and about 45 - 60 degrees
F. during the growing phase and 60 - 80 degrees F. for the bulbing and ripening
phase with dry conditions during the ripening phase.
Most of the bulb onions
are grown in Kula on the island of Maui, it has good growing conditions,
60 - 65 degrees F. with sufficient rainfall but may lack dryness and higher
temperatures during the maturing and ripening phases (April - August). Onions
gro wn at the higher elevations can be properly cured by harvesting and
transporting it to some hot dry locality for about a week. The leaves and
roots should be untrimmed. To prevent sunburning, the onions should be set
out to cure with overlapping tops.
Localities that approach
the ideal for bulb onion production are the Kualapuu and Maunaloa districts
on Molokai and Lanai. Onions grown at lower elevations are generally smaller,
more pungent and have a better shelf-life than those grown in higher elevat
ions because of the warmer growing conditions during bulbing and ripening.
Culture
Soil Preparation
Fertile, well-drained
soil which can retain some moisture and is reasonably free from weed seeds
is ideal for onion production. The soil should be plowed to a depth of at
least ten inches, preplant fertilizers and amendments incorporated, harrowed
or rot ovated, leveled and left in good tilth.
Irrigation
The shallow root system
requires frequent irrigations. Plantings in Hawaii are almost all sprinkler
irrigated. A general guideline to keep in mind is that after planting, the
soil should not be allowed to dry out below one inch from the surface. Onions
at the bulbing stage utilize substantial amounts of water. If the onion
is set back by lack of water during its growth, the new growth may, upon
irrigation cause splits or double bulbs. Lack of water also causes slowing
down of growth and increases the amount of bloom on the leaves, giving the
plants a greyer or bluer-green color. Over irrigation would cause the leaves
to take on a yellowish-green color and results in reduced yields and bulbs
with very poor shelf life.
When the plants/bulbs
start to mature, irrigation should be reduced and when the plants start
to ripen (tops topple over at the neck), irrigation should be discontinued
and the soil allowed to dry out. If moisture remains in the ground, adventitious
roots may regenerate from the stem and this may complicate the curing process
of the mature bulbs.
Weed Control
Onions are not good
competitors with weeds. Early cultivation may be employed for weed control,
but once direct seeded onions reach the four or five-leaf stage, fields
should not be cultivated because root damage would be unavoidable.
Common preemergent herbicides
used are DCPA or Goal and postemergent Poast or a 3 - 5% sulfuric acid solution
may be tried. It is important to control weeds as soon as possible.
Adapted Cultivars
Bulb Type -Sea level
to 4,000 feet elevation
Short daylength type - Yellow Granex Hybrid, Yellow Granex 33, Grano
1015Y, Early Texas Grano 502, Awahia, Red Creole (last two pungent types)
Intermediate daylength type - Early Harvest, San Joaquin, Pronto S (mildly
pungent types)
Green Type - Sea level
to 4,000 feet elevation
Local multiplier types - medium leaved types with varying heights between
18 - 30 inches and varying shades of green to blue-green. Normally does
not flower and seed, basically propagated by divisions.
Non-multiplier types - medium leaved types also with varying heights
between 14 - 24 inches and varying shades of green. Normally propagated
from seeds "Koba Strain", "Evergreen Bunching", "Beltsville Bunching".
Shallots - pink to purplish scales on bulblets, known for its delicate
flavor over the white scale types. "Hawaiian Onion". (Akakai)
Futo-negi - large leaved types, (generally greater than 3/4" cylindrical
leaf diameter). "Kyoto Market", "Common Bunching".
Planting Schedule
Bulb Type
Short day onions - September 1 through March 15 (Group I type)
Medium-short day onion - March 15 through May 31 (Group II type)
Medium, medium-long and long day onions will not bulb in Hawaii under
our natural daylength conditions. (Groups III, IV, V and VI types).
Fertilizer
Optimum
pH
6.0
- 6.8. Below 6.0 may cause trace element deficiencies.
Amount
N, P, K and Others Taken up by Average Bulb Onion Crop (lbs./acre)
N = 61
|
P = 12
|
K = 72
|
Ca = 8.5
|
Mg = 11.2
|
Na = 7.0
|
Zn = 0.0996
|
Mn = 0.3017
|
Fe = 0.8210
|
Cu = 0.0187
|
B = 0.1005
|
Amount N, P, K Taken up by Average Green Onion Crop (lbs./acre)
Soil Amendment and Fertilization
Start with a soil test,
if the soil pH is less than 6.0 or the available calcium is less than 2,000
lbs. per acre, apply and incorporate agricultural lime at the rate of 2,000
pounds per acre (4.5 lbs./100 square feet) about 8 - 12 weeks before planting.
