SEED STARTING CALCULATOR
When to start seeds indoorsFind your frost date by ZIP code, then count back.
Enter a US ZIP code to see NOAA frost dates for the nearest weather stations, or type your own last frost date. Seed Window works out sow-indoors, harden-off and plant-out dates for every crop you choose.
- NOAA 1991–2020 frost data
- 30 crops with cited sources
- No account needed
YOUR GROWING SEASON
A plan you can put to work.
Choose your date and crops to see the plan.
Choose your crops8 selected
Tap a crop to add or remove it. Timing comes from the source shown in the crop table.
Add your own crop or variety
Use the numbers on your seed packet. Custom rows are marked as your own assumption.
One task at a time.
Check off tasks here, or import the .ics file into Apple Calendar, Google Calendar or Outlook. Calendar imports and this checklist do not sync.
Season timeline
- Sow indoors
- Growing indoors
- Harden off
- Plant out
- Sow outside
- Last frost (avg)
- Today
When to start seeds indoors: count back from your last frost
Johnny’s Selected Seeds’ chart, which this calculator uses for transplanted crops, gives two numbers for each crop: how many weeks the seedlings need indoors, and when they can go outside relative to the last spring frost. Put them together and you have a sowing date. Tomatoes, for example, need 5 to 6 weeks indoors and go out 1 to 2 weeks after the frost date, so they are sown 3 to 5 weeks before it.
Type your ZIP code and Seed Window lists the three nearest NOAA weather stations with their 10%, 30%, 50% and 90% last-freeze dates. Choose the date that matches the risk you are willing to take and every crop in your plan moves with it.
Worked example: ZIP 10001, New York
- The nearest station is New York Central Park, 3.9 km away. Its 30% last-freeze date is April 3.
- Tomato: sow February 27 to March 13, plant out April 10 to 17.
- Pepper: sow February 20, plant out April 17.
- Choosing the more cautious 10% date (April 9) moves every task 6 days later.
Dates for spring 2027, using the crop rules in the seed chart below.
Your frost date is a probability, so pick your risk
NOAA’s 1991–2020 U.S. Climate Normals give, for thousands of weather stations, the date on or after which the last 32°F (0°C) freeze of spring still occurs with a given probability. A 10% date is cautious: one spring in ten has a freeze on or after it. A 50% date is the median, so a later frost is as likely as not. The Old Farmer’s Almanac uses the 30% date.
In the warmest places a freeze does not happen every year. NOAA puts the chance of 32°F at Tampa International Airport at about 35% of years, and its dates there apply only to the years that have a freeze; Seed Window shows that chance next to the dates. NOAA’s method needs at least a 10% yearly chance of a freeze to compute a date; Miami International Airport’s file has no date, and the lookup says so instead of borrowing one from a colder station.
A station is not your garden. Check the distance and elevation shown, follow your local extension service where it gives different advice, watch the 10-day forecast before planting out tender crops and keep row cover ready.
Outside the US, or want a second opinion?
These links open other sites.
- Old Farmer’s Almanac frost datesUnited States and Canada, by ZIP or postal code
- USDA NIFA extension mapFind your state’s land-grant university and county extension office
- Bureau of Meteorology potential frost daysAustralia: monthly frost-day maps rather than a single date
- Elsewhere: your national weather service, a local garden club or nursery usually publishes typical frost dates for your area. In the Southern Hemisphere a plan can cross New Year; Seed Window handles the change of year.
How the dates are worked out
- Plant-out dateYour frost date plus or minus the crop’s offset. Tomato: “1 to 2 weeks after”, so May 22 to May 29 for a May 15 frost date.
- Indoor sowing windowThe plant-out window minus the weeks indoors. Tomato: 5 to 6 weeks, so sow Apr 10 to Apr 24.
- Hardening offStarts 10 days before the earliest plant-out date. Nebraska Extension suggests 7 to 10 days, adding outdoor time each day.
