When to plant onion
Planting dates are not a calendar, they are a distance from your last frost — which is why an answer that works in one place is wrong two hundred miles away. Put your own frost date in and this works out every date for onion from it.
Enter your last frost date to see the dates for onion.
How hardy is onion?
Hardy. It takes a real freeze and several of these are better for it — cold improves the flavour of some and is required by others to form properly. Sow as soon as the ground can be worked.
Why frost dates, and not months
A month is only a planting date if you already know the climate it refers to. "Sow tomatoes in March" is sound advice in one place and ruinous two hundred miles north of it, and the advice does not carry its own context — so it gets copied between places where it does not hold.
Frost carries the context with it. The last spring frost is the moment the ground and the air stop being able to kill a tender plant, and every crop has a fixed relationship to it: some go in weeks before, some wait weeks after. Learn your own frost date once and every planting date you will ever need follows from it, wherever you are.
Finding your frost date
It is an average, not a guarantee — roughly the date after which there is a small chance of frost, usually a one-in-ten year. Your national met service publishes it, and in the US the NOAA freeze/frost maps give it by station. A local gardening group is often better than either, because frost is local: a valley floor can be several weeks behind a slope a mile away, and a walled town garden weeks ahead of open fields outside it.
If you are somewhere frost never comes, the limiting season is heat or rain rather than cold. Plant tender crops to avoid the hottest months, and time everything else to the start of the wet season.
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