Onion (Allium cepa)-Hort Answers - University of Illinois Extension
University of Illinois Extension

University of Illinois Extension

Hort Answers


Allium cepa

Onion 'Super Star' AAS Winner 2001
Onion 'Super Star' AAS Winner 2001

Onions From Sets: Growing green onions from sets is probably the simplest method for the home gardener. The plants are quickly established and become vigorous and strong. Onion sets may be used to produce both green onions and dry onion bulbs, though production of really premium dry onions requires methods described in the following section.

Onions from Transplants: Transplanting young onion seedlings is the method of growing that most regularly produces large, dry, attractive onions for slicing (as shown in catalog pictures). Transplants are purchased in bundles (usually 60 to 80 plants) from garden stores and through seed and nursery catalogs (though mail-order onion plants often cost as much as buying the 60 to 80 full-size mature bulbs they may produce). Keep onions free from weeds by shallow cultivation and hoeing. To develop long, white stems for green onions, slightly hill the row by pulling the loose soil toward the onions with a hoe when the tops are 4 inches tall. Do not hill onions that are to be used as dry onions. Hilling may cause the necks of the stored bulb to rot.

Weeds and grass compete with the onion plants for nutrients and moisture during the growing season. Remove all weeds and grass by diligent and repeated shallow cultivation and hoeing. Side-dressing with fertilizer may be necessary.


Several varieties are used for onion sets. All of these varieties are widely adaptable. The home gardener has little choice of varieties at the store, however, because sets are seldom sold under varietal names, merely by color: yellow, white or red. Yellow sets are sometimes sold as the varieties Ebenezer or Stuttgarter. Purchase firm, dormant sets early - before they begin growth in heated salesrooms. Store sets in a cool, dry, dark environment if planting must be delayed after purchase. Divide the sets into two sizes before planting. Large sets (larger than a dime in diameter) are best used for green onions. If allowed to grow, these sets may "bolt" and form flower stalks. The small sets (smaller than a dime in diameter) produce the best bulbs for large, dry onions; and they usually do not "bolt." Extremely cold weather during early season growth also may condition onions from sets to flower.

Round onion sets produce flat onions; elongated or torpedo-shaped sets mature into round onions. Most gardeners prefer white sets for green onions, although red or yellow sets are also acceptable. Gardeners should try to match varieties to their location. Long-day onions are bred for best performance in the North and short-day varieties perform best in southern locations. Short-day varieties may perform acceptably in the North if the plants can be set out very early in the season. Long-day types may not get the bulbing signal in the Deep South and so should be avoided there. The normal garden center may offer Yellow and White Sweet Spanish (long-day varieties), Yellow and White Bermuda (short-day varieties and a red variety that may or may not be named (Southport Red Globe, perhaps; a long-day variety). Catalog shoppers may choose from a slightly wider variety selection, which may include Texas Grano (short- day), Vidalia Sweet (really a Granex hybrid, short-day), Red Hamburger (short-day), Walla Walla Sweet (long-day) and Texas 1015Y Supersweet (short-day). Prices normally are two to three times as high through catalog sales and may be as much as ten times as high. Only individual consumers can judge if this cost is justified for trying a new variety.

Planting Time
Onions can be planted as soon as the garden can be tilled in the spring, usually late March or early April in prime regions for producing onions. Good fertility, adequate soil moisture and cool temperatures aid development.


Harvest Time
Pull green onions anytime after the tops are 6 inches tall. Green onions become stronger in flavor with age and increasing size. They may be used for cooking when they are too strong to eat raw. Though leaves are traditionally discarded, all parts above the roots are edible.


To produce green onions, plant the larger sets 1/4 inches deep and close enough to touch one another (green onions are harvested before crowding becomes a problem). To produce dry onions, plant the smaller sets one inch deep, with 2 to 4 inches between sets. Allow 12 to 18 inches between rows. If sets are two inches apart, harvest every other plant as green onions so that bulb development of the remaining sets is not impeded by neighboring plants.


Soil Conditions
Moist, Well-Drained
Exposure/Light Requirements
Full Sun
Pests and Problems

Environmental Damage

Fungal Disease

Herbicide Injury

Insect Damage

Additional pests and problems that may affect this plant:


  • Onion maggot
  • Thrips
  • Cutworms
  • Wireworm


  • Botrytis leaf blight (Botrytis squamosa)
  • Botrytis neck rots (Botrytis allii)
  • Pink rot (Phoma terrestris)
  • Purple blotch (Alternaria porri)
  • Smudge (Colletotrichum circinans)
  • Smut (Urocystis colchici)
  • White rot (Sclerotium cepivorum)
  • Slippery skin (Psuedomonas gladioli pv. alliicola)
  • Soft rot (Erwinia carotovora subsp. carotovora)
  • Sour skin (Psuedomonas cepacia)
  • Aster yellows (Phytoplasma)
  • Viral diseases: Onion yello dwarf, Shalot latent
  • Nematode diseases: Root knot nematode (Meloifogyne spp.), Root lesion nematode (Pratylenchus penetrans), Stem and bulb nematode (Ditylenchus dipsaci)
Additional Notes
Onion is a cool-season vegetable that can be grown successfully throughout most of temperate North America. Onions may be grown from sets, transplants or seeds. Onions start bulb formation when the day length is of the proper duration and different varieties of onions require different day lengths to initiate bulbing. In general, most common varieties fall into one of two classes, long-day (for northern latitudes) and short-day (for southern latitudes). For this reason, onion varieties that are grown in the South are not adaptable to the North and vice versa. Late plantings of the suggested varieties also result in small bulbs or lack of bulbing all together in any location. High temperatures and low humidity are advantageous during bulbing and curing. Onions have shallow roots and compete poorly with weeds and grasses. Timely shallow hoeing and cultivation are important, especially when the onions are small. Onions may be eaten raw, broiled, boiled, baked, creamed, steamed, fried, french fried and pickled. They are used in soups and stews and combination with vegetables and meats.


Related Resources
Home, Yard & Garden Pest Guide
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