Answer: Onions are a cool season plant and will express their best growth at temperatures from 55 to 75 F. High quality onions require cool temperatures during early development and warmer temperatures during maturity. Onions go through a process called `bulbing' to produce the onion bulb. Bulbing is affected by amount of daylight, not by plant age. Daylight necessary to initiate bulbing depends on the variety of onion and can range from 12 hours for early maturing types to 15 hours for late maturing types. Mature onion bulb size is highly correlated to the size of the onion plant at the time bulb formation begins. Thus, bulb size is influenced by the same factors which influenced plant growth prior to the beginning of bulbing, plus environmental conditions during the completion of bulbing and maturity. Factors affecting large bulb formation include early planting, space per plant, soil moisture, weed competition early in the growing season, and damage from blowing soil particles, insects and pests.
The ratio of double or split bulbs is influenced by variety and environment. For instance, the Sweet Spanish variety will produce a higher percentage of splits or doubles, if grown at wider spacings or in thin stands. Also, when plants are grown in uniform stands, fertilizing at a rate which produces large yields will result in more doubles than a lower rate. Larger onion sets will result in more doubles than smaller sets. Also, sets planted 3 inches deep versus 1.5 inches deep will produce more doubles and splits.
Hope this at least heads you in the right direction! Better luck with your next crop!
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