- Direct sowingCrops sown outside use CSU Extension’s “as early as” window. For bean and corn (“around” the frost date) the bar runs from the frost date to one week after.
All arithmetic uses calendar days, so leap years and the change of year are exact. A single plant-out date is drawn as a diamond: it is the earliest safe date, and later is fine.
Seed starting chart: when to start 30 crops indoors
“Sow indoors” is counted back from the last frost date using the weeks indoors and plant-out timing quoted in the next columns (Johnny’s and CSU checked 2026-10-19). The last column is a cross-check from University of Minnesota Extension’s seed-starting table (read 2026-11-08); it does not change your plan. To follow UMN’s numbers instead, add the crop under “Add your own crop or variety” with those weeks.
| Crop | Sow indoors (before last frost) | Weeks indoors | Plant out or sow outside, relative to frost | Source | UMN Extension: weeks indoors |
|---|---|---|---|---|---|
| Tomato | 3–5 weeks before | 5 to 6 | “1 to 2 weeks after” | Johnny's Seeds Seed company | 5-6 |
| Pepper | 6 weeks before | 8 | “2 weeks after” | Johnny's Seeds Seed company | 9 |
| Eggplant | 3–6 weeks before | 6 to 8 | “2 to 3 weeks after” | Johnny's Seeds Seed company | 9 |
| Okra | 4 weeks before to frost date | 4 to 6 | “2 to 4 weeks after” | Johnny's Seeds Seed company | 9 |
| Cucumber | 1–3 weeks before | 3 to 4 | “1 to 2 weeks after” | Johnny's Seeds Seed company | not listed |
| Melon | 1–2 weeks before | 3 to 4 | “2 weeks after” | Johnny's Seeds Seed company | not listed |
| Watermelon | 1–2 weeks before | 3 to 4 | “2 weeks after” | Johnny's Seeds Seed company | not listed |
| Squash | 1–2 weeks before | 3 to 4 | “2 weeks after” | Johnny's Seeds Seed company | not listed |
| Pumpkin | 1–2 weeks before | 3 to 4 | “2 weeks after” | Johnny's Seeds Seed company | not listed |
| Broccoli | 5–6 weeks before | 3 to 4 | “2 weeks before” | Johnny's Seeds Seed company | 10-11 |
| Cabbage | 8–10 weeks before | 4 to 6 | “4 weeks before” | Johnny's Seeds Seed company | 10-11 |
| Cauliflower | 6–8 weeks before | 4 to 6 | “2 weeks before” | Johnny's Seeds Seed company | 10-11 |
| Kale | 8–10 weeks before | 4 to 6 | “4 weeks before” | Johnny's Seeds Seed company | not listed |
| Lettuce | 7–9 weeks before | 4 to 5 | “3 to 4 weeks before” | Johnny's Seeds Seed company | 10-11 (head lettuce) |
| Swiss chard | 7–8 weeks before | 5 to 6 | “2 weeks before” | Johnny's Seeds Seed company | not listed |
| Celery & celeriac | 9–11 weeks before | 10 to 12 | “1 week after” | Johnny's Seeds Seed company | 12-13 |
| Spinach | Sow outside | — | “As early as 2-4 weeks before the date of the average last spring frost” | CSU Extension University extension | not listed |
| Onion | 12–14 weeks before | 8 to 10 | “4 weeks before” | Johnny's Seeds Seed company | 14-15 |
| Leek | 10–12 weeks before | 8 to 10 | “2 weeks before” | Johnny's Seeds Seed company | 14-15 |
| Carrot | Sow outside | — | “As early as 0-2 weeks before the date of the average last spring frost” | CSU Extension University extension | not listed |
| Beet | Sow outside | — | “As early as 0-2 weeks before the date of the average last spring frost” | CSU Extension University extension | not listed |
| Radish | Sow outside | — | “As early as 2-4 weeks before the date of the average last spring frost” | CSU Extension University extension | not listed |
| Pea | Sow outside | — | “As early as 2-4 weeks before the date of the average last spring frost” | CSU Extension University extension | not listed |
| Snap bean | Sow outside | — | “From seed: around the date of the average last spring frost” | CSU Extension University extension | not listed |
