The Crazy, Stupid, Love star's poor needle skills left him with a design on his left arm that looked like the plant.
Hollywood hunk Ryan told British Metro newspaper: 'One is supposed to be a monster's hand dropping a bloody heart but I did it myself with a tattoo kit, so it looks like a cactus.'
But that didn't put off the 30-year-old from getting more inkings, as he has the cover of children's book The Giving Tree on his left upper arm.
He added: 'Another one is a ghostly lady and a skeleton, and the other one is the cover of The Giving Tree, a children's book my mother used to read to my sister and me - so that one is significant.'