Answer: Sounds like the mature onion you planted is developing foliage and will eventually produce a seed stalk. Each of these events will sap the energy stored in the bulb, and the bulb will eventually wither away. You can cut the foliage off and use it as you would green onions, and after the and seeds form, you can collect them and start your own onion plants.
Here are some basic guidelines to follow:
Onions can be grown from seed or sets or transplants. Start seeds indoors about 8 weeks ahead or plant them outdoors in early spring as soon as the soil can be worked. Onions are usually seeded rather heavily and then thinned. Sets or transplants should be planted outdoors in early spring about two inches apart and one to two inches deep, and then eventually thinned to be about four inches apart (for larger bulbing type onions). You can eat the thinnings as green onions! Storage onions planted in late April should be ready in late summer -- average days to maturity is about 100 days.
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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