Thor never directly calls Groot by a specific name in the Marvel Cinematic Universe (MCU). However, he does refer to him as "the tree" or "the plant" in Guardians of the Galaxy and Avengers: Infinity War.
This is likely due to Groot's limited vocabulary and Thor's initial unfamiliarity with him. Groot only speaks in the phrase "I am Groot," making it difficult for Thor to understand him.
In Avengers: Endgame, Thor seems to have a better understanding of Groot, but he still refers to him as "the tree" or "the plant" during their interactions.
