In seasonal alpine ecosystems, drought impacts depend strongly on timing. Using a mesocosm experiment with six species typical of widespread Alpine grasslands, the study compared four two-month treatments: continuous irrigation (C), early-season drought (ED), mid-season drought (MD), and full-season drought (FD), measuring leaf growth and senescence onset/intensity. ED and FD reduced leaf growth for most species and at community level (−30% and −51% vs. C); only Carex curvula responded to MD (−24%). Senescence began ∼33 days earlier under ED and FD for most species. After two months, senescence increased under ED, MD and FD (+88%, +316%, +627%), with forbs more affected by MD than graminoids (+85% vs. +32%). Thus, drought most strongly reduced growth when early, but had the greatest senescence effects about one month after snowmelt, highlighting species-specific responses and drought timing as key controls on climate-change impacts in high-elevation grasslands.