A crop’s latest safe planting date is the first autumn frost date minus its days to maturity minus a safety margin, and its earliest is set by the last spring frost and soil warmth. For example, a 100-day crop with a first frost on 15 October and a 14-day margin must be in the ground by about 23 June; valleys that run 5-10 C colder than nearby hilltops shift the dates by weeks.
Part of Caloric Budget for a Community
A planting and harvest window is the range of dates in which a crop can be sown so that it matures before frost and can be harvested and stored in good condition.
Frost Dates Are The Boundaries
Every temperate crop has two boundaries: the spring date when frost no longer kills seedlings and the soil is warm enough, and the autumn date when frost ends growth. Between them lies the growing season. On a calm night cold air drains downhill like water and pools in valleys, which can run 5 to 10 degrees C colder than nearby hilltops (Frost Prediction). The same field can have a different frost date in a hollow than on a slope.
Worked Example: A Latest Planting Date
This example uses illustrative inputs, not data for any real site or crop.
- First autumn frost: 15 October, day 288 of the year (273 days to the end of September plus 15).
- Crop days to maturity: 100.
- Safety margin for a cool spell or slow germination: 14 days.
- Latest planting day = 288 − 100 − 14 = 174, which is 23 June (151 days to the end of May plus 23).
- A frost on 5 October instead (day 278) moves the date earlier by 10 days to 13 June.
Replace the three inputs with your own: your first frost from several years of records, the crop’s real days to maturity in your climate, and your own margin.
What You Need
- A frost log kept for several years: last spring frost and first autumn frost, by field
- Days to maturity for each crop, from your own seed and trial
- Soil temperature or experience for the spring start
- A labour calendar (Harvest Labor Calendar)
Method: Set A Window
Step 1: From the frost log, take the usual dates, not the best case. Plan to the early end of autumn and the late end of spring. Step 2: Subtract days to maturity and the margin from the autumn date to get the latest planting. Step 3: Set the earliest planting by the last spring frost and the crop’s cold tolerance. Step 4: Spread plantings across the window so one cold week does not take everything, and so harvest labour is not all in one week. Step 5: Record actual planting, emergence and harvest dates each year and revise.
Seasonal Planning Links
Grain, roots and legumes have different windows, and rotation moves them each year (Three-Field Calendar). Where a sequence of crops is planned across the whole year, see Seasonal Planning. Extending the season with frames and covers is in Four-Season Growing.
Common Mistakes
- Using the average frost date. Half of years are earlier.
- Planting everything on the same day. One bad week then costs everything.
- Ignoring local cold pockets. Valley fields frost earlier.
- Planting at the last moment. Late-planted crops are small and yield less.
What’s Next
- Learn next: Winter Calorie Gap Planning — what the harvest has to cover
- Combine with: Seasonal Production Calendar — the whole year in one view