The lime requires some time and moisture to chemically react with acid soil
to raise the pH.
Soils low in phosphorus,
apply and incorporate about 1,000 lbs./acre (2.25 lbs./100 square feet)
of treble super phosphate or its equivalent. It is suggested that the phosphate
be banded 2-1/2 - 3-1/2 inches below and to both sides of the row where
onion s will be planted.
Soils low in magnesium
(less than 500 lbs./acre), apply magnesium sulfate (epsom salt, 9.8% Mg.)
at the rate of 200 lbs. per acre as a soil application at planting.
Soils low in potassium,
apply murate of potash (0-0-61) at the rate of 300 - 350 lbs./acre (0.7
- 0.8 lbs./100 square feet). Apply half (150 - 175 lbs.) at time of planting
together with the epsom salt and or the complete fertilizer which is suggested
be low. The remaining half may be combined with the nitrogen and applied
4 - 6 weeks later as side dress.
For bulb onions, after
adjusting for pH, P, K, Ca and Mg, and for soils within the moderate range
of available nutrients, about 1,500 - 2,000 lbs. of 4-12-8 or 5-10-10 and
about 200 - 400 lbs. of sulfate of ammonia should be sufficient for an acre
of bulb onions. All the complete fertilizer is usually banded preplant below
the row to be planted.
Bulb onion has only
a single primary root, continuous plant growth depends almost entirely upon
the adventitious roots which are continually dying and must be continually
replaced by new adventitious roots arising from the stem (or stem plate).
Most onion roots are in a six inch radius from the stem and therefore are
shallow feeders.
Studies have determined
that it is best to apply banded preplant fertilizers 2-1/2 - 3-1/2 inches
below the seeded or transplanted row for best growth. Several applications
of fertilizers containing nitrogen and possibly potassium should be applied
especially in sandy, well drained soils because earlier applications may
leach from the root zone because of all the watering during the early growth
period. Normally a side dressing of 200 lbs. of sulfate of ammonia (21-0-0-)
per acre when the plants are about 1/4 inch thick at the base is given to
the seeded and set propagated plants. A second application of 200 lbs. of
sulfate of ammonia is given about a month after the first.
For transplanted
crops, one side dressing of 200 lbs. sulfate of ammonia per acre a month
after transplanting may be sufficient. The nitrogen may be injected into
the irrigation water and applied in several applications.
For green onions,
after adjusting for pH, P, K, Ca and Mg and for soils within the moderate
range of available nutrients, about 800 - 1,200 lbs. of 10-10-10 fertilizer
or equivalent should be sufficient for a one acre crop. Green onion roots
are similar to bulb onions and therefore are shallow feeders. Apply half
of the 10-10-10 fertilizer banded 4 - 5 inches below the row to be planted
(transplant divisions 1 - 1-1/2 inches deep) and the remaining half 4 -
5 weeks later side dressed. In sandy areas it may be necessary to supplement
the side dressing with about 200 lbs. of sulfate of ammonia.
Nutrient Deficiency Symptoms of Onion
Nitrogen
leaves light green
older leaves die, showing bleached yellow color
leaves are short and small in diameter
growth stiff and upright
Phosphorus
older leaves wilt
tip die back
green areas are mottled
dead leaves turn black
Potassium
older leaves first show slight yellowing
leaves then wilt and die; appears like crepe paper - dying and drying
begin at tips of older leaves
poor bulb formation
Calcium
tips of younger leaves die back
may show dry or brown tissue in bulb
leaves appear limp
root injury maybe evident
Magnesium
leaves die back from the tips
foliage dies prematurely
growth is slow
Sulfur
Iron
similar to Calcium except no root injury evident
Zinc
tips of leaves die back
slow growth
Copper
bulbs lack solidity
bulb scales are thin and pale colored
leaves are chlorotic
Manganese
growth stunted
poor production
Boron
leaves are deep blue green
youngest leaves become mottled, yellow and green with distorted shrunken
areas
ladderlike cracks appear on upper side of basal leaves
basal leaves become very stiff and brittle
Molybdenum
leaves light green similar to Nitrogen deficiency
die back of older leaves; bleached yellow color
not typical
Chlorine
no description available of deficiency
toxicity indicated by foliage die back and reduction of growth
Other facts of interest
are: insufficient nitrogen will induce early maturity and reduce bulb size.
