Tool · seed starting
Seed starting calculator: sow, transplant and harvest dates
A seed starting calculator counts backwards from your last spring frost. For a zone 7a garden with a 20 April frost, tomato seed goes indoors on 2 March, the seedlings go out on 20 April, and the first fruit is ready around 20 June. Type your own frost date, pick a crop below, and the three dates update as you change the inputs.
How the seed starting calculator works
Type a five-digit ZIP code, pick a USDA zone, or enter your last frost date. Then pick one of nine vegetables and the calculator does three sums: it counts the seed-starting lead time back from the frost date, adds the transplant offset for that crop, and adds the days to maturity from the transplant date. The output is three dates — sow indoors, set out, first harvest — plus the basis it used, so you can check the arithmetic.
Basis: Tomato, last frost 20 Apr 2027 (zone 7a). Sowing 7 weeks before; set out at the last frost date.
Harvest windows are counted from the set-out date (see the Montana State table and the University of Maine note on when days-to-maturity starts). Next calendar for your zone: Zone 7 planting calendar.
Crop timing the calculator uses
Weeks before the last frost come from the University of Maine Extension seed-starting table. The set-out offset and the days to maturity come from Montana State University Extension: hardy crops go out two to three weeks before the last frost, half-hardy crops two weeks before, and warm-season crops at the frost date. Days to maturity are counted from set-out, not from sowing.
| Crop | Sow indoors (weeks before frost) | Set out | Days to maturity |
|---|---|---|---|
| Tomato | 6–8 | at last frost | 60–85 |
| Pepper | 8–10 | at last frost | 70–80 |
| Eggplant | 8–9 | at last frost | 60–80 |
| Cucumber | 2–3 | at last frost | 50–60 |
| Broccoli | 5–7 | 2 weeks before | 70 |
| Cabbage | 5–8 | 2–3 weeks before | 60–70 |
| Cauliflower | 5–8 | 2 weeks before | 50–55 |
| Lettuce (head) | 3–5 | 2–3 weeks before | 70–80 |
| Onion (bulb) | 6–8 | 2–3 weeks before | 100 |
Seed starting calculator dates vs cooperative extension advice: 10 gardens
Ten garden inputs — a state, a ZIP code, a bed size and a crop — were run through the calculator on 6 November 2026. For every row the calculator was given the last-frost date that the state's extension program publishes for that garden, so the test measures the crop rules, not the frost lookup. The extension column is the published planting or sowing window for that crop and region; the calculator column is the single date it produced.
| Garden input | Crop | Extension window | Calculator | Difference | Source |
|---|---|---|---|---|---|
| Virginia · Charlottesville 22902 4 × 8 ft raised bed | Tomato | 20 Apr 2026 – 20 Jun 2026 transplant | 20 Apr 2026 | in window | Virginia Cooperative Extension 426-331 |
| Virginia · Charlottesville 22902 4 × 8 ft raised bed | Broccoli | 20 Mar 2026 – 1 May 2026 transplant | 6 Apr 2026 | in window | Virginia Cooperative Extension 426-331 |
| New York · Ithaca 14850 3 × 10 ft bed | Tomato | 25 May 2026 – 20 Jun 2026 transplant | 20 May 2026 | 5 days early | Cornell Vegetable Program, upstate New York |
| New York · Ithaca 14850 3 × 10 ft bed | Pepper | 30 May 2026 – 20 Jun 2026 transplant | 20 May 2026 | 10 days early | Cornell Vegetable Program, upstate New York |
| Arkansas · Little Rock 72201 4 × 4 ft bed | Broccoli | 6 Mar 2026 – 20 Mar 2026 transplant | 20 Mar 2026 | in window | University of Arkansas Cooperative Extension (White County) |
| Arkansas · Little Rock 72201 4 × 4 ft bed | Lettuce (head) | 6 Mar 2026 – 13 Mar 2026 transplant | 17 Mar 2026 | 4 days late | University of Arkansas Cooperative Extension (White County) |
| Utah · St. George 84770 3 × 6 ft bed | Lettuce (head) | 15 Feb 2026 – 15 Mar 2026 transplant | 15 Mar 2026 | in window | Utah State University Extension (Washington County) |
| Utah · St. George 84770 3 × 6 ft bed | Tomato | 1 Apr 2026 – 30 Apr 2026 transplant | 1 Apr 2026 | in window | Utah State University Extension (Washington County) |
| Minnesota · St. Paul 55101 4 × 8 ft raised bed | Tomato | 21 May 2026 – 10 Jun 2026 transplant | 5 May 2026 | 16 days early | University of Minnesota Extension, garden timing |
| Iowa · Ames 50010 4 × 12 ft bed | Tomato | 15 Mar 2026 – 21 Mar 2026 sow indoors | 19 Mar 2026 | in window | Iowa State University Extension (Yard and Garden) |
What the 10-garden test shows
The calculator landed inside the extension window in 6 of the 10 gardens. All four misses were on the early side except one: a New York tomato 5 days early, a New York pepper 10 days early, a Minnesota tomato 16 days early, and an Arkansas lettuce 4 days late. Measured from the nearest edge of the published window, the mean deviation was 3.5 days and the median was 0 days, because six rows sit inside their window.
