Answer: Onions prefer to grow in the cooler temperatures of autumn and spring. When the weather gets too hot, they bolt (send up flower stalks), which indicates the plants are putting all of their energy into producing seed, rather than enlarging bulbs. You should always cut any stalks off of onions as soon as they appear. I'd suggest planting onion sets in late August for harvest in the early winter months, or December for early spring harvest. If you plant during the cooler times of the year, your onions won't bolt.
The average days to maturity is about 100. 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. Hope this at least heads you in the right direction! Better luck with your next crop!
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