Growing Degree Day Calculator
Calculate daily and cumulative growing degree days from manual high and low temperatures to time crabgrass pre-emergent applications with sourced GDD base 50 thresholds.
How this is calculated
Growing degree days (GDD) measure accumulated heat above a base temperature using the simple average method:
daily GDD = max(0, ((daily high + daily low) / 2) - base temperature)
season total = starting total + sum of each day's GDDWorked example: a day with a high of 75 °F and a low of 55 °F at base 50 °F gives (75 + 55) / 2 - 50 = 15 GDD. A day averaging below the base contributes zero (the floor prevents negative accumulation). Three days of 15, 0, and 20 GDD with no starting total produce a season total of 35 GDD.
Purdue Turf notes that "most of the models use a base temperature of 50 (abbreviated as GDD50)." The same program reports that "research suggests that 200 GDD need to accumulate with a base of 50 °F before crabgrass germinates (source: Dr. Ron Calhoun)." The MSU GDD Tracker (gddtracker.msu.edu) tracks crabgrass pre-emergent and germination models and starts counting on February 15; the Purdue tracking page says to "download the data from the beginning of the year." This calculator uses manual daily high and low temperatures and does not pull weather data (no backend, no ZIP lookup). If you know your season total from a weather service, type it in the starting total field and add today's temperatures.
Once the timing is decided, the pre-emergent rate calculator converts the label's active ingredient rate into product per 1,000 sq ft for your formulation, and the herbicide tank mix calculator turns that into ounces per tank. For a season-long fertilization plan, the annual fertilization schedule template lays out the full program by region and turf type.
Sources
- 200 GDD base 50 F crabgrass germination threshold (Dr. Ron Calhoun). Purdue Turf. Retrieved .
- GDD base 50 is used by most turf models (Purdue Turf tracking page). Purdue Turf. Retrieved .
- MSU GDD Tracker (Crabgrass PRE and Crab Germination models, starts Feb 15). Michigan State University. Retrieved .
- Crabgrass: soil temp 55 F germination threshold (UW Extension). University of Wisconsin Extension. Retrieved .
- Crabgrass: soil temp 53 to 55 F for 5 days (UMD Extension). University of Maryland Extension. Retrieved .