Onions

Onions are not roots, even though they grow underground.

The layered sphere we call onion is a bulb. Edible onions form when shortened underground stems are surrounded by fleshy leaves, called scales. People have been growing onions for 7,000 years.

Onions are members of the Allium family, which includes leeks, chives, and garlic. All of these crops are best planted late summer and early fall. One of the nicest things about growing your own onions is that they can be eaten at any stage of development. Early green shoots are called scallions, or spring onions, while mid-stage development crops are called summer onions. Onions are a cool weather crop. Hot weather will cause onions (Allium cepa) to go to seed, or bolt.

Mixed onions (Colin) CC BY-SA 3.0

Onion varieties

​There are three common varieties of onion: white, yellow, and red. White and yellow onions are commonly sautéed or caramelized. This releases sugars that make onions taste sweet. Red onions are more often used fresh, in salads and when grilling. Onions are also classified according to the number of day-hours it takes to trigger the development of a bulb:

• long-day (L) onions - need 14 hours of daylight or more, planted late winter, are best suited for colder regions
• indeterminate-day (I) onions - need 12 to 13 hours, planted in fall, may work in warmer regions
• short-day (S) onions - need 11 to 12 hours, best choice for warmer regions

Some onions have been bred to be smaller. These are called boiler, pickler, or pearl onions. The term “pearl onion’ is incorrect because pearl onions are an entirely different species. True pearl onions are more closely related to leeks, than to common onions, and they do not have layers, more similar to garlic.

Scallions

Onion bulb, leaves, and roots (USDA) Public Domain

How to grow onions
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Onion seeds are relatively short-lived, when compared to other seeds. You are better off planting transplants or onion ‘sets’. Onion sets are seeds that were sown in early summer, but so close together that they could not continue their development at that time. As a biennial, these sets are more likely to bolt, rather than creating onion bulbs. Transplants are one or more seasons older than sets and usually available in garden stores in October and November. If growing seeds, cover them only lightly with soil. If seeds are started in small pots in September, they should be ready for transplanting in 2 months. Frequent thinning is necessary for a good harvest. Onions are heavy feeders, so it’s a good idea to add aged compost to the planting area a couple of weeks ahead of time. You can expect to harvest mature onions in 12 to 18 weeks, usually in June. Successively planting more onion seeds can ensure an ongoing crop of fresh, tasty onions. Generally grown as an annual, this biennial edible can reseed itself in your landscape indefinitely, given the opportunity, so choose a well drained spot that gets plenty of sun.

Onion pests and diseases

Onions are susceptible to attack by onion maggots, onion flies, and onion eelworms. Leafminers, bulb mites, thrips, and nematodes can also cause problems. Most onion diseases are related to excessive moisture. These include bacterial soft rot, downy mildews, garlic mosaic, rust, pink root, white rot, basal rot, and several botrytis infections. Proper irrigation and allowing the soil to dry out between waterings are your best defense.

Curing onions

Onions are mature when the tops die back naturally. When the top half of mature stems has turned brown, stop watering. When the lower stem begins to turn yellow, carefully dig them out of the ground. After mature onions are dug up, they need time to cure, or dry. This reduces the risk of rotting. Harvested onions can be braided, laid out flat, or placed in nets for several days to dry out properly.

Foodscaping with onions

Due to their ability to self-propagate, onions and other alliums make useful foodscape plants. Using mature sizes, mixed plantings of alliums can be lovely and useful. Mature onion plants can reach 3 feet in height, with impressive spherical flowers that attract hoverflies and other beneficial insects. Wide-leafed leeks are slightly shorter and can stay in place for several months before they must be harvested. Narrow-leafed garlic is more seasonal, providing green spikes for several winter and spring months, followed by browning that some find messy. The browned tops can be bent over and hidden from view behind spiky, clumping chives that tend to stay green year round, offering up miniature versions of the onion flowers.

Onions about to flower next to radishes in full bloom

Why do onions make me cry?

Onions contain protective enzymes that are released as an acid gas when cut. That’s what burns your eyes and causes you to tear up when cutting onions. And you can ignore all those great ideas about how to avoid the burn: bread in your mouth, sunglasses, lemon juice… none of them work. If you really need respite from onion burn, freeze or refrigerate your onions before cutting, or cut them under or in front of a fan.