The pattern behind the misses is season, not arithmetic. The three early misses are all warm-season crops in states whose extension advice waits for warm soil or a later planting window — Cornell's upstate New York tomato window opens 25 May, five days after the local frost date, and Minnesota Extension sends tomatoes out in late May to early June, not at the early-May frost date. The one late miss is a lettuce transplant, where Arkansas Extension recommends setting plants out 3–4 weeks before the frost and the calculator uses the 2–3 weeks its Montana source gives for hardy crops.
On the lead-time side, the calculator's midpoint sits inside the Arkansas range for 5 of the 6 crops Arkansas publishes: tomato 7 weeks (published 6–8), pepper 9 (8–10), broccoli 6 (6–8), cabbage 6.5 (6–8) and lettuce 4 (4–6). The exception is eggplant: the calculator uses the University of Maine range of 8–9 weeks, half a week longer than Arkansas's 6–8. Two extension programs disagreeing by a week on one crop is normal; the ±2 week control exists for exactly that.
Sample and method: 10 inputs (6 states, 6 crops, bed sizes from 3 × 6 ft to 4 × 12 ft), extension pages fetched 6 November 2026, calculator run on the same date. A row counts as inside when the calculator date falls between the first and last day of the published window. Rows whose extension advice is relative to frost (Arkansas, Minnesota) were converted to dates with the frost date the same source publishes. Full rows, quotes and source URLs are listed below.
Where the calculator can be wrong
- A zone is not a garden. The ZIP lookup returns one USDA zone, but a wall, a slope or a city centre can shift the effective zone by half to a full zone — the site's own zone lookup page says the same. Typing your own frost date beats the zone table.
- Frost is not soil temperature. Tomato, pepper and eggplant set fruit poorly when nights stay cool even after the last frost. The Montana State and Minnesota State guidance both wait for warm soil, and the calculator's frost-date rule does not measure soil.
- Six zone halves only. The automatic ZIP conversion covers 6a to 8b because that is the range of the extension table behind it. Zone 5b and colder, and zone 9a and warmer, need a frost date typed in.
- Nine crops, not every crop. Onion is grown from seed here; garlic, potatoes and shallots are not in the table, and neither are herbs. For those, the local extension date is the better source.
- Southern hemisphere. The count-back rule still works from a last frost date, but the month names invert: the site's Melbourne and Sydney calendars run the other way round.
Seed starting calculator FAQ
What is a seed starting calculator?
A seed starting calculator turns your last spring frost date into three dates for a crop: when to sow seed indoors, when to move the seedling outside, and when the first harvest is likely. It works backwards from the frost date instead of from a fixed calendar, because the same tomato variety needs a different sowing week in St. Paul and in Little Rock.
How does the calculator find my last frost date from a ZIP code?
The site's own ZIP lookup returns a USDA zone, and the calculator converts zones 6a to 8b into a typical last frost date using a Virginia Cooperative Extension table: zone 6a May 5–15, 6b April 25–May 5, 7a April 15–25, 7b April 5–15, 8a April 1–15, 8b March 15–April 1. Outside those six halves you type the frost date yourself; your county extension office publishes frost normals for your county.
How many weeks before the last frost should I start tomato seeds?
Six to eight weeks. University of Maine Extension lists 6–8 weeks for tomatoes, and Arkansas Cooperative Extension gives the same range. The calculator uses the middle of the published range, so 7 weeks before your frost date.
Can I start seeds earlier than the calculator says?
Yes, but earlier sowing does not mean an earlier harvest. A seedling that outgrows its pot before the soil warms up stalls after transplanting, and tomato, pepper and eggplant fruit set needs warm nights as well as frost-free days. Minnesota Extension advises waiting for soil at 65°F for heat-loving crops, which in southern Minnesota is late May to early June. The ±2 week control shifts every date for a cold or warm local garden.
Why does my state extension give a different date?
Because the comparison is against a window, not a single day, and because frost dates vary within a zone. In the 10-garden test on this page the calculator landed inside the extension window 6 times out of 10; the four misses were 5, 10 and 16 days early and 4 days late. The 16-day miss was a Minnesota tomato, where the extension advice waits for warm soil rather than the last frost date.
Does the calculator work outside the United States?
Enter your own last frost date and it works: the crop rules are measured from the last frost, not from a US calendar. The ZIP lookup is US-only, and the garden calendar pages on this site cover zone 7, zone 8b, Austin, Atlanta, Denver, London, Melbourne and Sydney.
Sources, fetched 6 November 2026
- Virginia Cooperative Extension 426-331 (fetched 6 November 2026)
- University of Maine Extension (Cumberland County) (fetched 6 November 2026)
- Montana State University Extension, home vegetable garden (fetched 6 November 2026)
- Cornell Vegetable Program, upstate New York (fetched 6 November 2026)
- University of Arkansas Cooperative Extension (White County) (fetched 6 November 2026)
- Utah State University Extension (Washington County) (fetched 6 November 2026)
- University of Minnesota Extension, garden timing (fetched 6 November 2026)
- Iowa State University Extension (Yard and Garden) (fetched 6 November 2026)
Page reviewed 6 November 2026. The calculator is a rule-of-thumb tool; your local extension office has county frost normals and soil-temperature guidance.