Similar to a mansplainer, one who explains plants to a horticultural professional, or one who has much more experience and knowledge in landscape and plant care than the explainer.
I got into an argument with a plantsplainer yesterday who insisted my pumpkin was a watermelon and scolded me to prune my perfectly healthy tomatoes full of fruit.
to condescendingly explain vegan or vegetarian theory or values to meat eaters, often using inappropriate or made-up language
"Someone had to die for you to eat that. Enjoy your rotting animal carcass, carnist."
"Don't plantsplain to me, hippy!"