Family: Liliaceae
Genus and Species: Allium cepa
Climate
The onion is adapted to a wide range of temperatures and is frost-tolerant. Onions produce best when cool temperatures (55°-75°) prevail for an extended period of time. This will permit the onions to generate considerable foliage and root development before bulbing starts. After bulbing begins, considerably warmer temperatures and low relative humidity into the harvest and curing period are desirable.
Onion development is very dependent upon the length of day or photoperiod. Photoperiod, along with temperature, controls when the onions form bulbs. Some onion varieties are short-day in response and form bulbs when the days are 12 hours or less in length. Other onion varieties are long-day plants forming bulbs when there are 15 or more hours of daylight. This effect of day length makes some onion varieties unsuitable for northern climates because they begin to bulb when the plants are too small. The influence of day length also requires that Sweet Spanish and Bermuda onions be grown from plants rather than seed in northern climates.
Soil
Soil is the key to growing good onions. Some onion varieties are very particular about soil type. In general, deep, loose, fertile, friable soil works well for onions. Mix in compost or rotted manure to your bed before you plant. Onions grow best when soil temperatures are between 55° and 75°. Avoid heavier soils such as clay and silt loams unless modified with organic matter to improve aeration and drainage. Onions do not like highly acidic soils. The ideal soil pH for onions is between 6.2 and 6.8.
Spacing
Plant several rows of onions in beds 3'-4' wide. Onions can be planted in a zig-zag pattern with 1-1/2" to 2" between plants for the highest yield in fertile soil.
Alternately, set onions in rows 4" apart.
Alternate spacings include 3"-4" for larger onions and 4" for mild or sweet types.
Direct Seeding
Direct seeding onions provides the least amount of disturbance to the growing cycle. Onions grown by direct seeding are more resistant to disease. Direct seeding takes longer so it is not always the best option in northern climates.
Direct seed in a prepared bed after the soil has warmed to 60°. Sow onion seeds in rows about 1/4" - 1/2" deep (plant onion sets 1" deep.). Keep rows 4" apart. After the seeds are up and the starts are 2"-4" all, thin onion plants to desired spacing.
Seeding For Transplants
Onion seeds can be started indoors underneath growing lights 8-10 weeks before the last frost.
Germination
These seeds germinate best in soils around 55°F-95°F.
Germination will take 5-7 days.
Transplanting Into the Garden
Onions grow best when the early part of their growth cycle is cool. Plant them in early spring in cool climates. In mild winter areas, plant onions in the fall to grow through the winter.
Watering
Onions need a constant supply of adequate moisture for best results. Onions started from plants will benefit from a light mulch to help conserve moisture for uniform growth.
Harvesting
Onions are mature when most of the tops fall over. Once several tops fall over, use the back of a rake to bend the rest of the onion tops to the ground. Leave the onions in the ground until the tops have dried.
Harvest onions by gently pulling on the tops. Use a garden fork to carefully loosen soil around the onions if needed.
Post-Harvest Handling
Clean the dirt off onion bulbs with a soft brush or your gloved hand. On sunny, breezy days, onions may be pulled and left in the garden for a day or two to dry before they are taken to a curing area. Curing must take place for the onions to be stored for any length of time. Cure onions by placing them in a warm, well-ventilated area until the necks are thoroughly dry. With warm temperatures, good air circulation and low humidity, curing should be completed within two weeks after harvest. Onions are cured when the outer skins are completely dry.
Storage
Storage types: Storage onions are best stored in a cool, moderately dry area in ventilated containers. Storage onions can keep for 1-6 months at 32° and 65%-70% relative humidity.
Fresh onions, like Walla Walla, are best used within several weeks of harvest.
Diseases
Smut, Onion Blast, Onion Neck Rot, Downy Mildew, Bacterial Soft Rot, and Pink Root.
Pests
Onion maggots and Thrips.
Comments
Growing onions from sets provides a bit of ease for the gardener but if you want variety and stronger onion plants, grow your onions from seed.
References
Riofrio, M and Wittmeyer, E.C., "Growing Onions in the Home Garden", Ohio State University Extension Fact Sheet, HYG-1616-92, http://ohioline.osu.edu/hyg-fact/1000/1616.html, Accessed 2003-06-17.
Bradley, F. M. and Ellis, B. W.(Ed.). (1992), Rodale's All-New Encyclopedia of Organic Gardening: The Indispensable Resource for Every Gardener, Emmaus, Pennsylvania: Rodale Press.
University of Californica, UC Pest Management Guidelines, Last modified 2002-06-04, http://www.oregonstate.edu/Dept/NWREC/onionb-w.html, Accessed 2003-5-19
Smith, E.C. (2000), The vegetable gardener's bible: discover Ed's high yield W-O-R-D system for all North American gardening regions., Storey Books: Pownal, VT.