Using SowWindow
Frost dates and planting windows
Use this guide when setting up the seed-starting calendar or checking a date in an existing plan. SowWindow uses the dates you supply. It does not choose a location or retrieve a forecast, and a calculated window does not confirm that conditions are ready in your garden.
Reviewed Oct 28, 2026
Enter your season
Enter the plan year and choose the season you are planning. Supply the last spring frost date for spring-based calculations. Supply the first autumn frost date when using a supported later-season rule. Read the full value in each field before calculating, especially when a browser uses a different display order for day and month.
- Choose the year that belongs to the start of your plan.
- Enter the relevant full frost dates, including years.
- Select the crops and methods you intend to use.
- Update the calendar and inspect the source notes beside the results.
Understand the inputs
A historical frost reference is a planning assumption. NOAA notes that actual annual weather can differ from its historical freeze patterns. Keep the source of your selected dates in a note so you can revisit the decision.
A hardiness-zone number cannot fill a date field. USDA’s zones describe winter temperature conditions, not a day on the calendar. Likewise, a statement such as “when the soil is workable” needs your judgment; the tool should not convert it into an invented frost offset.
Read a window and its paired date
A range such as March 27–April 10 is a window, not a task to repeat every day. Check which action it describes. For linked indoor sowing and transplanting, keep the nursery interval consistent with the particular transplant day you choose.
The tomato example with a May 1, 2027 frost input has a May 8–15 transplant window and a March 27–April 10 indoor envelope. Choosing May 8 as the transplant day narrows its paired five-to-six-week indoor window to March 27–April 3. Choosing April 10 and May 8 together would not preserve that interval.
Use full years across December
A growing period can cross the year boundary. Enter its actual end year instead of forcing its dates into the starting year. The order is determined by the complete dates, so December 2027 can correctly precede January 2028.
This calendar support does not establish that a source’s regional growing advice applies in another climate or hemisphere. Keep the source conditions and your local assumptions visible even when the arithmetic accepts the dates.
Keep manual dates and exports clear
When you change a frost input, review recalculated tasks separately from dates you have edited manually. Keep a manual date unless you deliberately reset it. A note explaining the edit helps distinguish a purposeful choice from an old value.
Calendar exports use all-day date events, so a planting task is not tied to a midnight time in a particular time zone. A displayed window includes its final date. Calendar file formats represent the end boundary differently; the export handles that conversion. Check the dates after importing into your calendar application.