UC Master Gardeners, Santa Clara County, CA
University of California
UC Master Gardeners, Santa Clara County, CA

Onion Handout

Garden Help > Vegetables > Onion

Common Name: Onion
Scientific Name and Family: Allium cepa and Amaryllis Family
Origin: Western Asia


Biennial monocot with prominent bulb, hollow cylindrical leaves and an odor when bruised. Roots shallow, 12-18"


Has been used for food since very early times; was eaten in Egypt before 3000 B.C. Also used as flavoring in nearly every current world culture. Botanically, there are three groups. Many claims are made for the medicinal benefits of onions.


  • Cepa: Onion; single bulb, produced from seed, lacking bulbils in inflorescence
  • Aggregatum: Shallot, Multiplier Onion, Potato Onion; similar to onion but smaller, reproduced by vegetative division instead of by seed and lacking bulbils in inflorescence.
  • Proliferum: Egyptian or Tree Onion, (top set onions), propagated by bulbils which grow in place of flowers in the flower cluster. These are treated as sets and planted the next season.


Full sun, humus enriched soil, will grow in wide range of soil types with appropriate adjustments in water nutrition management, sufficient moisture since shallow rooted, frequent light irrigation desirable (grown as a winter vegetable in mild climates). Cool season vegetable, usually planted in cool temps and harvested in late spring, to late summer.


As there are short-day and long-day types of onions, grow only those adapted to your day length. If a line were drawn between San Francisco and Washington, D.C., residents on the north side would grow long-day onions while those living on the south side would rely on the short-day onions for best results. Most seed catalogs indicate (S) short or (L) long-day types.

Usually, short day types are planted in the fall, long day types in the late winter, early spring. (Short-day onions develop bulbs in no more than 12 hours of daylight. Consequently, in the north where summer days are longer, these plants form extremely small bulbs prematurely.

Long-day onions require 14-16 hrs of daylight and thus fail to form bulbs under short-day conditions.) The "middle half" of California can also grow intermediate day length varieties (13-15 hrs). These are planted in the fall and mature after short day varieties in the late spring, early summer.


Onions may be grown from seed (reliably viable one year only), sets, or transplants. However, seed for homegrown transplants allows for the greatest variety for the home gardener and has less chance of bolting prematurely than sets.

In California at lower elevations with mild winters, plant seed in potting mix in containers in September. The plants should be ready for transplanting 50-60 days later. Two weeks after the seed has germinated, fertilize with 1/2 strength fertilizer, followed by twice monthly fertilizing until planted out. Incorporate plenty of manure (preferably horse manure) in planting bed and mulch with manure after planting. To encourage resident mycorrhizae, organic fertilizers are more compatible). If winter rains are not sufficient, apply water as needed. Do not allow the soil to completely dry out during the growing season.


These are small onions arrested in their development. They are grown from seed, planted in late spring and so close together that they are unable to expand and mature early in the season. When planted the following season, they will mature sooner than onions grown from seed of the same kind. However, few kinds of onions are offered as sets and as a biennial, their natural tendency is to go to seed in the second season.


Check with your local nursery stores in October or November to find a source of transplants or as discussed earlier, grow your own. Buy and plant immediately. The sooner the onions are planted the larger the bulbs will be when lifted from the soil in early June.


When 1/2 the tops have bent over, stop watering, bend the remainder with the back of a rake. Never break onions neck,

Dig out of the ground when bent tops begin to yellow. Keep tops on as long as possible (even hanging them up with tops on until used); Dry on a wire screen in a shaded airy spot until tops are brittle. Cut tops at 1" and store bulbs in a cool dry dark area with good ventilation.

If mild onions are grown, do not anticipate keeping qualities beyond late August unless cool dry conditions are available. A warm garage is not adequate. (Share with friends before August.) Be aware that longer day onions are usually better keepers.


If a sterile medium is not available, pour boiling water twice through the planting mix and drain. This leaches out humic acid and many desirable onions. Baking or microwaving moist soil is better.

If soil pH is high adding sulfur will lower the ph. Do not add sulfur if mild onions are your goal. Sulfur adds to pungency.

Withhold watering as onions mature (as first tops begin to bend). Otherwise, the additional water uptake may cause splitting of the bulbs and increase the chance of rotting after harvest.

The colors used in catalogs are based on the dried skin color of the onion not the bulb color in cross section.

Recommended Varieties for Santa Clara County

Many new hybrids now available to commercial growers; these are slowly becoming available to home gardeners.

Short-day Onions - Mild

  • Yellow Bermuda
  • White Bermuda
  • Crystal White Wax
  • Excel (yellow)
  • Maui Granex (tan)
  • Red Granex
  • Sweet Vidalia (tan, granex)
  • Early Grano (tan)
  • Texas Grano (tan)
  • Torpedo (red)
  • Torpedo onions are generally not mild
  • (Grano is open pollinated; Granex is a hybrid)

Intermediate-day Onions - Mild to Pungent

  • Long Yellow Sweet Spanish, medium, between mild & hot
  • California Early Red
  • Inter or Long Day Southport
  • Yellow globe
  • Long Yellow Globe Danvers (med size)
  • Stockton Red (lg size)
  • Stockton White (lg size)
  • Stockton Yellow (lg size)
  • Early Red Burger

Long-day Onions - Mild to Pungent

  • Giant Walla Walla - intermediate and long strains both exist
  • Sweet Sandwich - may be too long; not mild
  • White Sweet Spanish
  • Yellow Sweet Spanish
  • Fiesta (hybrid - good storage)
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