How would I implement this fold function?

Given are the two datatypes Color and Plant.

data Color = Red | Pink | White | Blue | Purple | Green | Yellow
   deriving (Show, Eq)

data Plant =
     Leaf
   | Blossom Color
   | Stalk Plant Plant
   deriving (Show, Eq)

Now I'm supposed to implement a function fold_plant of following type:

(x -> x -> x) -> (Color -> x) -> x -> Plant -> x

The way I understand a fold function is that it takes a list and for each iteration it removes the first element from the list and does something with that element.

Apparently fold_plant Stalk Blossom Leaf is the identity on plants.

Now I know that in Haskell you make functions like this:

fold_plant :: (x -> x -> x) -> (Color -> x) -> x -> Plant -> x
fold_plant = do something

But from here on I don't know how a fold function would work an a plant.