Plant growth regulator
A product that slows vertical leaf growth, so the turf needs less mowing and puts more into roots and density.
The common cool-season active is trinexapac-ethyl, applied to greens on a growing-degree-day interval. Others work at different points in the same hormone pathway and behave differently on the way out.
The important part is the end of the interval, not the start. When a regulator wears off the plant can rebound — growth comes back faster than baseline for a short window — so an interval that quietly stretched into a heat wave shows up as a mowing problem and a green-speed problem at the same time.
Track it on accumulated heat and the reapplication date stops being a guess. That is the single most common use of a GDD tracker on a small course.