Guide

Growing degree days (GDD): what they mean and how to use them

Heat accumulation, base temperature, and tying them to weeds and pests.

Growing degree days (GDD) summarize how much heat has accumulated above a baseline (often 50°F for many turf/insect models). Higher cumulative GDD ≈ more biological activity—weed germination windows, insect emergence, disease risk periods.

Placeholder: example cumulative GDD curve through the year.

What “base 50°F” means

Each day contributes roughly max(0, daily_mean_temp_F − 50). Cool days add little; warm days add a lot. Models stack those into cumulative GDD from a start date (often Jan 1 for northern lawns—your extension may differ).

How people use GDD in turf

Align pre-emergent discussions with soil temperature—but GDD stacks help compare one season to another.
Track pest emergence (grubs, armyworms) vs calendar dates that drift year to year.
Compare regional reports (“we hit 300 GDD by early May this year”) even when spring is early or late.

In GrassGuidePro

The yard chart shows an illustrative cumulative GDD curve from your ZIP and hemisphere—it’s for context next to playbook load and logs, not a replacement for soil thermometers or label directions.