Marin Master Gardeners
University of California
Marin Master Gardeners

Growing Onions in Marin County

Onions, (Allium cepa) are grown for their edible bulbs. A UC Marin Master Gardener in Novato shares her experience growing them. Read on to learn about how you can, too. Growing Onions

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My first experiences

The first year I planted onions, I picked up a six-pack of onion starts from the nursery, put them in the ground and all winter long the rain took care of everything. It was a wet spring so I really only needed to water for about a month, and I got the most beautiful crop. I could hardly believe it; growing onions had to be the easiest thing I ever did. What a success story. I grew more onions than I could eat before they started to go bad. The next year I grabbed a six-pack stuck them in the ground, and most of them bolted….hmmm, this will need a bit of research. Thus began my onion journey. Through many years of research, trial, error and success, this is what I’ve learned about growing onions in Marin County.

Choosing varieties

The tricky part about onions is there are MANY varieties developed for different purposes and locations.

Some onions are grown in the South (35 latitude) and only need 11-13 daylight hours to develop a nice bulb. These varieties are classified as short day onions

Onions adapted to the 35 – 38 latitude are known as intermediate day onions requiring 12-14 daylight hours.

Long day onions are grown in latitudes 37 – 42. This type requires 14-16 hours of daylight.

All this means the onion plant will start developing its bulb when the hours of daylight reach the variety’s requirement.

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Marin County’s latitude range is 37.8 – 38.1

Basically, our gardens are at the top end of the intermediate day spectrum and the low end of the long day spectrum. You might be asking yourself, ‘why does this even matter?’ Well let me tell you. The intermediate day onions are pretty easy. If you get them into the ground in the spring and they are no bigger than ¼ inch, you will get some good onions. After harvest, they can be stored for one to four months depending upon the variety.

The long day onions are a bit trickier; you will have to order seeds because it is rare for the local nurseries to sell long day onion starts. You can also order sets from companies specializing in onions. Sets are small onions that were planted from seed in the previous year – they are more likely to bolt, but they shorten the growing season. I’ve had great success growing Copra, Cortland, and Patterson. I’ve had mixed success growing Yellow of Parma but I plan to give it another try.

If you want to try growing the long day onion from seed, you need to start the seeds indoors by mid-October. I like to transplant them into a compost potting mix in early December and then get them into the ground in January. You really need to get them into the ground while they are still less than a ¼ inch wide. Once in the ground, the plants will be growing their root system. Ideally they will grow another ‘leaf’ every week or so. The longer they have to develop while the days are shorter, the larger the bulb will be. As soon as the day length hits the required number, the bulbs will begin to develop. Usually this happens by the end of May or the beginning of June.

Planting requirements

Find the sunniest plot in your garden. Onions really need the sun.

Onions will be in the ground for up to seven months. Start with a rich friable soil that retains moisture. They will need frequent irrigation throughout the season, ideally provided by a rainy winter and spring. The root system is shallow so very little water is extracted from deep in the soil. Also, the roots all originate at the stem, or basal plate of the plant. This means that the upper soil areas must be kept moist to stimulate root growth. Rates of transpiration, photosynthesis, and growth are lowered by even mild water stress. Water once or twice a week, depending upon the weather. Use the knuckle rule to tell when it’s time to water.

I’ve grown onions successfully in a heavily composted soil with monthly addition of compost. I’ve also grown them adding a little blood meal into the soil to provide the nitrogen they need for green growth and a little bone meal to boost the phosphorous, which will help their initial root growth. DO NOT fertilize onions after the weather warms and they are nearing their bulb forming stage.

One last thing about transplanting - the onions do not need to go in very deep. It’s actually nice to plant them shallow so when the bulbs start forming they are partially above the soil.


Once the tops become yellow and begin to fall over, bend the tops down to speed the final ripening process. Loosen the soil to encourage drying and after a few days, turn them up and let them cure on dry ground. Allow the onions to dry for several weeks before you store them in a root cellar or any other storage area. Ideally store at 40-50 degrees in braids or with the stems broken off. Mature dry-skinned bulbs like it dry; so don’t store them with apples or potatoes.

I have stored Copra onions under my house, at about 65 degrees, for ten months. I ran out just in time to start using the new crop. It’s quite rewarding eating my homegrown onions for the whole year.

Give it a try because now you too “know your onions”


Life is an onion - you peel it year by year and sometimes cry.”
- Carl Sandburg, Remembrance Rock


Article written by Laura Colvin, September 27, 2018
Photo credits:
Photo 1: Laura Colvin
Photo 2: Kathy Keatley Garvey

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