GDD calculator and tracker for lawns

How growing degree days work

A growing degree day (GDD) is a heat unit. To get one day's GDD you average that day's high and low temperature, then subtract a base temperature (the temperature below which the plant does not meaningfully grow). If the answer is below zero it counts as zero, and each day's units are added to a running total. Turf managers use that running total to time crabgrass pre-emergent, track spring greenup, and schedule plant growth regulator (PGR) sprays.

What base temperature to use

Base temperature is the cutoff below which growth stalls, and it changes with what you are tracking. Seasonal tracking for pre-emergent and greenup uses a 50°F base with an 86°F high cap. PGR tracking uses Celsius: a 0°C base for cool-season grass and a 10°C base for warm-season grass. Using Fahrenheit for a PGR would count too much heat and tell you to respray far too often.

PGR reapplication targets

A PGR like trinexapac-ethyl (the active ingredient in Primo Maxx) is re-sprayed once enough heat has accumulated, not on a fixed number of days. These research targets come from low-mown golf greens, so a home lawn mowed higher will differ. Use them as a starting point and always follow the product label.

• Cornell Turfgrass Program: suppression begins rebounding after about 200 GDD on creeping bentgrass greens.
• Reasor et al. 2018, Crop Science: estimated trinexapac-ethyl reapplication interval on ultradwarf bermudagrass greens (about 262 GDD when measured on a 0°C base).

New to timing Primo Maxx by heat instead of the calendar? Read the Primo Maxx GDD calculator guide.

Common questions

What are growing degree days for lawn care?

Growing degree days (GDD) measure accumulated heat above a base temperature. Seasonal warm-season turf tracking often uses base 50°F with a high-temperature cap, while PGR tracking in Lawn Dominator uses Celsius-scale GDD from the application date. Lawn care professionals use GDD to time pre-emergent herbicide applications, track grass greenup, and schedule PGR reapplication windows.

When does bermuda grass green up based on GDD?

No universally sourced GDD threshold has been established for bermuda greenup in peer-reviewed extension literature. Texas A&M AgriLife Extension identifies 65°F soil temperature (at depth, with consistent nighttime temps above 60°F) as the threshold for active bermuda growth and recovery, which is the most reliable signal to track. GDD accumulation can serve as a secondary trend indicator alongside soil temperature and visual green-up, but specific GDD numbers for bermuda greenup vary significantly by cultivar, location, and winter injury.

When do I reapply Primo Maxx based on GDD?

Primo Maxx (trinexapac-ethyl) is typically tracked from the previous application date using Celsius-scale GDD: 0°C for cool-season trinexapac sessions and 10°C for warm-season trinexapac sessions. Higher application rates can extend the interval. Always refer to the product label for your specific rate and conditions, as the exact interval varies with grass type, growth rate, weather stress, and application rate.

What base temperature is used for GDD in lawn care?

Seasonal GDD accumulation for pre-emergent and greenup timing uses base 50°F with an 86°F high-temperature cap. For PGR reapplication timing, this calculator converts the displayed Fahrenheit high and low temperatures to Celsius first, then applies max(avg°C - base°C, 0). Cool-season trinexapac sessions use a 0°C base; warm-season trinexapac sessions use a 10°C base.