Growing onions

Growing onions


Now is a good time to plant onions in Napa Valley. Walla Walla, Red Torpedo and Red Burger are just some of the seedling varieties I have seen in local nurseries, and a variety of seeds and “sets” (small bulbs) for onion relatives—such as chives, pickling onions and scallions — is available, too.

Onions are members of the Amaryllis family. Instead of a flamboyant flower, they have strappy edible leaves and edible bulbs that store energy and moisture to help the plant survive summer. Allium cepa is the species we grow as onions, and it has been cultivated and eaten for centuries all over the globe.

Young onions need cool temperatures to thrive. They grow to pencil size through the fall and winter, then develop bulbs as the days grow longer again in the spring.

Onions need soil amended with organic matter with low sulfur content, as sulfur is thought to contribute to pungency. Manures and compost are good sources of organic amendments that rain won’t leach. 

You can plant large storage onions now, use the thinnings over the winter as scallions, and then harvest large bulbs in the spring or summer for storing. Many other smaller varieties can be started now and over the next few months, for harvest through the winter, spring and summer.

University of California experts recommend growing onions in raised beds, preferably in sandy soil and with drip irrigation. But amended beds in the ground are also fine, and many scallion varieties are successful in large pots.

To grow large onions, the University recommends planting onion transplants or sets three to four inches apart, in rows one to two feet apart. Other sources recommend three to four inches all around, then thinning and weeding onions to provide room as they grow. This method requires frequent light feedings.

After the Napa County Master Gardener onion trials last year, we agreed that drip irrigation not only conserved water but also minimized weeds. Overhead watering encouraged weeds, direct competition for the nutrients your onions need.

Onion seedlings look like miniature green onions. To harvest big bulbs, plant seedlings  three to four inches apart. They need full sun and frequent light watering for the 12- to 18-inch root zone. They also appreciate a light monthly feeding of fish emulsion or other fertilizer. 

About 130 to 150 days after transplanting, your large onions will start to yellow. Stop watering to prevent the bulbs from splitting. After at least two weeks, and after the leaves have fallen over and begun to dry, carefully dig your onions and brush off the dirt. Don’t get them wet! Cure them in a dry, shady spot until the tops are brittle before storing in a cool. dry place. In Napa, onions tend to have a short storage life.

Onion “sets,” which look like miniature bulb onions, can also be planted now. The same spacing and requirements apply. Onions grown from sets mature a little earlier than the green transplants, but they are also prone to bolting more quickly when the weather warms. 

You can grow large storage onions from seed, although you need to start the seed by September to have transplants ready by November.

Home-grown onions can provide variety for your kitchen without specialty store prices.  We are fortunate to have mild winters in Napa Valley, so we can do successive plantings — either by seed, transplants or sets—from September through March. Try growing mini white, golden or red cipolline onions. These small flat onions are good when fresh but may also be pickled, or dried and braided, for later use.

Seed catalogs and local nurseries have seed for purple and red scallions, in addition to the green and white variety in most stores. Small, cocktail-sized onions come in a variety of colors and a range of flavor from pungent to sweet. They can enhance everything from a martini to a pot pie. 

Don’t forget chives (Allium schoenoprasum) or garlic chives (Allium tuberosum) in pots or planted four to eight inches apart in the ground as an edible edging in your garden.  Seeds for chives and green onions are inexpensive, and the plants don’t need much room to grow; start some in pots for gifts this winter. 

Onions have few pests and deer don’t eat them. Watch for thrips, mites, pink rot and gray mold. If you have a problem with your onions, call the Napa County Master Gardener help desk and ask for Pest Notes and other helpful handouts.

Planting a mix of storage, mini, bunching and spring onions can provide you with an aromatic harvest all year.

Napa County Master Gardeners ( answer gardening questions Monday, Wednesday and Friday, 9 a.m. to noon, at the UC Cooperative Extension office, 1710 Soscol Ave., Suite 4, Napa, 253-4221, or (877) 279-3065.

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