Calculator
Planting date helper
This tool turns one date into a planting calendar. You enter the average last spring frost date for your own area, and it adds the weeks-before and weeks-after offsets that university extension services publish for each crop. It does not look anything up and it contains no regional dates, because frost dates differ even between neighbouring towns. Nothing you type leaves your device.
Enter your average last spring frost date to see the windows. Worked example (shown when scripts are off): if your date is May 1, tomatoes are set out May 8 to May 15 after starting seed indoors roughly March 13 to April 3, and lettuce goes out about April 3 to April 10.
Before you plant
An average last frost date is the date after which a killing frost becomes unlikely, not impossible. Treat it as the middle of a range. Watch the forecast, and if a cold night is predicted, hold tender plants back or cover them.
Where to find your frost date
Ask your local cooperative extension office, check a national weather service resource that lists frost probabilities for your area, or ask a long-standing local nursery. Your own garden notes are the best source of all: write down the date of the last frost each spring and the pattern will show within a few years. Your hardiness zone is a different number. It describes winter cold for perennials and does not tell you when to plant annual vegetables.
How the calculator works
Each crop has a start and an end offset in weeks from the last frost date, where a negative number is before and a positive number is after. The planting window runs from the earliest offset to the latest, with each week counted as seven days. For crops started indoors, the tool also counts back from the planting window: the earliest sowing date is the earliest planting date minus the longest seed-to-transplant time, and the latest sowing date is the latest planting date minus the shortest time.
Example offsets used here: lettuce 3 to 4 weeks before the last frost, broccoli 2 to 4 weeks before, tomato 1 to 2 weeks after, pepper 2 weeks after, eggplant 2 to 3 weeks after. Tomatoes take 6 to 8 weeks from seed to transplant and peppers 8 to 10 weeks.
Crops that go by soil, not the calendar
Peas go in as soon as the soil can be worked. Beets, carrots and corn are planted when the soil reaches a set temperature rather than a number of weeks, with extension guidance quoting about 45 degrees F for beets and about 60 degrees F for carrots and corn. A cheap soil thermometer pushed 2 to 4 inches down in the morning settles it. These crops are left out of the table on purpose.
Limits
- Offsets are typical, not guaranteed. A cold spring, a sheltered wall or a low frost pocket can move your real safe date by a week or more.
- Beans are listed from the frost date to one week after. The extension tables say simply "after the last frost", and the extra week is a cautious margin.
- Warm-season crops also want warm nights and warm soil. A date on the calendar does not warm the ground.
- Seedlings raised indoors need hardening off over 7 to 10 days before they go outside.
- This is general information, not advice for your site. Local extension planting calendars take precedence.
Where this comes from
The week offsets and seed-to-transplant times follow the University of New Hampshire Extension fact sheet on when to plant a vegetable garden, and were cross-checked against vegetable planting guides from Colorado State University Extension, the University of Maine Cooperative Extension and the University of Illinois Extension, which agree on the main crops: cool-season crops out 2 to 4 or more weeks before the last frost, tomatoes 1 to 2 weeks after, peppers about 2 weeks after, and beans after the last frost. Where sources gave a single figure, the tool uses that one figure for both ends of the window. The soil temperatures come from the same extension material. The date arithmetic was tested in code across many dates, including leap years.