Answer: Allium cepa includes many types of onions -- green onions, mini onions, sweet onions and storage onions! In general, onions need full sun and a well drained soil (not soggy) which has been well enriched with organic matter. They require about an inch of water a week and should be kept well weeded. The onion bulbs grow close to the soil surface so be careful not to damage them when weeding.
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 depend on day length to produce a bulb, so if you are growing a bulbing type onion you will need to select a variety suited to either medium or "widely adapted" day length rather than a short day variety better suited to the south.
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