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USDA Zones, Frost Dates & Calendar FoundationsGrowing Degree Days for Crop Scheduling
Growing degree days measure accumulated usable heat, which is closer to how plants and insects actually develop than the calendar is. This guide explains the math and the practical uses.
Key points
- Degree days track usable heat, not calendar days.
- Match the base temperature to the crop or pest.
- Use extension-published targets for your region and variety.
- Heat is only one factor, so confirm with field observation.
What are growing degree days?
A growing degree day is a unit of heat. For each day, you take the average of the high and low temperature and subtract a base temperature below which the crop makes little progress. Add up the daily values from a start date and you have a running total.
Crops and pests need a fairly consistent total to reach a given stage. That is why two seasons with different calendars can still bring the same stage at the same degree day count.
Why does the base temperature matter?
Different crops use different base temperatures. Cool-season crops start growing at lower temperatures than warm-season crops, so using the wrong base gives misleading totals. Extension sources list the base used for each crop or pest.
Many calculators also cap very high temperatures, since heat beyond a point does not speed growth. If you use an online tool, read how it handles the upper limit and which start date it assumes.
How do you calculate them step by step?
Pick a start date, often January 1 or the planting date, depending on the model. Each day, add the high and low, divide by two, and subtract the base. If the result is negative, count it as zero. Keep a running total in a notebook or spreadsheet.
For example, if a day has a high of 70 and a low of 50 and the base is 50, the average is 60 and the day adds 10 degree days. Your extension service can tell you which method it uses, since small differences exist between the simple average and the sine-wave methods.
Some university tools let you enter a ZIP code or station and read off accumulated degree days with a chosen base and start date. These save arithmetic, but check that the base and start date match the crop or pest you are tracking before you trust the number.
- Choose the base temperature for your crop or pest
- Decide the start date the model expects
- Cap or floor daily values as the method directs
How do you use degree days to schedule plantings?
Seed companies and extension programs often give days to maturity, but degree day targets can be more accurate in unusual years. If you know the total a variety needs, you can stagger sowings so harvests land when you want them, rather than all at once.
For sweet corn or processing crops, degree day tracking helps time successive plantings. Check your state extension for the published targets, and do not borrow a number from another region or variety.
How do degree days help with pests and scouting?
Many insects emerge or reach a damaging stage at known accumulated totals. University pest programs publish these thresholds so growers know when to scout, not when to spray by habit.
This article is education only. Control decisions depend on the pest, the crop, and the product label, which is the law. Ask extension for current recommendations, observe pre-harvest intervals, and protect pollinators during bloom.
What are the limits of the method?
Degree days measure heat only. Drought, waterlogging, day length, and nutrition all affect development too. A crop can lag its predicted stage in a dry spell even when heat totals are on track.
Treat the number as a guide. Compare it with what you see in the field, and keep notes so you can refine your own targets year to year.
What does a season of record keeping look like?
Write the start date, the base, and the running total in one column each week. Beside it, note what you observed: emergence, first flower, first harvest, the first sighting of a pest. By the end of the season you will have a table linking field events to totals.
Next year, set alerts for yourself a little before those totals arrive. For example, if first harvest came a few days after a certain total, begin harvest preparations when the running number is approaching it. Over several years the table becomes your own calibrated schedule. Writing the numbers down matters more than the tool you use. A column in a notebook, filled in each week through the season, is enough to build a personal reference. After a few years, you may find that your site runs consistently ahead of or behind the regional average, and that offset is worth knowing. Share the table with neighbors who grow the same crops, since comparing notes across several sites makes the pattern clearer and shows which differences come from the weather and which from the site itself.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.