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


Garden Help > Vegetables

UC Links

Yellow Onions (Photo: Evett Kilmartin)
Yellow Onions (Photo: Evett Kilmartin)
How to grow: Onion, UC Davis, Vegetable Research and Information Center

Pest management: Vegetable cultural tips, pests, and diseases

The Basics

Onions can be divided into two categories: strong-flavored ones that store well and mild, sweet ones that don't last long in storage. There are yellow, white, and red varieties in both categories.

Onions can also be divided into long day, intermediate, and short day. Long day varieties require 15–16 hours of daylight to form bulbs, short day as few as 12. The Bay Area is on the edge of the long/short day division, so with care, any of them may be grown.

Onion sets are small, immature onion bulbs that can be used to produce green onions, but are not recommended for onion bulbs because they will often bolt (go to flower) instead.

When to plant

  • For onion bulbs, transplant seedlings (not sets) in Nov–Dec
  • To grow your own transplants, seed in pots about 8 weeks earlier, in Sept–Oct
  • Onion seeds don't last well in storage and are best bought fresh every year.
  • For green onions, space transplants or set 4 to 6 times closer than bulb onions.

Harvest window

  • Bulbing onions are ready to harvest when the tops bend over and become dry, about 6 months from transplanting to harvest.
  • Green onions may be harvested as desired but before they bolt, typically when they are 1/4 to 1/2 inch in diameter.

Care overview

  • Onions need rich, well-drained soil and good weed control.
  • Water to keep soil evenly moist, but not wet. Mulch will help keep the soil moist and discourage weed competition.

More links

Recommended Varieties for Santa Clara County*

Early Red Burger, OP Improved version of Stockton Early Red. Deep red, flat, and sweet. 10–12" tall. Plant 4–6" apart.
Sierra Blanca, Hybrid Sweet and mild Spanish white onion with thick rings, stores well for 2–3 months. Day-neutral. 10–12" tall. Plant 4–6" apart.
Torpedo Rossa Lunga di Firenze, OP These purple-red torpedo-shaped onions are an Italian heirloom with a rich, mild, sweet flavor. Intermediate day. 10–12" tall. Plant 4–6" apart.

Other recommended varieties: Early Yellow Globe, Fiesta, Granex, Grano, Southport White Globe, Stockton Red, White Sweet Spanish,Yellow Sweet Spanish

* Many other varieties may also do well here in Santa Clara County. This list is based on UC Master Gardener trials, taste tests, and feedback from local growers.

Webmaster Email: webmaster-mgsantaclara@ucanr.edu