High nitrogen may increase bulb size and cause large necks and soft bulbs
with poor storage quality. High rates of fertilizers containing ammonium
ion ( NH+4) applied close to the plant should be avoided as ammonia (NH3)
is toxic to onion plants.
Handling
Storage of Bulb Onions
Storage of the types
of bulb onions grown in Hawaii is usually of minimum concern. It is a fact
that the granex, grano, bermuda and early harvest types are not good storage
onions, therefore, the production and marketing are done accordingly to
minimize the necessity for storage. If storage is desired, cure well and
gradually bring temperature down to 32 degrees F with relative humidity
at 65 - 70 percent.
To prevent disease from
spreading in stored onions, a relative humidity of 70 percent must not be
surpassed. Above 70 percent, bulbs tend to root more readily, however, above
70 percent scale color for the yellow and brown types may be enhanced.
Harvesting
and Postharvest Storage of Green Onions
Shallot Types - If harvesting for the small mature bulbs that cluster
at the base, wait at least till leaves start to turn yellowish or may
harvest when the tops eventually dry off. Store like bulb onions. If harvesting
for the young bulbs with gr een leaves, it should be pulled, older leaves
stripped and washed before leaves start to turn yellowish.
Multiplier Types - The locally grown types mature approximately 70 - 85
days after planting. The whole cluster of divisions (usually 6 - 12 divisions)
is pulled washed and discolored/damaged leaves stripped/topped. Many wholesalers
prefer packing in ten pound bundles or boxed.
Non-multiplier Types - These are generally white bulb cultivars harvested
at the immature/miniature bulb stage. The local or Asian non-multiplier
green onions are not of the bulbing type but flowers very readily year
round and need to be harvested before flowering (70 - 90 days). In harvesting,
entire planted areas are pulled, plants and roots washed, discolored/damaged
leaves stripped if necessary. Like the multiplier types, wholesalers prefer
packing in ten pound bundles or boxed. Storage cond itions for green onions
are 32 degrees F and 90 - 95 percent relative humidity.
Planting to Harvest
a. Bulb onions
seed to harvest, 160 - 180 days
sets to harvest, 90 - 120 days
b.
Shallot, multiplier and non-multiplier green onion
seed to harvest, 75 - 120 days
sets/division to harvest, 50 - 75 days
c.
Futo-negi
seed to harvest, 160 - 200 days
Estimated Yield
Bulb Onions - 15,000 lbs. per acre (300 bags)
Non-multiplier green onion - 12,000 lbs. per acre
Multiplier green onions - 10,000 lbs. per acre
Futo-negi - 6,000 - 9,000 lbs. per acre
Harvesting and Curing Bulb Onions
Bulb onions should be
harvested when or after the necks weaken and the tops fall over --- when
elongation of new leaves into the neck stops. However, the nutrients from
the tops continue to increase the weight and dry matter content of the bulbs.
This i s an advantage when growers pull onions and do not trim tops off
immediately.
Curing is the drying
of neck, roots and outer scale tissue. Curing is essential to prevent disease
infection, particularly neck rot.
Generally, growers start
to harvest when 25 - 50 percent of the tops have fallen over. Many growers
pull the crop when the bulbs have reached its maximum size and place it
in windrows and allowed to cure, some growers clip the tops off as they
pull, bag bulbs immediately and cure bulbs in the bags under shelter. If
pulled crop is windrowed, bulbs should be protected from sunscald. The tops
should be clipped to leave a short neck; cutting too close to the bulb is
undesirable as a large open wound does n ot dry as well and decay organisms
may enter. The roots are trimmed close to the stem or base.
The length of curing
depends on temperature, humidity, wind or air movement and neck moisture
at topping. If the onions are mature, humidity low and air movement good,
couple days may be sufficient.
The use of forced air
in artificial drying systems can be advantageous if rapid curing is desirable,
if large bulbs are harvested before natural curing is complete, or if weather
is too damp. If artificial curing system is employed, using 90 to 95 degree
s F (32 to 35 degrees C) air moving through onions at the rate of 1 - 2
cubic feet of air per minute per cubic foot of onions should be sufficient.
If temperatures that high cannot be achieved, higher air-flow rates can
compensate. Relative humidity of 60 - 70 percent is desirable. Lower humidity
allows faster drying but gives poorer scale color and greater loss of outer
scales, whereas higher humidity slows drying and creates a more suitable
environment for disease establishment.
