Climate

Growing degree days

Plants and insects grow faster in warm weather and slower in cool weather. A growing degree day, or GDD, counts that warmth, so extension services time crabgrass control, pest scouting and plantings by degree days instead of the date. This guide covers how to work them out, which base temperature to use, when to start counting, and how to put them to work. The math is the same as the Degree Day Calculator.

Updated October 2026 6 min read By Connor Schmitz

Degree days

Every crop and insect has a base temperature, the point below which it barely develops. For sweet corn and many insects that base is 50 °F. A degree day is one day spent one degree above the base. Most extension models work it out from the day's high and low:

GDD = (high + low) ÷ 2 − base

A day that averages below the base counts as zero, never as a negative. UConn Extension gives two days from a sweet corn season:

Cool spring day 5 GDD High 62, low 48. The average is 55, which is 5 above the 50 °F base.

Summer day 21 GDD High 82, low 60. The average is 71, so 21 above the base.

One summer day gives the crop four times the heat of that spring day. Add the days up from a start date and you get a running total. A crop or insect reaches each stage at about the same total every year, whatever the date.

Base temperature

Each model is built on one base, and its numbers only work with that base. Most crops and pests in extension models use 50 °F. Cool-season plants and some insects use a lower one. The calculator sets the right base for each preset.

Corn adds two limits. Purdue's method caps the day's high at 86 °F and raises a low below 50 up to 50 before averaging. A day of 90 and 72 counts as (86 + 72) ÷ 2 − 50 = 29 GDD, not 31. The calculator's pest models leave the low alone, so a cold spring night does not count heat the insect never had.

Try it

Set a day's high and low

High90 °F Low72 °F

Base 5031

Corn, 86/5029

For corn, the 90 high is capped at 86, so it counts 2 fewer.

Average method, as in the Degree Day Calculator.

Most common mistake

Mixing Fahrenheit and Celsius totals. A degree day in Celsius is 1.8 degree days in Fahrenheit, and the base changes too: 10 °C is 50 °F. A turf study that says 200 degree days base 0 °C means 360 base 32 °F. Check both the unit and the base before comparing to a published number.

Quick check

A day hits 95 °F with a low of 70. How many corn degree days is that, by Purdue's 86/50 method?

Answer: 28. The high is capped at 86, so (86 + 70) ÷ 2 = 78, and 78 minus 50 is 28. The plain average would give 32.5.

Start date

A total means nothing without the day counting began. Each model names it.

January 1 Most pests Japanese beetle adults, for one, are counted from January 1. Winter adds almost nothing in cold places, so the date mostly sets a common starting line.

Planting day Crops Sweet corn and field corn count from the day they are planted.

An event Some pests Some insect models start at a trap catch or first eggs found, which you have to see for yourself.

Place and year

The same date carries very different heat from one place to the next. Here is the average year in three places, base 50 °F, added up from January 1:

Average-year totals from the Degree Day Calculator for Raleigh (ZIP 27601), Des Moines (50309) and Bozeman (59715). The dots mark where each place passes 1,030, the total Iowa State gives for Japanese beetle adults to start coming out.

Japanese beetles show up around mid-May in Raleigh, mid-June in Des Moines and late July in Bozeman. A spray calendar copied from another state would be weeks off. Bozeman gathers about 2,000 degree days in a whole year, while Raleigh passes that by the end of June.

Years differ too. A warm spring runs ahead of the average and a cold one behind. That is why the calculator tracks this year's total next to the average, and why a date from last year's notes is only a rough guide.

Degree Day Calculator This year's degree days for your location against an average year, with presets for crops, lawn work and pests, each from an extension source.

Open the calculator

Timing work

Extension services publish degree day totals for common pests and crops. A few of them, all base 50 °F:

Job	Total	Counted from
Crabgrass starts sprouting	200	Jan 1
Japanese beetles start	1,030	Jan 1
Japanese beetles end	2,150	Jan 1
Sweet corn ready, example variety	1,302	Planting

Crabgrass: Purdue. Japanese beetle: Iowa State. Sweet corn: UConn's example. Varieties differ, so use the rating from the seed supplier.

Crabgrass. Purdue puts crabgrass germination at 200 degree days, base 50 °F. A pre-emergent only stops seeds that have not sprouted yet, so it has to be down and watered in before the total gets there.

Sweet corn plantings. UConn Extension uses degree days to space plantings for a steady harvest. If one planting will be picked over three days at about 19 degree days a day, that is 57. Make the next planting once 57 degree days have added up since the last one. In UConn's spring example, at about 5 degree days a day, that took 12 to 13 days.

Quick check

A source says a pest hatches at 200 degree days, base 10 °C. What is that in Fahrenheit?

Answer: 360. Each Celsius degree is 1.8 Fahrenheit degrees, so 200 × 1.8 = 360, and the 10 °C base is 50 °F. Adding 32 is only for a temperature, not a difference.

Seed packets count days, but a plant counts heat. The same variety takes longer in a cool summer than a hot one. The seed starting guide shows how that plays out in a short mountain season.

Summary

A degree day is the day's average temperature above a base, never below zero.
Use the base and the method the model was built on. Corn caps the high at 86 and raises the low to 50.
Every total counts from a start date. That is January 1, the planting day, or an event you see for yourself.
The same total comes weeks apart in different places, and shifts from year to year.
Convert Celsius totals by 1.8, and convert the base too.

Sources

University extension services, the same ones the Degree Day Calculator uses. The three curves are the calculator's own average-year totals.

UConn Extension, "Scheduling Sweet Corn Plantings" ↗. The average formula, the 50 °F base, the two example days, 1,302 degree days to maturity, and the 57 degree day spacing.
Nielsen, R.L., Purdue University (2020), "Heat Unit Concepts Related to Corn Development" ↗. The 86/50 modified method and the 90/72 example.
Patton, A., Purdue Turfgrass Science (2013), "When should I apply my preemergence herbicide for crabgrass control?" ↗. 200 degree days base 50 °F, and applying before germination.
Iowa State University Extension (2024), "Japanese beetle adults emerging now" ↗. Adults from 1,030 to 2,150 degree days base 50 °F, from January 1.
Kreuser, W.C. and Soldat, D.J., University of Wisconsin-Madison, Crop Science ↗. Trinexapac-ethyl reapplied at 200 degree days base 0 °C.

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