How Seed Clock counts a date.

1. The frost dates

From NOAA's U.S. Climate Normals for 1991-2020, station USW00014820, Cleveland Hopkins International Airport (41.41°N, 81.86°W), about 13 miles west of Garfield Heights. NOAA gives the date by which the last spring night at or below 32°F has passed, and the first fall one has come, at chances from 10% to 90%. Seed Clock offers three: Bold (50%: spring Apr 20, fall Nov 3), Usual (30%: spring Apr 27, fall Oct 30), Careful (10%: spring May 5, fall Oct 21). The same calendar dates are used every year.

NOAA data file for USW00014820

2. The planting rules

From Montana State University Extension, MontGuide MT199308AG, "Can I Grow That Here? Vegetable Seed and Transplant Schedules for Garden or Container" (reviewed August 2013). The guide writes each vegetable's planting time relative to the last spring frost and the first fall frost, for example "1 week before last frost to 12 weeks before first frost". The guide was written for Montana; Seed Clock uses only its frost-relative timing and counts those weeks from the Cleveland Hopkins frost dates above. For Ohio-specific growing advice (soil, varieties, pests), see Ohio State University Extension at ohioline.osu.edu. We use the guide's planting windows, days to maturity, weeks to transplant size, frost tolerance and germination soil temperatures; the wording on this site is our own.

The guide (PDF)

3. Reading rules

• Where the guide gives a range for the start or end of a window ("8 to 10 weeks before first frost"), Seed Clock uses the outer end, so the window is the wider one. Each vegetable page shows the guide's own words beside the dates.
• "Mid-summer" in the guide (spinach and turnip) is read as the June solstice, June 21.
• A rule with a single date ("Fall: 11 weeks before first frost") is shown as "around" that date.
• For vegetables started indoors, the indoor window runs from the opening of the planting window minus the most weeks to transplant size, to the close of the window minus the fewest weeks.
• "Days to first harvest" counts from planting out for transplanted crops and from sowing for the rest, as the guide does. Two exceptions count from sowing indoors: celery, whose 125 days the guide gives "after direct seeding", and eggplant, whose 100-120 days the guide gives without saying from when (its last set-out date, 14 weeks before first frost, only leaves room for them if they count from seed). For those two the Garden Book uses your recorded indoor sowing date, or else the indoor window.

4. What Seed Clock does not claim

No yields, no variety advice, no pest or soil advice, and no Garfield Heights weather station. The dates are odds from a nearby long record, not a forecast. Check the forecast before setting out tender plants.