Insects Recorded in Hawaii for Onions
| Acrolepia assectella (Zeller) |
leek moth |
| Acrolepiopsis sapporensis (Matsumura) |
Asiatic onion |
|
leaf miner |
| Atractomorpha sinensis Bolivar |
pinkwinged |
|
grasshopper |
| Hylemya (Delia) platura (Meigen) |
seedcorn |
|
maggot |
| Liriomyza huidobrensis (Blanchard) |
pea leafminer |
| Liriomyza sativae Blanchard |
vegetable leafminer |
| Neotoxoptera formosana (Takahashi) |
onion aphid |
| Spodoptera exigua (Hubner) |
beet armyworm |
| Thrips alliorum (Priesner) |
thrips |
| Thrips tabaci Lindeman |
onion thrips |
Planting Schedule
Bulb Type
Short day onions - September 1 through March 15 (Group I type)
Medium-short day onion - March 15 through May 31 (Group II type)
Medium, medium-long and long day onions will not bulb in Hawaii under
our natural daylength conditions. (Groups III, IV, V and VI types).
Green Type
Local multiplier types - year round
Non-multiplier types - year round. Certain cultivars like "Koba strain"
may be harvested two weeks earlier than when planted during summer as
opposed to winter planting.
Shallots - Best to plant matured (dried) bulblets if available year round.
May plant immature bulblets (divisions) when mature bulblets not available.
It takes two to three weeks longer when immature bulblets are planted
as opposed to mature bulbl ets.
Futo-negi - year round, spring plantings generally matures earlier than
when planted fall or winter.
Propagation Method
Direct seeded or transplanted
for bulb onions, non-multiplier and futo-negi. Planting by sets for local
multiplier green onions, shallots and bulb onions. Planting from divisions
for local multiplier and shallot type green onions.
Bulb
Onion Sets
Bulb onion sets
are small bulbs 1/3 - 1-1/8 inches in diameter. They are generally produced
the previous season by seeding thickly or growing under conditions favoring
rapid bulbing. The small bulbs is the result of planting seeds densely (2
- 3 oz. seed per square yard). The ideal size of sets are between 5/8 -
3/4 inches in diameter.
These sets are
harvested and cured in dry well-ventilated areas until ready to be planted.
Advantage of planting sets --- matures about two months earlier than from
seeds. Disadvantage of planting sets --- high labor for planting and sets
are costly.
Bulb Onion Transplants
To grow enough bulb
onion seedlings to transplant into one acre, sow two pounds of seeds in
approximately 4,500 square feet of prepared seed bed. Seeds may be sown
by broadcasting or planted in rows 6 - 8 inches apart and 1/4 - 3/4 inches
deep. Transpla nts are usually ready 8 - 10 weeks after seeding, when the
majority of the seedling necks are pencil size (1/4 - 5/16 inch) in diameter,
7 - 13 inches tall and have 4 - 5 leaves. Seedlings are hardened about 1
- 1-1/2 weeks before transplanting by reduci ng irrigation supply.
Spacing
|
Onion
Type
|
"
Between Rows
|
"
Between Plants
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Bulb Onion
|
15 - 24
|
3 - 6
|
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Non-multiplying
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Green Onion
|
12 - 20
|
1/3 - 3/4
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Multiplying
Green Onion
|
12 - 20
|
8 - 10
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Shallots
|
12 - 16
|
8 - 12
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Futo-Negi
|
15 - 30
|
3 - 4
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Amount of Seed/Planting
Material
Bulb Onions - 4 - 5 lbs. seed per acre direct seeded; 3/4 oz. per 100
feet row. Estimate two lbs. of seed to produce transplants or sets for
one acre. Also estimate about 400 - 500 lbs. of sets to plant one acre
(1/3 - 1/2 inch diameter).
Green Onions - 12 - 18 lbs. seed per acre direct seeded. 3 - 4 oz. per
100 feet row. Estimate about 1,000 lbs. dried sets/divisions per acre
(usually 2 - 3 planted per hole/hill).
Seeding Depth
1/4
- 3/4 inch, heavier soil require less depth for seeds and sets. Divisions
are usually planted 1 - 1-1/2 inches deep.
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Green - Seeds Co., Ltd.
81/10B Ho Van Hue Street, Phu Nhuan District, Ward 9, Ho Chi Minh City, Vietnam
Tel: +84 (8) 847 6901 - Fax: +84 (8) 844 1392 - Email: info@green-seeds.com
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