Propagation
Onions may be grown from seed or, more commonly today, from sets started from seed the previous year. Onion seeds are notoriously short lived.,, Onion sets are produced by sowing seed very thickly one year, resulting in stunted plants that produce very small bulbs. These bulbs are very easy to set out and grow into mature bulbs the following year, but they have the reputation of producing a less durable bulb than onions grown directly from seed and thinned.
Seed-bearing onions are day-length sensitive; their bulbs begin growing only after the number of daylight hours has surpassed some minimal quantity. Most traditional European onions are what is referred to as "long-day" onions, producing bulbs only after 15+ hours of daylight occurs. Southern European and North African varieties are often known as "intermediate day" types, requiring only 12–13 hours of daylight to stimulate bulb formation. Finally, "short-day" onions, which have been developed in more recent times, are planted in mild-winter areas in the fall and form bulbs in the early spring, and require only 9–10 hours of sunlight to stimulate bulb formation.
Either planting method may be used to produce spring onions or green onions, which are the leaves of immature plants. Green onion is a name also used to refer to another species, Allium fistulosum, the Welsh onion, which does not form bulbs.
Onions are a cool-weather plant; hot temperatures cause them to "bolt" (meaning that the stem quickly grows and begins flowering). Since they take up to 120 days to maturity, seeds will often have to be started indoors. The seedlings or bulbs are planted outside as soon as the soil is workable (i.e. not frozen or soggy) and harvested in early summer. Snowfalls in March or April are often known as "onion snows" because they fall on freshly planted onions.
All members of the Allium family are prone to the deadly disease of white rot mildew (typically spread through infected garlic cloves), which can linger in soil for many years and destroys the plants.
The tree onion produces bulblets instead of flowers and seeds, which can be planted directly in the ground.
I'itoi onion (Allium cepa) is a prolific multiplier onion cultivated near Baboquiviri, Arizona. They have a shallot-like flavour. They are easy to grow and ideal for hot, dry climates. To grow them, bulbs are separated, and planted in the fall 1 inch below surface and 12 inches apart. Bulbs will multiply into clumps and can be harvested throughout the cooler months. Tops will die back in the heat of summer and may return with monsoon rains; bulbs can remain in the ground or be harvested and stored in a cool dry place for planting in the fall. The plants rarely flower; propagation is by division.
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