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Zones A1-A3, 1-45, H1, H2
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Full
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Moderate

Onion

Liliaceae
Annuals, Bulbs and bulblike plants, Vegetables

Botanically, the onion is Allium cepa, a species not known in the wild. More so than for other vegetables, growing onions successfully requires proper variety selection, appropriate planting methods, and good timing. Consult a local nursery or your Cooperative Extension Office for advice on varieties that grow well in your area.

Onion varieties differ in size, shape, color, flavor, and storage life. More important, different types form bulbs in response to varying day lengths. If you choose a type inappropriate for your area, it may bolt, form small premature bulbs, or not bulb up at all.

Long-day varieties need 14 to 16 hours of daylight to form bulbs and are best adapted to northern latitudes, such as Alaska, Canada, and the northern tier of the United States. They tend to be pungent, and they store well; examples include ‘Early Yellow Globe’, ‘Ebenezer’, ‘Ruby’, ‘Southport White Globe’, ‘Sweet Spanish’. Generally, long-day onions are planted in early spring and form bulbs as the days get longer in summer.

Short-day varieties need 10 to 12 hours of daylight and are best adapted to the southern third of the United States and Hawaii. They are typically planted in fall to early winter, grow vegetatively through winter, then begin to bulb up in spring. These onions tend to be sweet and are poor keepers; examples are ‘Bermuda’, ‘California Red’, ‘Granex’, ‘Grano’, and ‘Super Sweet’.

Intermediate-day onions, requiring 12 to 14 hours of daylight, are best suited to interior valleys of central California and the middle third of the United States. They store moderately well. Examples are ‘Autumn Spice’, ‘Red Torpedo’, ‘Ringmaker’. They are usually planted in early spring.

(True bunching onions, also sold as scallions, are varieties that do not form bulbs and are harvested like scallions or green onions; they can be grown anywhere. )

Onions can be grown from seed, sets (small bulbs), or transplants. Sets and transplants are easiest for beginners, though starting from seed gives a larger crop for a smaller investment and offers a wider choice of varieties.

In mild-winter climates, onions grow well from seed planted in fall to early winter. Sets and transplants can be planted all winter long and through early spring, although sets are generally long-day varieties that will not bulb up well in southern regions. They can, however, be grown for scallions or green onions—standard onions harvested before they form bulbs. In Hawaii, short-day varieties are usually started from seed from earliest part of cool season; intermediate types are planted later in cool season for warm-season harvest. Types called Maui onions are actually any sweet, bulbing onion grown in Hawaii. In cold-winter climates, planting can begin in early spring as soon as the soil is workable; or start seed indoors in late winter and transplant later.

Soil should be loose, rich, and well drained. If planting sets, push them just under the soil surface so the point of the bulb is visible. Space sets and transplants 4–5 in. apart (closer if you want to harvest some as green onions). Sow seed 1/4 in. deep, in rows 15–18 in. apart. Thin seedlings to separate them by 4–5 in.; they can be eaten or transplanted to extend planting. Trim back tops of transplants about halfway.

Onions are shallow rooted and need moisture fairly near the surface. Feed plants regularly, especially early in the season: the larger and stronger the plant, the bigger the bulb it forms. Carefully eliminate weeds. When most of the tops have begun to yellow and fall over, dig bulbs and let them cure and dry on top of the ground for several days. Cover bulbs with tops to prevent sunburn. When the tops and necks are completely dry, pull off the tops and brush dirt from bulbs; then store bulbs in dark, cool, airy place.

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