Literal opposites:
* Black thumb: This is the most common and humorous opposite, implying a complete inability to keep plants alive.
* Brown thumb: This conveys a similar meaning to black thumb, but might be used specifically for someone who overwaters and kills plants.
More nuanced opposites:
* Lack of green thumb: This is a straightforward way to say someone doesn't have the talent for gardening.
* Unwillingness to garden: This implies someone is capable of gardening but chooses not to.
* Aversion to plants: This suggests a dislike or fear of plants.
* Indifference to plants: This describes someone who simply doesn't care about plants.
The best opposite will depend on the specific situation and the level of humor you want to convey.
