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Regional Calendars, Climate & DataComparing Regional Planting Calendars
Two planting calendars can look similar and disagree by weeks. Before you compare them, you need to know what each one was built on.
Key points
- Regional calendars differ because of climate, risk level, threshold and intended use.
- Ask what frost risk level, threshold and years of data each calendar uses.
- Convert to relative timing, such as weeks from your last frost, to compare fairly.
- Use other regions for ideas, and your own records and extension for dates.
- Use relative timing, not absolute dates, whenever you borrow from another calendar.
Why do planting calendars from different regions disagree?
Calendars reflect local climate, and they also reflect local choices. One may list the earliest date that works in a warm year, while another lists a conservative date meant to avoid replanting. A third may be written for home gardens, with protection assumed, and a fourth for field crops with insurance dates in mind.
University of California guidance on planting times says directly that regional planting dates are approximate and that your own site may differ, even within a small area.
Another source of disagreement is the audience. A calendar for a home garden may assume row covers and frequent watering, while a calendar for a farm with limited labor may assume none.
What should you check before comparing two calendars?
Ask four questions of each calendar. What frost risk level does it assume: the 90, 50 or 10 percent date? What threshold temperature: 36, 32 or 28 degrees F? Which years of data: the NOAA normals now in use cover 1991 to 2020, and older ones cover 1981 to 2010. And is it written for transplants, direct seeding, or both?
If the source does not say, treat its dates as rough guidance and verify with extension.
A quick way to test two calendars is to ask each one the same question: what is the earliest date you would plant a tender crop, and how often would a freeze catch it? A source that cannot answer is giving you a guess.
- Risk level and threshold temperature
- Years of data behind it
- Direct seed or transplant
- Home garden, market farm or field crop
How can you put two regions on the same footing?
Convert dates to relative timing: weeks before or after the last spring frost at a chosen risk level, or the soil temperature at which the crop is planted. For example, a calendar that says plant a crop two weeks after the average last frost can be applied anywhere if you know your own frost date and risk level.
Relative timing travels well. Absolute dates do not. Write the conversion down so you can reuse it each season, such as two weeks after your chosen last frost date.
What do soil temperature and heat units add?
Soil temperature tells you when seed can germinate, and growing degree days tell you whether a crop can finish before fall frost. Iowa State Extension advises planting corn once soil is above 50 degrees F and expected to keep rising, rather than by a fixed date, and Kansas State and Ohio State publish guides to calculating growing degree days.
If two regions have similar frost dates but different heat accumulation, a variety that suits one may fall short in the other.
Length of day and heat also differ between regions, and some crops respond to both, so dates that work in one place may produce poor results in another.
When is copying another region a bad idea?
It fails most often when the other region has a different rainfall pattern, a different elevation, or a different distance from large water. A coastal calendar and an inland calendar at the same latitude can differ by weeks, because cool coastal soils warm slowly, as Oregon State Extension describes.
Use outside calendars for ideas and crop lists, not for exact dates.
Differences in day length also matter for some crops, so a calendar from far to the north or south can mislead even when frost dates match.
How do you make a comparison useful?
Put your own frost probabilities, soil temperature readings and past planting dates next to the calendar you are testing. Where they disagree, ask why. Often the answer is a different risk level or threshold, which you can adjust for.
Then check the result against a county extension planting guide, and plant a small test block before committing a large area. Ask your extension office which published calendar fits your area best before you compare any others.
How do you record the comparison for future use?
Make a small table with one column per calendar, and rows for the crop, the planting date, the assumed risk level and the source. Add a final column for the date you actually used and how it worked out.
After a season, you will know which calendar tends to run early, which runs late and which came closest to your farm. That knowledge is worth more than any single table.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.