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Onions, Green Bunching
Allium fistulosum and Allium cepa
Last revised February 12, 2010
Note: This file contains only information specific to the production of green bunching onions. For more information on onion culture, see the file Dry Bulb Onions -- Western Oregon.
Bunching onions may be produced from immature, thickly planted white onion varieties of Allium cepa and from Allium fistulosum(commonly known as Japanese bunching types), or, rarely, from an interspecific hybrid of cepa x fistulosum. Allium fistulosumvarieties are often referred to as "Welsh" onion, derived from the German word "welsche", meaning foreign.
Green bunching onions: Photo credit: Alex Stone, Oregon State University
Green bunching onions are known by several names depending on the region of the country. Some of the names used are "scallions," "green onions," and "spring onions." All these terms can be used for immature onions, but in reality, the "green bunching onion" of commerce that one purchases in the store in the Northwest today is most likely a different species from that of the bulb onion. Green onions today are most often Allium fistulosum, which is further classified into four taxonomic groups. For a complete description, refer to "Alliums and Allied Crops" Vol. III by H. Rabinowitch and D. Brewster.
A considerable amount of Allium cepa (the regular bulb onion) is still used for green onion production under certain conditions and in certain areas. Allium cepa can be planted thickly and harvested immature for this purpose.
Allium fistulosum is a perennial widely cultivated throughout the world, from Siberia to tropical Asia. It produces long cylindrical plants. There are dividing and non-dividing types. They are generally non-bulbing; however, some types may develop a slight swelling at the base of the plant. Many varieties are available in catalogs. Some years ago, an interspecific cross between A. fistulosum and A. cepa was made by the USDA and an amphidiploid variety (a tetraploid having a complete set of chromosomes ofA. cepa and A. fistulosum) was produced and called "Beltsville Bunching". It is still used a little today, but the seed is expensive and hard to produce.
One can identify the species by looking carefully at the bottom of the green leaves near where they turn white. If the leaf cross section is "D" shaped (or has a flat side), it is A. cepa. If "O" or round, it is A. fistulosum.
Onions are daylength sensitive. Differences in this sensitivity (as well as stand densities) are used in onion production to produce things like pearl onions or small boiler types. Pearl onions are short day types grown in the northern states (under the long days of summer), for early bulbing and pearl production.
Allium cepa types - for early crop: White Lisbon. For trial: Southport White Globe, White Spanish Bunching, White Knight.
A. cepa x A. fistulosum types: Beltsville Bunching.
A. fistulosum types - non-bulbing; resistant to pink root and botrytis leaf blight; cold hardy; for later crops and for overwintering: Hishiko, Ishikura, Kincho, Tokyo Bunching, Tokyo Long White. For trial: Japanese Bunching (reported tolerant to high temperatures).
Red bunching onion: Santa Claus, an "Ishikura" type, but with rose-red stem that can add color to salads. Red color intensifies with colder weather.
SEEDING AND GERMINATION
Seeding usually takes place from March through mid-August.
Green onion seed numbers approximately 9,500 per ounce. Use high quality fungicide-treated seed. Since onion seed has poor useful longevity (less than 2 years), unless stored under ideal conditions. Germination will occur at temperatures from 46 to 86 F with the optimum at over 60 F.
Seeding rates for bunching onions depends on the spacing between rows. Approximately 10 to 15 lb of seed are used at about 15-inch spacing, but at the closest spacings up to 25 lb of seed per acre may be needed.
The early crop may be clump-seeded, 8-9 seeds/cell, in plug trays and grown to transplant size in plant-houses, greenhouses, or cold frames. When ready the seedlings are planted in clumps in rows that are 12 to 15 inches apart. This practice is not currently being used in Oregon.
Later crops: Seed as soon as the soil can be worked. Four to 7 rows, 12-15 inches apart, are seeded to each set, or bed. Use a scatter shoe to plant a band 2-4 inches wide and seed 35-50 plants per foot of row. Approximately 6-8 lb of seed may be needed per acre for direct-seeding.
A pH of 5.3 to 6.0 is suitable for organic soils but a pH of 6.5 to 7.0 is preferred for mineral soils. When the pH is below the range considered suitable, lime should be applied and incorporated thoroughly to a depth of 6 in. or more. Lime should be applied in the fall or as early in spring as possible.
A late summer green-manure crop of oats, plowed down or worked in, before the ground becomes too wet to work is beneficial. An application of ammonium nitrate when the green-manure crop is being turned in will help break down the material.
A soil test is the most accurate guide to fertilizer requirements. The following recommendations are general guidelines.
Before transplanting or seeding broadcast and incorporate the following:
Nitrogen: 50-70 (N) lb/acre. Add other N as needed to keep tops green and vigorous under different temperature and rainfall conditions. It may be necessary to side-dress the crop with an additional 75-100 lb N/acre.
For Nitrogen liquid fertilizer formulations providing weed control effects in bunching onion and other alliums, see the file Nitrogen Fertilizer Solutions Providing Ancillary Weed Control in Alliums.
Phosphate: 145-155 (P2O5) lb/acre.
Potash: 150-170 (K2O) lb/acre.
Onions are shallow-rooted. Maintain the top foot of soil at 65-70% of field capacity or more. In western Oregon 12-15 inches of water may be necessary for a 60 to 70 day crop.
HARVESTING, HANDLING, AND STORAGE
Yields average 800-1000 11-lb cartons/acre.
Harvest should begin when onions are 1/4 to 1/2 inch in diameter at the base. There should be at least 2 inches of white shank. Onions are hand pulled and bunched with 6-9 onions, or 1/4 lb, held together with rubber bands, usually two per bunch. Pulling is usually done without undercutting. Bunching is usually done in the field.
Field boxes are moved to the processing sheds within two to three hours of being harvested. The onions are run through a washer/cooler machine which automatically washes them in a 33 to 35 F water bath. Green tops are usually trimmed to 12 inches. In some cases harvested onions are bunched in the packing shed.
Chilling the wash water removes field and ambient heat from the onions. They are then immediately packed, 4 dozen bunches to a paraffin coated box, containing about 11-12 pounds of onions.
STORAGE (Quoted or modified from USDA Ag. Handbook 66 and other sources)
Hold green onions at 32 F and 95 to 100 % relative humidity. Green onions (scallions) and green shallots are quite perishable and are normally marketed promptly. They can be stored 3 to 4 weeks at 32 F if moisture loss is prevented. Crushed ice spread over the onions aids in supplying moisture. Packaging green onions in perforated polyethylene film also will aid in preventing moisture loss. Storage life at 50 F for green onions is only about 1 week. Higher temperatures favor more rapid yellowing and decay of the leaves.
Vacuum cooling is effective for removing field heat, but the onions should be wetted first and packed in polyethylene-lined cartons to minimize moisture loss.
For maximum storage, a controlled atmosphere of 1 % oxygen with 5% carbon dioxide at 32 F should allow 6 to 8 weeks storage. However, the green onions must be properly packed in waxed cartons or poly-lined containers. The freezing point of green onions is about 30 F.
Green bunching onions are commonly packaged in cartons holding 4 dozen bunches weighing 11-12 lb.
Fresh Market Vegetable Production, Onions