| Sweet corn | Sow outside | — | “From seed: around the date of the average last spring frost” | CSU Extension University extension | not listed |
| Basil | 5 weeks before | 6 | “1 week after” | Johnny's Seeds Seed company | not listed |
| Parsley | 8–9 weeks before | 9 to 10 | “1 week after” | Johnny's Seeds Seed company | not listed |
| Thyme | 7–9 weeks before | 8 to 10 | “1 week after” | Johnny's Seeds Seed company | not listed |
| Oregano | 7–9 weeks before | 8 to 10 | “1 week after” | Johnny's Seeds Seed company | not listed |
| Sage | 5–7 weeks before | 6 to 8 | “1 week after” | Johnny's Seeds Seed company | not listed |
| Hardening off (all transplants) | — | — | “7-10 day” period before setting out | Nebraska Extension University extension | “1-2 weeks” |
Seed starting questions
When should I start seeds indoors?
Count back from your last spring frost date. In the seed chart on this page, onions start earliest, 12–14 weeks before the frost date; peppers 6 weeks before; tomatoes 3–5 weeks before; cucumbers 1–3 weeks before; and squash and melons 1–2 weeks before. Enter your ZIP code or frost date above to turn those offsets into calendar dates for your crops.
How does the ZIP code lookup find my frost date?
It takes the Census Bureau’s 2024 internal point for your ZIP Code Tabulation Area and lists the three nearest weather stations with published freeze dates in NOAA’s 1991–2020 U.S. Climate Normals, with distance and elevation. The dates are NOAA’s published 32°F freeze probabilities for that station, unchanged. Only the first three digits of your ZIP are used to load the data file; the full ZIP never leaves your browser.
What do the 10%, 30% and 50% frost dates mean?
NOAA gives, for each probability, the date on or after which the last 32°F freeze of spring still occurs that often. At New York’s Central Park station there is a 10% chance of a freeze on or after April 9, 30% on or after April 3 and 50% on or after March 30. The Old Farmer’s Almanac uses the 30% date. Pick 10% if losing tender transplants to a late frost would set you back.
When should I start tomato seeds indoors?
Johnny’s Selected Seeds gives tomatoes 5 to 6 weeks indoors and a plant-out date 1 to 2 weeks after the last frost, which means sowing 3 to 5 weeks before your frost date: April 10 to 24 for a May 15 frost date. University of Minnesota Extension’s table also keeps tomatoes indoors for 5–6 weeks. The tomato and pepper report works through both crops.
Why do seed starting charts disagree?
Johnny’s lists 3–4 weeks indoors for broccoli and 4–6 for cabbage, while UMN Extension’s table, a month-by-month calendar for Minnesota, lists 10–11 weeks for both. The chart shows both so you can see the gap. If you follow a different source, add the crop under “Add your own crop or variety” with your numbers.
Is my USDA hardiness zone the same as my frost date?
No. USDA hardiness zones are based on the average annual extreme minimum winter temperature. They tell you which perennials can survive winter, not when the last spring frost usually happens. Use a frost date for seed starting; the frost date vs hardiness zone report explains the difference.
My ZIP code isn’t found, or the nearest station looks wrong.
The Census Bureau notes that not every valid ZIP code has a ZIP Code Tabulation Area. For those, Seed Window shows the stations nearest the average point of the ZIP areas that share your first three digits, and says so. A station can sit lower, higher, nearer the coast or more urban than your garden, so check the distance and elevation shown and choose the station most like your site, or type your own date.
WORKED REPORTS
Understand the dates before you plant.
Worked calculations, real examples and source checks for the decisions behind your plan.