What the calendar calculates
The calendar first places the transplant window using your entered frost date and the signed days you set: a negative value places transplanting before frost, a positive value after. Indoor sowing dates are then counted back from that transplant window using the indoor weeks you set, not directly from the frost date. The result is an outer indoor sowing window and a transplant window, both inclusive.
The arithmetic is a date calculation, not a weather prediction. Historical frost probabilities do not guarantee any particular season, and your seed packet or plant tag supplies variety-specific information. Adjust the crop values to match your packet and local guidance rather than treating the defaults as fixed rules.
- Enter your last spring frost date for the year you are planning.
- Select one or more of the eight crops.
- Review each crop's indoor weeks and signed transplant days, and edit them if your packet or local source differs.
- Read the outer indoor sowing window and transplant window as inclusive planning ranges.
Source context: University of New Hampshire Extension · University of Illinois Extension
Later rounds are arithmetic, not succession advice
You can set the total number of rounds, from 1 to 6, including the first round, and choose an interval. The tool repeats the same date arithmetic at that interval so you can see additional planning windows. This is arithmetic only, not an agronomic succession recommendation.
The tool does not calculate fall-frost feasibility or heat feasibility for later rounds. If you plan multiple rounds, check each one against your own local conditions and the crop's needs before relying on it.
- Choose the total number of rounds, up to 6, counting the first round.
- Set the interval between rounds.
- Treat each later window as a planning range to verify yourself, not as a recommendation.
Source context: University of New Hampshire Extension · University of Illinois Extension
Defaults, downloads, and journal handoff
The default crop values are a named New Hampshire Extension reference. For example, tomato defaults are 6 to 8 indoor weeks with transplanting 1 to 2 weeks after frost, and pepper defaults are 8 to 10 weeks with transplanting 2 weeks after frost. The editable UI supplies the full crop reference list. These are regional planning references that need local and packet adjustments.
You can print the calendar, download a CSV, or download an all-day ICS file. You can also send a single planned date and crop to the journal through a browser-only URL fragment; it only prefills an unsaved record.
- Check the default values against your seed packet before relying on them.
- Use print, CSV, or all-day ICS to keep a copy of the calendar.
- Send one planned date and crop to the journal if you want to start an unsaved record.
Source context: University of New Hampshire Extension · University of Illinois Extension
Reading the supplied tomato example
The example uses the New Hampshire tomato reference with a user-entered last spring frost of May 15, 2027. Transplanting 7 to 14 days later gives May 22 to May 29. Counting back 6 to 8 weeks from that transplant window gives an outer indoor sowing window of March 27 to April 17.
This is a date-arithmetic example, not crop advice for every site. The outer sowing window is not a promise that every date suits every location. Your own frost date, packet, and local guidance should govern the values you enter.
- Compare your entered frost date with the example to see how the windows shift.
- Confirm that your edited crop values match your packet before planning.
Source context: University of New Hampshire Extension · University of Illinois Extension
Check your measured example
Illustrative New Hampshire tomato reference, anchored to a user-entered last spring frost of May 15, 2027: transplant 7–14 days later and count back 6–8 weeks. The outer sowing window is not a promise that every date suits every site.
| Quantity | Calculated result |
|---|---|
| last spring frost | 2027-05-15 |
| transplant earliest | 2027-05-22 |
| transplant latest | 2027-05-29 |
| indoor sow earliest | 2027-03-27 |
| indoor sow latest | 2027-04-17 |
The supplied example is a date-arithmetic calculation using the New Hampshire tomato reference and a user-entered last spring frost of May 15, 2027. Transplanting 7 to 14 days later gives May 22 to May 29, and counting back 6 to 8 weeks from that transplant window gives an outer indoor sowing window of March 27 to April 17. It is not a promise that every date suits every site, and no additional numerical results are implied.
Example inputs explain the method. Review the source context, chosen cultivar and actual site before planting or ordering.
Your before-you-plant checklist
- Enter the last spring frost date for the year you are planning.
- Check each crop's indoor weeks and signed transplant days against your seed packet or local source.
- Read both windows as inclusive planning ranges, not daily tasks.
- Verify any later rounds yourself for fall frost and heat feasibility.
- Confirm whether you need print, CSV, or all-day ICS before relying on the calendar.
Questions that come up while planning
Does the calculator predict my frost date or weather?
No. You enter your own last spring frost date. The tool counts backward and forward from that date using the crop values you set. Historical frost probabilities are not a guarantee for a particular season.
Are the default crop values nationwide instructions?
No. The defaults are a named New Hampshire Extension reference. They are regional planning references that need local and packet adjustments.
What do later rounds mean?
Rounds means the total number of rounds including the first, from 1 to 6. Later rounds repeat the same date arithmetic at an interval you choose. They are not an agronomic succession recommendation, and the tool does not calculate fall-frost or heat feasibility.
Sources & method
Horticultural guidance is linked to its original source and regional context. Geometry and unit conversions are calculated separately. Read how we prepare and check a page.
- Growing Vegetables: When to Plant Your Vegetable GardenUniversity of New Hampshire Extension
- When to PlantUniversity of Illinois Extension