'Green Thumb' means to be skilled at gardening.
Example of use: "Catelyn has a green thumb - she can make the desert bloom!!"
Not surprisingly, the origin of the idiom “green thumb” has its roots buried deep in the garden. The term, which is sometimes quite literal, is associated with England’s King Edward I, who enjoyed eating fresh green peas and who has been said to regularly give prizes to the serf who could display the greenest thumb from shelling them.