Growing Onions
Onions are grown for commercial purposes in twenty-six states. Of these states, California, Oregon, Idaho, Texas, New York, Michigan, and Colorado are major suppliers.
Onions are usually grown on loam or silt loam soils that have good water-holding capacities. Good crop rotation is a necessity. Rotated crops may include small-grains, potatoes, and sugarbeets. The fields are prepared in the fall, fumigated, bedded, and the residue plowed under. They are left to mellow over the winter. Onions grow well in soil with a pH of 5.5 to 7.0, though the onion's sensitivity to acid soil leads them to grow best in soil with a pH between 6.2 and 6.8.
Bulbs can reproduce by means of underground offsets called bulblets (called "sets"). However, most growers use onion seed. Onions grown from seed will mature into bulbs in about five months.
Seed-raised onions are the most important commercially, because it is their ripe bulbs that can be shipped or used for storage. Most seed varieties used today are proprietary and seed producers are contracted. Very, very little seed used for planting is in the public domain. Most seed comes from growers in the U.S. though some growers are now using seeds from Chile and other parts of the world. More than 100 varieties are grown all across the U.S.
Onion seeds are usually planted in the spring before the last frost. In the large production areas of Western Idaho and eastern Oregon, they are planted as early as late February. Growers can usually plant for a successful bulb harvest until early-to-mid April. Onions grown for their seed are usually planted the fall prior to harvest; growers will usually plant bulblets in this case.
There are approximately 9,500 onion seeds per ounce. An average of one to three pounds are used per acre, depending on the seeding method and desired size. Most onions are direct seeded, though sometimes overwintered sweets are transplanted in the spring. Onion seeds are usually planted at a depth of 1/2 to 1 inch. Growers usually use precision seeding techniques, placing individual seeds at a predetermined spacing within a row. The result is a highly uniform crop with a high yield of the desired size.
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. The most common planting practice is two rows planted per raised bed. These are often split into 2 lines per row with rows 12 to 18 inches apart on top of beds that are on 30 to 44 inch centers. This is equivalent to about 150,000 plants per acre.
Cultivation is usually not necessary with excellent chemical control, but when there are problems, the grower may cultivate as many as three to four times.
Growers usually prefer to not have to irrigate until the onion plant has emerged from the soil. Then the fields are irrigated regularly until the plant has matured, typically around mid-August. Soil conditioning is typically necessary. Onion plants are shallow-rooted and plant growth is good when nitrogen is carefully applied with irrigation water.
It is important that excellent plant development is achieved before bulbing starts. The best foliage and root development occurs when the temperatures are cool (55 to 75 degrees F.). After bulbing begins, high temperatures and low humidity continuing through the harvest process are important. Through the entire growing process, adequate soil moisture must remain constant.
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Growers must closely monitor onion plants for pests and diseases. Onion thrips are a widespread pest. Onion maggots are another problem pest. Diseases include pink root, botrytis blast, downy mildew, purple blotch, white rot and neck rot.
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Photoperiod is an important factor in bulb development. This term describes the effect of day length on bulb formation. Some onion varieties have a short day length response, forming bulbs when the days are 12 hours or less. Other varieties have a long day length response, forming bulbs when there are 15 or more hours of daylight. Short day length varieties are generally unsuitable for growing in all but the southern parts of the U.S. Long day length and a few intermediate day length varieties are grown in the large production areas of Oregon and Idaho.
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A plant is considered to be mature when it stops growing. When this happens, the grower stops irrigating. The bulb will continue to grow even after the water is withheld. When 25-50% of the onion leaf tops have fallen over, the plant is ready to be lifted from the ground. The lifters move carefully through the fields so as not to create clods. The plants are carefully lifted from the ground and are left to wilt. They must dry rapidly (cure) before they are topped. If an onion has not cured properly, it is very susceptible to neck rot.
Ten or more days later, when the leaves are dry, a mechanical onion topper cuts the tops off the onion bulbs. Clods are removed or covered with a drag so that the bulbs can drop softly to the ground. Any breaks in the skin can leave the bulb susceptible to rot and other diseases. Again, the onions are left to lie in the fields. They will cure for at least three more weeks before they are lifted again and bulked for storage.
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