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 varieties. Long day varieties require 15 to 16 hours of daylight to form bulbs while short day varieties require as few as 12 hours of daylight. 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 often bolt (go to flower) instead.
Cultural tips for growing onions and garlic, UC IPM
Planting
- For onion bulbs, transplant seedlings (not sets) 4 to 6 inches apart in November to December.
- For green onions, space transplants or sets 2 to 3 inches apart.
- To grow your own transplants, seed in pots about 8 weeks earlier, in September to October. Onion seedlings are relatively easy to separate, so can be seeded quite densely.
- Onion seeds don’t last well in storage and are best bought fresh every year.
Harvesting
- Bulbing onions are ready to harvest when the tops bend over and become dry, about 6 months after transplanting.
- Green onions may be harvested as desired, but before they bolt, typically when they are ¼ to ½ inch in diameter.
Pest management
Pests and disorders of onions and garlic, UC IPM
More links
- How to Grow and Harvest Bulb Onions [video, 4:23]
- Onion handout
Recommended varieties for Santa Clara County*
| Early Red Burger, OP | Improved version of Stockton Early Red. Deep red, flat, and sweet. 10 to 12 inches tall. Plant 4 to 6 inches apart. |
| Sierra Blanca, Hybrid | Sweet and mild Spanish white onion with thick rings, stores well for 2 to 3 months. Day-neutral. 10 to 12 inches tall. Plant 4 to 6 inches 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 to 12 inches tall. Plant 4 to 6 inches 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.