Understanding your result
The sowing window is counted backward from the target transplant date. The earlier sowing date uses the longer growing period. Dates are shown as year-month-day and use calendar days, avoiding daylight-saving shifts. Hardening off finishes on the transplant date.
When the transplant plan is ready, use the Plant Spacing Calculator to check how many planting positions the bed can hold.
The formula
A positive offset delays planting until after the expected frost; a negative offset targets planting before it. Indoor weeks include the hardening-off period, so that period is not added again. This is a calendar planner: the supplied date and growing intervals determine the outputs without a location or weather lookup.
Worked example
With an expected frost on April 15, 2027 and a 14-day delay, transplanting is planned for April 29. An indoor interval of six to eight weeks gives a March 4–18 sowing window. Seven hardening-off days start on April 22.
How to use this calculator
- Enter an expected last spring frost date appropriate to your location.
- Choose the crop’s transplant offset and minimum/maximum indoor weeks.
- Enter hardening-off days and record the three calendar results.
Read what the seed packet counts from
Some instructions count weeks before transplanting; others explicitly count before the last frost. This planner uses transplanting as its starting point for indoor weeks. If your packet gives frost-based sowing instructions, use a zero transplant offset for that particular backward calculation or adjust the interval deliberately.
An expected frost date is not a forecast
A historical frost date can be useful for planning but cannot guarantee a frost-free night in a future year. Temperature, soil conditions, light, and seedling development still determine readiness. Revisit the target as the date approaches; changing a calendar entry does not change the crop’s temperature tolerance.
Different crops deserve different runs
The calculator does not supply universal crop presets. Use variety-specific information and calculate each crop separately. Hardening off gradually prepares seedlings for outdoor conditions; the entered number of days is included in the overall growth interval. Zero weeks with a zero hardening period simply makes sowing and transplanting the same date.
Assumptions & limitations
What this calculation assumes
- The indoor growing interval includes germination and hardening off as entered by the user.
What to keep in mind
- No ZIP-code frost lookup, weather forecast, or crop-specific planting recommendation is provided.
Common questions
Can this work in the Southern Hemisphere?
Yes. Enter the relevant local spring date; there is no built-in northern season assumption.
Why does the earliest sowing date use more weeks?
A seedling given eight weeks must be sown earlier than one given six weeks if both share the same transplant date.