Q: Why was the Incredible Hulk so good at gardening?
A: Because he had a green thumb!
Explanation: Dr. David/Robert Bruce Banner becomes the superhero known as the Hulk, or the Incredible Hulk, when he becomes angry. In that process of becoming the Hulk, his body transforms, becoming larger with lots of muscles, and his skin turns green. When his skin turns green, his whole body turns green, including his thumb.
To have a green thumb is an idiom that means to be really good at taking care of plants and gardens. My wife has a green thumb, which you can see when you admire the succulents, bonsai, orchid, and geraniums in the window garden. OK, you don’t really need a green thumb to grow geraniums, but you get the idea.
This joke is funny because it plays with the literal and the figurative meanings of green thumb.
Watch here to see a transformation of Dr. Banner into the Hulk. Better yet, watch this video to learn how to grow geraniums: