Onions can require an early indoor start, but timing cannot compensate for choosing the wrong day-length type. Before finding a sowing date, confirm that the variety is suited to the latitude and growing season where you will plant it.
Choose an appropriate onion type
Suppliers describe onions as short-day, intermediate-day, or long-day types. Those categories relate to bulbing response to daylight. Use regional recommendations and the variety description rather than selecting solely for storage claims, color, or packet price.
Plan the longer indoor period
The calculator uses ten to twelve weeks indoors and an outdoor target four weeks before the entered spring frost date. With May 1 as frost, that means an April 3 planting target and approximately January 9 through January 23 sowing. This is a planning example, not a universal onion schedule.
Compare seeds with other starting options
If your indoor setup is limited, compare growing from seed with locally appropriate transplants or sets. Include the weeks of lighting, container space, and handling in that decision. Keep different onion varieties labeled because your results will be more useful if you know what was actually planted.
- Verify day-length suitability before ordering.
- Check local guidance for spring versus fall planting.
- Reserve shelf space for the full seedling period.
Select Onion in the planner only after checking regional variety guidance. Compare its early sowing date with the date you can realistically start your setup.
Plan my gardenOur guides offer general planning advice. Check the instructions for your seed variety and your local extension service before planting. We do not claim hands-on tests of products mentioned in buying guides.