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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.
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)
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.
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.
Bulb Type -Sea level to 4,000 feet elevation
Bulb Type
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)
| N = 30 | P = 5 | K = 40 |
|---|
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
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
a. 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.
| 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 |
Bulb Type
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 |
|---|---|---|
| Bulb Onion | 15 - 24 | 3 - 6 |
| Non-multiplying | ||
| Green Onion | 12 - 20 | 1/3 - 3/4 |
| Multiplying Green Onion | 12 - 20 | 8 - 10 |
| Shallots | 12 - 16 | 8 - 12 |
| Futo-Negi | 15 - 30 | 3 - 4 |
Amount of Seed/Planting Material
1/4 - 3/4 inch, heavier soil require less depth for seeds and sets. Divisions are usually planted 1 - 1-1/2 inches deep.
