This page explains how to start and complete the Watch Out for the Flowers shrine quest.
How to Start the Quest
There is an island in Hylia river called Floret Sandbar, east of Wahgo Katta Shrine. Go to Floret Sandbar and to Magda (who has been nicknamed "Flowerblight Ganon" by fans) to start the quest.
How to Complete the Quest
Check the Hila Rao Shrine page for details on completing the quest.