People have been celebrating the summer solstice with elaborate rituals since prehistoric times. But humans aren’t the only species to take mark June 21 as a special time. Studies are showing the summer solstice is an important cue for plants too.

During the Northern Hemisphere’s summer solstice, the upper half of the earth is tilted toward the sun, creating the longest day and shortest night of the year. This solstice falls between June 20 and 22.