If a person is described as having green fingers, or a green thumb as goes the American version of the idiom, it means the described person has a talent for gardening or otherwise dealing with plants. She might have a knack for growing healthy flowers and other plants, or she might have a talent for reviving struggling ones. Such plants might grown indoors or outdoors, in pots and baskets or sprawling gardens. The origin of these English sayings likely stems from the association of the word “green” with the color of most plants, but it also is a word connected to most of the natural and organic world. Similar to other sayings, this idiom can be meant as a compliment in its true meaning or insult when used ironically.
Similar to other idiomatic expressions, this term has different versions depending on the location. For example, saying that someone who has a talent for gardening has green fingers is primarily an Australian and British version of the saying. In the United States, people tend to say those who are skilled with plant care have green thumbs.
The English sayings likely originate from the fact that most plants are green. If a person is talented at handling such green things, it's like she has chlorophyll — which makes plants green — in her blood. Many herbs, bushes, ferns, vines, and trees are green. Even most colorful flowers have green stems, stalks, and leaves. Plus, “green” is a word often used in connection with something natural, regardless of color.
Sometimes, people will say someone has green fingers or a green thumb and mean it in a sarcastic or ironic way, rather than truthful or as a compliment. Not everyone has a talent for gardening, and some people are actually bad at it. In fact, some people are so untalented at keeping flowers and plants alive, much less helping them thrive, it might seem as if they are cursed. So, people may say such a person has a green thumb, but actually mean the opposite. Depending on the speaker, using the idiom in this way can be construed as an insult or just playful banter.