A Rhododendron is a flower picked by hand from rhododendron bushes, 2 to 4 at a time. The game groups these bushes with the berry bushes, which is why the source table says “Berry Bushes”. There are 11 of them around your base, including one at Riverside, and a picked bush flowers again after about 20 minutes. Picking needs Farming mastery 1, which a shovel on your toolbar gives.
At first the only thing to do with it is study it: 3 Science at the Study Table for 30 red tech points.
Two more uses come later: Fragrance at the Distillation Cube (2 Rhododendrons and 2 Clay Urns), which comes with Church Smells I, and the graveyard Flower Beds (3 or 6 each).