How to grow great onions

Last Updated: 
August 4, 2008

CORVALLIS - Onions can be confusing to the home gardener because there are so many to choose from. Some are hot and strong. Others are mild and savory. Then there are different ways to start them – from seed, starts or bulbs, sometimes called "sets."

Certain varieties of onions can be stored over the fall into the winter. Others spoil more quickly. Some make good scallions, others don't.

Confused? Here's a primer on onions from vegetable researchers in Oregon State University's Department of Horticulture.

Almost all onions, scallions and shallots are members of the species called "Allium cepa," explained Jim Myers, vegetable breeder at OSU. Scientists divide Allium cepa into three major groups of onion crops.

There are common bulb onions, including the popular sweet Spanish, Walla Walla, Bermuda and red onions as well as green onions or scallions. There are onions that "multiply" by forming a clump of bulbs upon maturity, including "potato" onions, shallots and "ever-ready" onions. And there are onions that reproduce by topsetting "bulbils," tiny onions at the top of a flowering stalk, termed "walking onions," or "Egyptian" onions.

All onions are biennial, meaning that they normally take two years to grow from seed, flower and set seed again.

Onions first grow "vegetatively," forming roots, leaves and other plant growth. Once these basics are established, the bulb, the part that is generally known as an onion, forms when the day and night lengths reach the proper length for each particular type of onion.

Once an onion bulb forms, there are enough nutrients stored for flowering, or "bolting," usually occurring during the second year of an onion's two-year life. Flower formation usually ruins the bulb for harvest as an onion, as the resources of the bulb usually are absorbed to send up the flowering stem. Hot and strong onions generally keep longer than mild and sweet types like Walla Walla Sweets.

The OSU Extension Service recommends the following varieties as performing well in Oregon:

(yellow), Copra, Prince, First Edition, Millennium, Frontier, New York Early, Candy.

(overwintering) Buffalo, Walla Walla Sweet.

(red) Red Wing, Mars.

(white) White Sweet Spanish, Superstar, Blanco Duro.

(green bunching) Ishikura, Tokyo Long White, He-shi-ko.

The earlier you get your onions in the ground in the spring, the better the chance they will have to grow nice big bulbs, explained Deborah Kean, OSU vegetable researcher.

Plant onion bulbs or seeds as soon as the soil is dry enough to work. March through April are good times to plant summer harvested onions in most areas of Oregon. In Oregon's milder areas, overwintering onion seed can be planted in August to mid-September for harvest next June or July. These can be harvested as scallions all season long.

To help eliminate difficulties with onion seed germination, or to avoid waiting for soil to dry out or warm up in the spring, start onion seeds in flowerpots indoors. Transplant these when the tops are two to three inches tall.

Use good potting soil, a container with a drain hole, and provide plenty of light. Seed-planted onions need a longer period of development than with onion sets.

Plant onion seeds a half-inch deep at a rate of one to five seeds per inch. Thin seedlings after they are established. For large dry onions, thin seedlings to two to three inches apart; for medium-sized onions, one to two inches; and for boilers and green onions, a half-inch to an inch. The key to getting good seed establishment is to keep soil moist so it doesn't form a hard crust.

Onions can be grown in almost any type of soil as long as it has good fertility, drainage and tilth. Onions respond to both compost and commercial fertilizers.

A handful of complete fertilizer, such as 16-16-16, applied along the row at planting time will get the plants off to a good start. A good compost or organic fertilizer will also supply the needed nutrients for onions.

Author: Carol Savonen
Source: Jim Myers, Deborah Kean