Growing Degree Days Calculator
Accumulate growing degree days from your temperatures, and see how long a crop target takes.
Plants do not read calendars; they read heat. Growing degree days (GDD) add up each day’s warmth above a base temperature, and crops hit their stages (germination, flowering, harvest) at characteristic GDD totals wherever they are grown. This calculator turns your average daily highs and lows into GDD per day, the total over a period, and the days needed to reach a target, with the sweet corn milestones that gardeners use most.
Enter recent or forecast average temperatures. The base is 50 F for most warm season crops; peas and cool crops use 40 F, and some tropical crops 55 to 60 F.
How this is worked out
One day’s GDD = (high + low) / 2 minus the base temperature, floored at zero: a day averaging at or below the base contributes nothing, which is the mathematical form of ‘the crop just sat there’. The corn method also caps the high at 86 F, because corn development stops accelerating above it and heat beyond that is stress, not progress. Totals are what matter: a crop stage arrives when its accumulated GDD does, in a hot June or a slow one.
Assumptions:
- Averaged highs and lows stand in for daily records. Real accumulation from daily data differs slightly; for planning, the average method is within a few percent over two weeks.
- Base temperatures are crop physiology, not preference: 50 F underlies most warm season vegetables and corn, 40 F the cool crops, 55 to 60 the tropical ones.
- GDD predicts development, not health: drought, waterlogging and infertility stall crops that the heat sum says should be moving.
Worked examples
1. A warm fortnight: highs 85, lows 65, base 50. Mean 75, so 25 GDD per day, 350 over the fortnight. Sweet corn sown at the start emerges (90 GDD) before the first week ends.
2. A cold spring week: highs 58, lows 42. Mean 50, base 50: zero GDD a day. The calendar advanced a week; the corn did not, and the replant decision writes itself from this number.
3. Chasing 1,400 GDD for sweet corn harvest. At 25 per day it takes 56 days; at a cooler site’s 18 per day, 78 days. Same variety, same sowing date, three weeks apart at harvest: GDD is why.
Base temperatures by crop group
| Group | Base | Notes |
|---|---|---|
| Peas, spinach, cool crops | 40 F | They grow in weather warm crops call cold |
| Most vegetables, corn | 50 F | The standard garden base; corn also caps at 86 F |
| Beans, squash, cucumbers | 50 to 55 F | Use 50 for planning, expect checks below 55 nights |
| Okra, sweet potato, tropical crops | 55 to 60 F | They mark time until real warmth arrives |
Seed catalogues increasingly print GDD to harvest beside days to harvest for corn and some vegetables; where both are printed, the GDD figure travels between gardens and the day figure does not.
Common mistakes
- Using one base for everything. A pea accumulating at base 50 looks frozen while it is actually growing at base 40 arithmetic. Match the base to the crop group.
- Forgetting the floor at zero. Cold days do not subtract GDD; development pauses, it does not reverse. Negative daily values are a calculation error, not a setback model.
- Expecting GDD to rescue drowned or starved crops. Heat is one input. A crop short of water or nitrogen accumulates the days and not the growth.
- Comparing GDD across cap methods. Corn figures with the 86 F cap and uncapped figures diverge in hot climates. Compare like with like.
Putting the result to work
Track GDD from sowing for corn and you will call harvest within days by the third season. The same totals explain transplant behaviour: the Harvest Date Calculator gives calendar expectations, and GDD tells you whether this year’s weather is running ahead of or behind them. Insect pests also run on GDD, which is the advanced use: many extension services publish pest emergence by accumulated GDD for your region.
How do I calculate growing degree days?
- Enter your average daily high and low for the period (recent days or a forecast stretch).
- Set the base temperature for the crop group.
- Optionally cap highs at 86 F for the corn method.
- Read GDD per day, the period total, and days to reach your target.
- Compare against crop milestones: corn emergence near 90, harvest near 1,300 to 1,500 (base 50).
What are growing degree days in plain terms?
A running total of warmth. Each day contributes its average temperature minus a base below which the crop does not develop, and crop stages (emergence, flowering, ripening) happen at characteristic totals. Two gardens with different weather reach the same total on different calendar dates, which is why GDD predicts better than days.
What base temperature should I use?
50 F for most warm season vegetables and sweet corn, 40 F for cool season crops like peas, and 55 to 60 F for tropical crops like okra. When a catalogue prints a GDD figure, it states its base; use that base with that figure.
Why cap the high at 86 F for corn?
Corn development speeds up with warmth only to about 86 F; above that, extra heat adds stress, not growth. The standard corn GDD method therefore caps the daily high at 86 (and floors the low at 50). In mild climates the cap rarely binds; in hot ones it changes totals materially.
Can GDD tell me when pests arrive?
Yes, and professionals use it exactly so: many insects emerge or hatch at published GDD accumulations for a region. Your extension service’s GDD tracker, matched to these same calculations with local weather, times scouting and controls far better than calendar dates.
My crop is behind its GDD schedule. What is wrong?
If the heat total is genuinely reached but the crop lags, the limit is elsewhere: water, nitrogen, root damage, or a variety whose printed figure assumes different conditions. GDD isolates the weather variable, so a lag with the heat accounted for is a useful diagnosis in itself.
Method (daily mean minus base, floored at zero; corn cap 86 F) follows the standard growing degree day convention used by US extension and the seed trade.
