Blog · Agriculture
Growing degree days explained
Growing degree days (GDD) are a way of measuring accumulated heat so that you can estimate how far a crop or an insect has developed. For each day you take the average of the maximum and minimum temperature, subtract a base temperature below which the organism does not develop, and add up the results over the season.
The idea rests on a simple observation. Plants and insects do not develop according to the calendar. They develop according to temperature, within limits. A warm spring brings a crop to flowering sooner than a cool one, even if both were sown on the same date. Counting heat instead of days gives a steadier clock.
Other names for the same idea include heat units, thermal time and growing degree units.
The formula
The daily calculation is:
GDD = ((Tmax + Tmin) / 2) − Tbase
where Tmax is the day's maximum temperature, Tmin is the day's minimum, and Tbase is the base temperature for the organism in question. If the result is negative, it is set to zero. A cold day adds nothing, but it does not take anything away either.
You then add the daily values together from a starting point, often the sowing date, emergence, or a fixed date used for a pest model. The running total is what gets compared with a published figure for a growth stage.
Base temperatures vary by crop
The base temperature is the point below which development is taken to stop. It differs between species, and different sources sometimes use slightly different values for the same crop.
- Maize is commonly calculated with a base of 10 °C (50 °F).
- Cool-season cereals such as wheat and barley are usually given a much lower base, often at or near 0 °C.
- Insect pests each have their own base temperature, set out in the model for that pest.
Always use the base temperature that goes with the target figure you are comparing against. A total calculated with one base cannot be compared with a threshold published for another.
A worked example
Take maize with a base of 10 °C.
| Day | Tmax | Tmin | Average | GDD |
|---|---|---|---|---|
| Mild day | 28 °C | 14 °C | 21 °C | 11 |
| Cold day | 12 °C | 2 °C | 7 °C | 0 (the result is −3, so it is set to zero) |
Over those two days the total is 11.
The modified method and upper cut-offs
The simple average has two weaknesses. On a cold night the minimum drags the average down, even though the crop grew well during the warm afternoon. On a very hot day the maximum pushes the average up, even though development does not keep speeding up in extreme heat.
The modified method deals with this by adjusting the temperatures before averaging. For maize the usual version is:
- If Tmin is below 10 °C, use 10 °C.
- If Tmax is above 30 °C (86 °F), use 30 °C.
Then apply the formula as normal. This is sometimes called the 86/50 method after the Fahrenheit limits.
Here is how the two methods compare on the same days:
| Day | Tmax | Tmin | Simple GDD | Modified GDD |
|---|---|---|---|---|
| Hot day, cool night | 34 °C | 8 °C | 11 | 10 (uses 30 and 10) |
| Cold day | 12 °C | 2 °C | 0 | 1 (uses 12 and 10) |
The daily differences are small, but they add up over a season. The rule is the same as for base temperatures. Use the method that was used to produce the target figure you are working to, and do not mix the two.
Celsius and Fahrenheit totals are not the same
A GDD total calculated in Fahrenheit is 1.8 times the total calculated in Celsius, because a Fahrenheit degree is smaller. A maize hybrid rated in Fahrenheit units will have a much larger figure than the same hybrid rated in Celsius units. Check the units before comparing anything.
What growing degree days are used for
- Crop staging. Estimating when a crop will reach emergence, a given leaf stage, flowering or maturity.
- Variety choice. Seed suppliers often rate varieties or hybrids by the heat units needed to reach maturity, which can be compared with what a location typically provides.
- Pest and disease timing. Many insect models predict egg hatch or adult emergence from accumulated degree days, which helps to time scouting.
- Planning field work. Estimating when a growth stage that matters for an operation is likely to arrive.
- Comparing seasons. Seeing whether this season is running ahead of or behind a typical one.
In each case the total tells you when to go and look. It does not replace looking.
The limits of the method
Growing degree days are a model, and a deliberately simple one.
- Temperature is not the only driver. Day length, water stress, nutrition, waterlogging and variety all affect development. Some crops need a period of cold before they will flower, which heat totals do not capture.
- The average of two readings is an approximation. The real temperature curve over a day is not symmetrical. More detailed methods use hourly data or fit a curve to the day.
- Air is not soil. Before and shortly after emergence, the growing point of many crops is at or below the soil surface, where soil temperature matters more than air temperature.
- The temperature source matters. A station some distance away may run warmer or cooler than your field. A steady bias of one degree adds or removes about one Celsius GDD every day, which becomes a large gap by midsummer. See our guide to microclimate and weather stations.
- Thresholds are averages. Published totals for a growth stage come from trials. Individual fields and seasons scatter around them.
Doing it yourself
You need a daily maximum and minimum for your location, a start date, the right base temperature and the right method. A spreadsheet with one row per day is enough. Many national weather services and extension services also publish degree day calculators for their region.
Treat the result as a guide to timing. Decisions about crop inputs and operations should follow the product label and the advice of your agronomist or local extension service, which take precedence over any calculated figure.
Where Prism fits
Prism Microclimate, built with FarmGrid, covers 961k fields across one whole state and includes growing degree days alongside field-level weather, soil moisture, frost, spray and field-work windows, and a morning brief per field. There is more on our agriculture page. If you are interested in how farm data fits together more broadly, see what a digital twin of a farm looks like.