Planting Onions?

Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County

Client's Request: Hello! My family will be moving to Contra Costa County this summer. I am wondering (many things, but firstly) what "length" of day onions are best for this area? It seems on the border for long day varieties,,, would long day ones still do ok, or should we stick to intermediate day varieties?

MGCC Help Desk Response: Thank you for contacting the UC Master Gardener Program Help Desk. As you are aware, onion varieties have different requirements regarding the number of daylight hours required to initiate bulb development and form a bulb. An onion plant will bulb up only after experiencing its critical day length for several weeks; the bigger the plant receiving the light stimulus, the bigger the bulb.

If an onion variety is described as long day, it sets bulbs when it receives 14 or more hours of daylight. Intermediate day onions set bulbs with about 13-14 hours of daylight. There are also short day varieties that require 12 hours of daylight, but they are not recommended for this area.

The following varieties are recommended for Contra Costa County:
Intermediate Day
- Italian Red
- Stockton (red, white or yellow)
Long Day
- Fiesta (yellow, sweet Spanish type)
- Yellow Sweet Spanish
- White Sweet Spanish
- Southport Globe (red or white)

Onions are also affected by temperature in addition to day length. Average daytime temperatures less than 70 degrees retard bulb formation. Seedling or young transplants exposed to several days of temperatures well below this, followed by a period of weather at or above this temperature while day length is increasing, may produce flower stalks before the mature or form bulbs (bolt). Plant seed or transplant intermediate or long day varieties from January through March for late summer or fall harvest.

Below are links with additional UC information on cultural tips for onions:
http://vric.ucdavis.edu/pdf/onion.pdf
http://ipm.ucanr.edu/PMG/GARDEN/VEGES/onions.html

Please let us know if you have any questions or would like additional information.
Good luck on your move to and growing onions in Contra Costa County.

Help Desk of the UC Master Gardener Program of Contra Costa County (JMA)

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Note: The  UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer your gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523, although we will be moving this spring. We will notify you if/when that occurs. We can also be reached via telephone:  (925)646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/  MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog  (//ucanr.edu/blogs/CCMGBlog/)

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