Onion planting guide
When to start onion seeds
Choose an outdoor planting date that fits your local conditions, then count backward for homegrown transplants. Onion day-length type still matters, so the date calculation cannot choose a variety for your latitude.
Homegrown transplants
10-12 weeks indoors
Count backward from a locally suitable outdoor planting date.
Direct seeding
Workable spring soil
Sow outdoors when the soil can be prepared.
Purchased transplants
Around 50°F
The source says transplants tolerate light frost.
Variety choice
Check day-length type
A variety must fit the daylight pattern at your latitude.
Seed-starting calculator
Find when to start seeds indoors
We count backward from the outdoor planting date using university timing guidance.
Choose the outdoor date
Use local conditions, not your Zone number
Direct seed or plant sets when spring soil is workable. Purchased transplants can tolerate light frost, and the reviewed source gives 50°F as a planting temperature. Current weather and wet soil can still delay the work.
Choose the variety
Day length controls bulb formation
The source explains that Minnesota storage onions are generally long-day types requiring at least 14 hours of daylight. That is a state example, not a nationwide recommendation. Use the packet or local Extension guidance for your latitude.
Why this site uses 10-12 weeks
The university's general indoor-seed table lists onions at 14-15 weeks, while its crop-specific onion guide says to start homegrown transplants 10-12 weeks before outdoor planting. This calculator uses the more specific crop guide and discloses the difference instead of averaging the two.
Harvest and storage signals
Watch the tops
The source recommends harvesting when about half of the tops have fallen over and dried. A calculated seed date does not predict that field signal.
Cure before storage
Storage onions need a warm, dry and well-ventilated curing area. Skip storage promises when bulbs have not cured properly.